Computations and Output After all the information has been entered, the ungaged inflows can be computed by running RAS normally (clicking the
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Figure 8-58. HEC-DSS Viewer inside of HEC-RAS
Once a record is selected to view, press the Plot/Tabulate Selected Pathname(s) button and the plot will appear as shown in Figure 859.
Figure 8-59. Plot of Computed Ungaged Inflows.
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CHAPTER
9
Viewing Results After the model has finished the steady or unsteady flow computations the user can begin to view the output. Output is available in a graphical and tabular format. The current version of the program allows the user to view cross sections, water surface profiles, general profiles, rating curves, hydrographs, X-Y-Z perspective plots, detailed tabular output at a single location, and summary tabular output at many cross sections. Users also have the ability to develop their own output tables.
Contents
Cross Sections, Profiles, and Rating Curves
X-Y-Z Perspective Plots
Tabular Output
Viewing Results From The River System Schematic
Stage and Flow Hydrographs
Viewing Computational Level Output for Unsteady Flow
Viewing Ice Information
Viewing Data Contained in an HEC-DSS File
Exporting Results to HEC-DSS
Cross Sections, Profiles, and Rating Curves Graphical displays are often the most effective method of presenting input data and computed results. Graphics allow the user to easily spot errors in the input data, as well as providing an overview of the results in a way that tables of numbers cannot.
Viewing Graphics on the Screen To view a graphic on the screen, select Cross Sections, Water Surface Profiles, or Rating Curves from the View menu on the HEC-RAS main window. Once you have selected one of these options, a window will appear with the graphic plotted in the viewing area. An example cross-section plot is shown in Figure 9-1. The user can plot 9-1
Chapter 9 Viewing Results any cross section by simply selecting the appropriate reach and river station from the list boxes at the top of the plot. The user can also step through the cross section plots by using the up and down arrows.
Figure 9-1 Example Cross Section Plot
An example profile plot is shown in Figure 9-2. The profile plot displays the water surface profile for the first reach in the river system. If there is more than one reach, additional reaches can be selected from the Options menu on or the reach button at the top of the window. An example rating curve plot is shown in Figure 9-3. The rating curve is a plot of the water surface elevation versus flow rate for the profiles that were computed. A rating curve can be plotted at any location by selecting the appropriate reach and river station from the list boxes at the top of the plot.
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Figure 9-2 Example Profile Plot
Figure 9-3 Example Rating Curve Plot
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Graphical Plot Options Several plotting features are available from the Options menu on all of the graphical plots. These options include: zoom in; zoom out; selecting which plans, profiles, reaches and variables to plot; and control over labels, lines, symbols, scaling, grid options, zoom window location, font sizes, and land marks. In addition to using the options menu at the top of each graphic window, if a user presses the right mouse button while the cursor is over a graphic, the options menu will appear right at the cursor location. In general, the options are about the same on all of the graphics. Zoom In. This option allows the user to zoom in on a portion of the graphic. This is accomplished by selecting Zoom In from the Options menu, then specifying the area to zoom in on with the mouse. Defining the zoom area is accomplished by placing the mouse pointer at a corner of the desired zoom area. Then press down on the left mouse button and drag the mouse to define a box containing the desired zoom area. Finally, release the left mouse button and the viewing area will display the zoomed-in graphic. A small window showing the entire graphic will be placed in one of the corners of the graphic. This window is called the Zoom Window. The Zoom Window shows the entire graphic with a box around the zoomed in area. The user can move the zoom box or resize it in order to change the viewing area. Zoom Previous. This option will re-display the graphic back to the size that it was one operation previous (i.e. if I zoomed in three times, then select Zoom Previous, the window would go back to the size it was after the second zoom in). HEC-RAS will remember the last 10 window sizes of the graphic and allow the user to use the Zoom Previous to go back through them. Zoom Out. This option doubles the size of the currently zoomed in graphic. Full Plot. This option re-displays the graphic back into its original size before you zoomed in. Using the Full Plot option is accomplished by selecting Full Plot from the Options menu. Pan. This option allows the user to move the graphic around while in a zoomed in mode. After zooming in, to move the graphic around, select Pan from the Options menu. Press and hold the left mouse button down over the graphic, then move the graphic in the desired direction. A shortcut to selecting the Pan option is to hold down the Shift Key to put the pointer into the Pan mode. Simply release the Shift Key to turn off the Pan mode. Measuring Tool. On any of the HEC-RAS graphics, even the river system schematic, the user can turn on a measuring tool and draw a multi point line (Called a polyline), and HEC-RAS will report back the 9-4
Chapter 9 Viewing Results length of the line, the area of the polygon formed by connecting the first and last point drawn, the dx length, the dy length, and the slope (dx/dy). To use this option simply hold down the Control Key while over the graphic, then draw the multi point line by pressing the left mouse button at each location you want to have a point. To end the line, simple release the Control key after the last point is drawn. Animate. This option was developed for unsteady flow output analysis, but can also be used for steady flow output. This option works with the cross section, profile, and X, Y, Z perspective plots. When this option is selected, a window will appear that allows the user to control the animation of any currently opened graphics. The user has the option to too “play” a graphic, which means to step through the time sequence of computed profiles. In a steady flow analysis, it can be used to switch between the profiles conveniently. Plans. This option allows the user to select from the available Plans for plotting. The default plan is the currently opened plan. The user can select additional plans to view for comparison of results graphically. Profiles. This option allows the user to select which profiles they would like to have displayed on the graphic. This option does not apply to the rating curve, it automatically plots all of the profiles. Reaches. This option allows the user to select which river reaches they would like to have displayed. This option only applies to the profile plot. Variables. This option allows the user to select whatever variables are available for plotting. The number and type of variables depends on what type of graphic is being displayed. The following is a list of variables that can be found on the profile plot: water surface, energy, critical water surface, observed water surfaces, Left main channel bank elevation, right main channel bank elevation, reach labels, ice cover, left and right levees, pilot channels, sediment elevations, and left and right lateral structures. The cross section plot is has the following eight variables: water surface, filled in water surface, energy, critical depth, observed water surface, ice cover, Manning’s n values, and pilot channels. Labels. This option allows the user to change the labels for the plot caption, as well as the labels used for the axis. The user can select any or all of the following items to be added to the caption: project title, plan title, run date, run time, geometry title, flow title, river and reach names, cross section descriptions, cross section river stationing, cross section node names, and any user defined additional text. Lines and Symbols. This option allows the user to change the line types, line colors, line widths, symbol types, symbol sizes, and symbol colors, fill patterns, and the line labels. When the user selects this option, a window will appear as shown in Figure 9-4.
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Figure 9-4 Line and Symbol Options Window
When the Line and Symbol Options window comes up, it will list only the information from the current plot. When this window is in the "Current Plot Line Styles" mode, the user can only change the information for the current plot. If the user wants to change the default line and symbol options for all of the plots, they must select Default Line Styles at the top of the window. When this option is selected, the user will be able to change the label, line, and symbol options for every variable that is plotted in the program. To use this option, the user finds the variable that they want to change from the list on the left side of the window. Select that variable by clicking the left mouse button while over top of the variable. Once a variable is selected, the options that are set for that variable will be highlighted with a red box around each option. The user can change whatever option they want, as well as changing the label for that variable. If a variable does not have a default label, you cannot enter one for that variable. Once the user has made all of the changes that they want to all of the desired variables, they should press the OK button. The changes will be saved permanently, and any plot that is displayed within HEC-RAS will reflect the user-entered changes. Scaling. This option allows the user to define the scaling used for the plot. Users are allowed to set the minimum, maximum, and labeling increment for the X and Y axis. Scaling can be set temporarily, or scaling can be set to be persistent (scaling stays constant for all cross sections). Persistent scaling is only available for the cross section and rating curve plots. Grid. This option allows the user to overlay a grid on top of the graphic. Users have the option to have both major and minor tics displayed, as well as a border around the plot.
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Chapter 9 Viewing Results Zoom Window Location. This option allows the user to control which corner of the plot that the zoom window will be placed, and the size of the window. Font Sizes. This option allows the user to control the size of all of the text displayed on the graphic. Land Marks. This option is specific to profile plots. With this option the user can turn on additional labels that will be displayed as land marks below the invert of the channel. Three types of land marks can be displayed: cross section river stations; node names; or cross section descriptions. In addition to these three variables, once one of the three are displayed, the user can select to edit the land mark labels. This will allow the user to put a label at a specific location on a plot.
Plotting Velocity Distribution Output The user has the option of plotting velocity distribution output from the cross section viewer. Velocity distributions can only be plotted at locations in which the user has specified that flow distribution output be calculated during the computations. To view the velocity distribution plot, first bring up a cross section plot (select "Cross Sections" from the view menu of the main HEC-RAS window). Next, select the cross section in which you would like to see the velocity distribution output. Select Velocity Distribution from the Options menu of the cross section window. This will bring up a pop up window (Figure 9-5) that will allow you to set the minimum velocity, maximum velocity, and velocity increment for plotting. In general, it is better to let the program use the maximum velocity range for plotting. Next, the user selects Plot Velocity Distribution, then press the "OK" button and the velocity distribution plot will appear as shown in Figure 9-6. For details on how to select the locations for computing the velocity distribution, see Chapter 7 and 8 of the User’s Manual. For information on how the velocity distribution is actually calculated, see Chapter 4 of the Hydraulic Reference Manual.
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Figure 9-5 Velocity Distribution Options
Figure 9-6 Velocity Distribution Plot
Plotting Other Variables in Profile To plot variables other than the water surface in profile, select General Profile Plot from the View menu of the main HEC-RAS window. Any variable that is computed at a cross section can be displayed in profile. An example would be to plot velocity versus distance. Other variables can be selected from the Plot Variables option under the Options menu of the plot. The user can plot several different variable types at one time ( e.g., velocity and area versus distance), but the scaling may not be appropriate when this is done. 9-8
Chapter 9 Viewing Results Once a user has selected variables for plotting in profile, the plot can be saved as a User Defined Plot. This is accomplished by selecting Save Plot from the Options menu at the top of the window. Once a user saves a plot, the plot can be recalled for any data set from the User Plots menu at the top of the window. An example of plotting variables in profile is shown in Figure 9-7. Additionally, the general profile plot has some predefined plots that the user can pick from. The predefined plots can be found under the Standard Plots menu at the top of the graphic window.
Figure 9-7 General Profile Plot of Variables Versus Distance
Plotting One Variable versus Another The rating curve plotting window has the ability to plot other variables besides discharge versus water surface elevation. Any variable that is computed at a cross section can be displayed against another computed variable (or variables). An example of this capability is shown in Figure 9-8. In this example, Discharge (x-axis) is being plotted against total flow area and main channel flow area (y-axis). To plot other variables, the user selects the X Axis Variable and Y Axis Variables from the Options menu of the rating curve plotting window. When selected variables to plot, keep in mind that all variables selected for a particular axis should have a similar range in magnitude.
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Figure 9-8 Example of Plotting One Variable Against Other Variables
Sending Graphics to the Printer or Plotter All of the graphical plots in HEC-RAS can be sent directly to a printer or plotter. The printer or plotter used depends on what you currently have set as the default printer or plotter in the Windows Print Manager. To send a graphic to the printer or plotter, do the following: 1. Display the graphic of interest (cross section, profile, rating curve, X-Y-Z, or river system schematic) onto the screen. 2. Using the available graphics options (scaling, labels, grid, etc.), modify the plot to be exactly what you would like printed. 3. Select Print from the File menu of the displayed graphic. When this option is selected, a pop up window will appear allowing you to modify the default print options. Change any desired options and press the Print button. The graphic will be sent to the Windows Print Manager. The print manager will then send the plot to the default printer or plotter. Note: The user can print multiple cross-sections at one time by using the Print Multiple option from the File Menu of the cross section and rating curve plots. This option also allows the user to establish how many cross sections or rating curves they would like to have printed on each page. 9-10
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Sending Graphics to the Windows Clipboard All of the HEC-RAS graphics can be sent to the Windows Clipboard. Passing a graphic to the clipboard allows that graphic to then be pasted into another piece of software (i.e., a word processor or another graphics program). To pass a graphic to the windows clipboard, and then to another program, do the following: 1. Display the graphic of interest on the screen. 2. Using the options menu, modify the plot to be exactly what you want. 3. Select Copy Plot to Clipboard from the File menu of the displayed graphic. The plot will automatically be sent to the Windows Clipboard. 4. Bring up the program that you want to paste the graphic into. Select Paste from the Edit menu of the receiving program. Once the graphic is pasted in, it can be re-sized to the desired dimensions. HEC-RAS sends and displays all graphics in a Window's Meta file format. Since Meta files are vector based graphics, the graphic can be resized without causing the image to distort.
X-Y-Z Perspective Plots Another type of graphic available to the user is the X-Y-Z Perspective Plot. The X-Y-Z plot is a 3-dimensional plot of multiple cross sections within a reach. An example X-Y-Z Perspective plot is shown in Figure 9-9.
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Figure 9-9. Example X-Y-Z Perspective Plot.
The user has the ability to select which reaches to be plotted, the range of the river stations, and which plans and profiles to be displayed. The plot can be rotated left and right, as well as up and down, in order to get different perspectives of the river system. Zoom in and zoom out features are available, as well as the ability to move around with scroll bars. The user can choose to overlay the water surface or not. The user has the ability to overlay a grid on the plot, as well as a legend and labels at the top. The graphic can be sent to the printer/plotter or the clipboard just like any other plot. Sending the graphic to the printer or clipboard is accomplished by selecting the Print or Clipboard options from the File menu. The user also has the option to reverse the order in which the water surface profiles are displayed. This option allows the user to display the higher water surfaces first, such that the lower profiles are not covered up.
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Tabular Output Summary tables of the detailed water surface profile computations are often necessary to analyze and document simulation results. Tabular output allows the user to display large amounts of detailed information in a concise format. HEC-RAS has two basic types of tabular output, detailed output tables and profile summary tables.
Detailed Output Tables Detailed output tables show hydraulic information at a single location, for a single profile. To display a detailed output table on the screen, select Detailed Output Tables from the View menu of the main HEC-RAS window. An example detailed output table is shown in Figure 9-10.
Figure 9-10 Example Cross Section Detailed Output Table
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Chapter 9 Viewing Results By default, this table comes up displaying detailed output for cross sections. Any cross section can be displayed in the table by selecting the appropriate river, reach and river station from the list boxes at the top of the table. Also, any of the computed profiles can be displayed by selecting the desired profile from the profile list box. Additionally, different plans can be viewed by selecting a plan from the plan list box. Users can also view detailed hydraulic information for other types of nodes. Other table types are selected from the Type menu on the detailed output table window. The following types are available in addition to the normal cross section table (which is the default): Culvert. The culvert table type brings up detailed culvert information. This table can be selected for normal culverts, or for culverts that are part of a multiple opening river crossing. An example culvert specific table is shown in Figure 9-11.
Figure 9-11 Example Culvert Type of Cross Section Table
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Chapter 9 Viewing Results Bridge. The bridge table type brings up detailed output for the cross sections inside the bridge as well as just upstream of the bridge. The bridge table type can be selected for normal bridge crossings, or for bridges that are part of a multiple opening river crossing. An example of the bridge specific cross section table is shown in Figure 9-12.
Figure 9-12 Example Bridge Type of Cross Section Table
Multiple Opening. The multiple opening type of table is a combination of the cross section table and the bridge and culvert tables. That is, if the user has defined multiple opening (bridges, culverts, and conveyance areas), then this table can be used to view the hydraulic results for each specific opening. Inline Structure. The Inline Structure type of table can be used to view detailed output for any inline weirs and/or gated spillways that have been entered by the user.
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Chapter 9 Viewing Results Lateral Structure. The Lateral Structure type of table can be used for viewing detailed output from a lateral weir, gated spillway, culvert, and rating curves. Storage Area. This table provides output about an individual storage area. Information includes water surface elevation, total inflow, total outflow, and net inflow. Storage Area Connection. This table provides detailed information about storage area connections. Storage area connections can consist of weirs, gated spillways, and culverts. Pump Stations. This table provides detailed information about pump stations. Pump station output includes to and from water surface elevations, total flow, flow through each pump group, flow through each pump, head difference, and efficiency. Flow Distribution In Cross Sections. The Flow Distribution table type can be used to view the computed flow distribution output at any cross section where this type of output was requested. An example of the flow distribution table output is shown in Figure 9-13.
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Figure 9-13 Example of the Flow Distribution Type of Table
At the bottom of each of the detailed output tables are two text boxes for displaying messages. The bottom text box is used to display the definition of the variables listed in the table. When the user presses the left mouse button over any data field, the description for that field is displayed in the bottom text box. The other text box is used to display any Errors, Warnings, and Notes that may have occurred during the computations for the displayed cross section.
Detailed Output Table Options Plans. This option allows the user to select which plan, and therefore output file, they would like to view. This option is available from a list box at the upper right hand side of the window.
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Chapter 9 Viewing Results Under the Options menu of the cross section table window, the user has the following options: Include Interpolated XS’s. This option allows the user to either view interpolated cross-section output or not. Turning the "include interpolated XS’s" option on (which is the default), allows interpolated sections to be selected from the river station box. Turning this option off gets rid of all the interpolated sections from the river station selection box, and only the user entered cross-sections are displayed. Include Errors, Warnings, and Notes in Printout. This option allows the user to have the errors, warnings, and notes information printed below the table, when the option to print the table is selected. Units System for Viewing. This option allows the user to view the output in either English or Metric units. It does not matter whether the input data is in English or Metric, the output can be viewed in either system.
Profile Summary Tables Profile summary tables are used to show a limited number of hydraulic variables for several cross sections. To display a profile summary table on the screen, select Profile Summary Table from the View menu of the main HEC-RAS window. An example profile summary table is shown in Figure 9-14.
Figure 9-14 Example Profile Table
There are several standard table (Std. Tables) types available to the user. Some of the tables are designed to provide specific information at hydraulic structures (e.g., bridges and culverts), while others provide generic information at all cross sections. The standard table types available to the user are: Standard Table 1. The is the default profile type of table. This table gives you a summary of some of the key output variables. 9-18
Chapter 9 Viewing Results Standard Table 2. This is the second of the standard summary tables. This table provides information on the distribution of flow between the left overbank, main channel, and right overbank. This table also shows the friction losses, as well as contraction and expansion losses that occurred between each section. Energy losses displayed at a particular cross section are for the losses that occurred between that section and the next section downstream. Four XS Culvert. This standard table provides summary results for the four cross sections around each of the culverts in the model. The four cross sections are the two immediately downstream and the two immediately upstream of the culvert. This table will list all of the culverts in the model for the selected reaches. Culvert Only. This standard table provides hydraulic information about the culvert, as well as the inlet control and outlet control computations that were performed. Six XS Bridge. This table provides summary results for the six cross sections that make up the transition of flow around a bridge. The six cross sections include the two cross sections just downstream of the bridge; the two cross sections inside of the bridge; and the two cross sections just upstream of the bridge. The program will display results for all the bridges in the model within the selected reaches. When viewing this table, on occasion there will be no displayed results for the cross sections inside of the bridge. This occurs only when the user has selected a bridge modeling approach that does not compute results inside of the bridge. This includes: Yarnell’s method; both pressure flow equations; and pressure and weir flow solutions. Bridge Only. The bridge only table shows summary information specifically for bridges. Bridge Comparison. The bridge comparison table shows the results for all of the user selected bridge modeling approaches that were computed during the computations. For example, the program can calculate low flow bridge hydraulics by four different methods. The resulting upstream energy for the user selected methods will be displayed in this table. Multiple Opening. This table shows a limited number of output variables for each opening of a multiple opening river crossing. Four XS Inline Structure. This table displays summary results of the four cross sections immediately around an inline weir and/or gated spillway. The four cross sections are the two immediately upstream and the two immediately downstream of the inline weir and/or gated spillway. Inline Structure. This table shows the final computed water surface and energy just upstream of each of the inline weir and/or gated spillways. In addition to these elevations, the table displays the total flow, the flow over the weir, and the total flow through all of the gates. 9-19
Chapter 9 Viewing Results Lateral Structure. This table shows a limited set of output variables for all of the lateral weir/spillway structures within the selected reaches. Encroachment 1, 2, and 3. These three standard tables provide various types of output for the computations of floodway encroachments. HEC-FDA. This table provides information that can be exported to the HEC Flood Damage Analysis (FDA) program. The table displays total flow, channel invert elevation, and water surface elevation. HEC-5Q. This table provides information that can be exported to the HEC-5Q (river and reservoir water quality analysis) program. The table displays only the specific parameters required by the HEC-5Q program. Ice Cover. This table shows summary output of ice information. This table was designed for performing a study that includes ice cover. Junctions. This summary table provides a limited set of output for all of the cross sections that bound a junction. This table will show this output for all of the junctions found in the model. Storage Areas. This table shows a limited amount of output for all of the storage areas in the model. Output includes: water surface elevation; minimum storage area elevation; surface area; and volume. Conn with Culverts. This table will show summary output for storage area connections that contain culverts. Pump Stations. This table shows a limited amount of output for any of the pump stations contained within the model. To view one of the types of tables, select the desired table type from the Std. Tables menu on the profile summary table. In addition to the various types of profile tables, the user can specify which plans, profiles and reaches to include in the table. The plans, profiles and reaches options are available from the Options menu on the profile plot. The user also has the ability to turn the viewing of interpolated cross sections on or off. The default is to view all cross-sections, including the interpolated ones. To prevent the interpolated sections from showing up in the table, de-select Include Interpolated XS's from the Options menu. Another feature available to users is the ability to set the number of decimal places that will be displayed for any variable of the predefined tables. Once a pre-defined table is selected from the Tables menu, select Standard Table # Dec Places from the Options menu. A window will appear displaying the current number of decimal places
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Chapter 9 Viewing Results for each variable. The user can change the number of decimal places to what ever they wish. User’s also have the ability to view summary output tables in either English or metric units. This is available from the Options menu on the profile tables. It does not matter whether the input data is in English or metric, the output can be viewed in either system.
User Defined Output Tables A special feature of the profile summary tables is the ability for users to define their own output tables. User defined output tables are available by selecting Define Table from the Options menu of the profile table. When this option is selected, a window will appear, as shown in Figure 9-15. At the top of the window is a table for the user selected variable headings (Table Column Headings), the units, and the number of decimal places to be displayed for each variable. Below this table is a table containing all of the available variables that can be included in your user-defined table. The variables are listed in alphabetical order. In addition to the variable names, to the right of each variable is a description. To add variables to the column headings, simply double click the left mouse button while the mouse pointer is over the desired variable. The variable will be placed in the active field of the table column headings. To select a specific column to place a variable in, click the left mouse button once while the mouse pointer is over the desired table column field. To delete a variable from the table headings, double click the left mouse button while the mouse pointer is over the variable that you want to delete. The number of decimal places for each variable can be changed by simply typing in a new value. User defined tables are limited to 15 variables. Once you have selected all of the variables that you want, press the OK button at the bottom of the window. The profile table will automatically be updated to display the new table.
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Figure 9-15 User Defined Tables Window
Once you have the table displayed in the profile table window, you can save the table headings for future use. To save a table heading, select Save Table from the Options menu on the profile table window. When this option is selected, a pop up window will appear, prompting you to enter a name for the table. Once you enter the name, press the OK button at the bottom of the pop up window. The table name will then be added to a list of tables included under the User Tables menu on the profile table window. To delete a table from the list of user defined tables, select Remove Table from the Options menu of the profile table window. When this option is selected, a pop up window will appear displaying a list of all the user-defined tables. Click the left mouse button over the tables that you want to delete, then press the OK button. The selected tables will then be deleted from the User Tables menu list.
Sending Tables to the Printer To send a table to the printer, do the following: 1. Bring up the desired table from the tabular output (cross section or profile tables) section of the program.
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Chapter 9 Viewing Results 2. Select Print from the File menu of the displayed table. When this option is selected, a pop up window will appear allowing you to modify the default print options. Once you have set the printer with the desired options, press the Print button. The table will be sent to the Windows Print Manager. The Windows Print Manager will control the printing of the table. The profile summary type of tables, allow you to print a specific portion of the table, rather than the entire table. If you desire to only print a portion of the table, do the following: 1. Display the desired profile type table on the screen. 2. Using the mouse, press down on the left mouse button and highlight the area of the table that you would like to print. To get an entire row or column, press down on the left mouse button while moving the pointer across the desired row or column headings. 3. Select Printer from the File menu of the displayed table. Only the highlighted portion of the table and the row and column headings will be sent to the Windows Print Manager.
Sending Tables to the Windows Clipboard To pass a table to the Windows Clipboard, and then to another program, do the following: 1. Display the desired table on the screen. 2. Select Copy to Clipboard from the File menu of the displayed table. 3. Bring up the program that you want to pass the table into. Select Paste from the Edit menu of the receiving program. Portions of the profile tables can be sent to the Clipboard in the same manner as sending them to the printer.
Viewing Results From the River System Schematic The user has the option of either bringing up graphics and tables from the View menu on the main HEC-RAS window (as discussed above), or from the river system schematic (found under geometric data). Once data have been entered, and a successful simulation has been made, the user can interact with the river system schematic. When the left mouse button is pressed over the river system schematic, a pop up menu will appear listing options that are relevant to the area of
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Chapter 9 Viewing Results the schematic that is located under the mouse pointer. An example of this is shown in Figure 9-16. In Figure 9-16, the pop up menu shown comes up whenever the user presses the left mouse button over a cross section. In this particular example, the mouse button was pressed over the cross section located at river station 9.9 of the Upper reach of Fall river. As shown in the menu, the user has the choice of editing the cross section data; plotting the cross section; plotting the profile for the reach containing this cross section; bringing up the XYZ plot for that reach; viewing tabular output; plotting the computed rating curve at this cross section; or viewing a picture of the location. Other pop up menus are available for bridges; culverts; junctions; and reach data.
Figure 9-16 Geometric Data Window with Pop up Menu
Stage and Flow Hydrographs If the user has performed an unsteady flow analysis, then stage and flow hydrographs will be available for viewing. To view a stage and/or flow hydrograph, the user selects Stage and Flow Hydrographs 9-24
Chapter 9 Viewing Results from the View menu of the main HEC-RAS window. When this option is selected a plot will appear as shown in Figure 9-17. The user has the option to plot just the stage hydrograph, just the flow hydrograph, or both as shown in the figure. Additionally, there are three tabs on the plot. The tabs are for plotting (Plot), viewing the data in tabular form (Table), and plotting a rating curve of the event (Rating Curve). By default the window comes up in a plotting mode. The stage and flow hydrograph plot also has a menu option to select the specific node types to be viewed. By default the plot comes up with a node type of cross section selected. This allows the user to view hydrographs at cross sections only. Other available node types include: Bridges/Culverts; Inline Structures; Lateral Structures; Storage Areas; Storage Area Connections; and Pump Stations. There are several options available for viewing this graphic. These options are the same as described previously for the cross section, profile, and rating curve plots. Additionally, this graphic can be sent to the windows clipboard, or the printer, as described under the previous plots. Additional output for the hydrograph plot includes statistics about the hydrographs (peak stage and flow, time of peak, and volume). Also, the user can simultaneously plot observed hydrograph data at locations where they have gaged information stored in a DSS file. The user attaches gaged hydrograph information to cross section locations from the Unsteady Flow Data editor.
Figure 9-17 Stage and Flow Hydrograph Plot
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Chapter 9 Viewing Results
Viewing Computational Level Output for Unsteady Flow When performing an unsteady flow analysis the user can optionally turn on the ability to view output at the computation interval level. This is accomplished by checking the box labeled Computation Level Output on the Unsteady Flow Analysis window (In the Computations Settings area on the window). When this option is selected an additional binary file containing output at the computation interval is written out. After the simulation the user can view computation level output by selecting either Unsteady Flow Spatial Plot or Unsteady Flow Time Series Plot from the View menu of the main HEC-RAS window. Shown in Figure 9-18 is an example of the Spatial Plot.
Figure 9-18. Unsteady Flow Spatial Plot for Computational Interval Output
As shown in Figure 9-18, the user can view either a profile plot, a spatial plot of the schematic, or tabular output. The user can select from a limited list of variables that are available at the computation level output. These are water surface elevation (XS WSEl); Flow (XS Flow); computed maximum error in the water surface elevation (XS WSEL ERROR); computed maximum error in the flow (XS FLOW ERROR); and maximum depth of water in the channel (DEPTH). Each of the plots can be animated in time by using the video player buttons at the top right of the window. This type of output can often be very useful in debugging problems within an unsteady flow run. Especially plotting the water surface error and animating it in time.
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Chapter 9 Viewing Results The other type of plot available at the computation interval output level is the Unsteady Flow Time Series Plot. When this option is selected the user will get a plot as shown in Figure 9-19.
Figure 9-19. Unsteady Flow Time Series Plot at Computation Interval Level
As shown in Figure 9-19, the user has the option to plot or tabulate the time series output. Additionally, the user can select from five variables to display on the plot/table. The variables are chosen from the Variables button at the top of the window.
Viewing Ice Information River ice information can be viewed both in a graphical and tabular format.
Viewing Graphical Ice Information on the Screen To view graphical ice information on the screen, select either Cross Sections, Profiles, or X-Y-Z Perspective Plot from the View menu on the HEC-RAS main window. Cross Section Plot. Figure 9-20 is an example cross section plot displaying ice. The ice cover is displayed by selecting Variables under the Options menu, then selecting the Ice Cover option. The ice thicknesses in the right overbank, main channel, and left overbank are displayed. The default color and fill pattern can be changed by the user
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Chapter 9 Viewing Results by selecting Lines and Symbols under the Options menu. Note that multiple profiles and multiple plans can be displayed on the same plot.
Figure 9-20 Cross Section Plot with Ice
Profiles Plot. An example of a profile plot with ice is shown in Figure 9-21. In this case, the WS-EG Profile was selected. As with the Cross Section plot, the ice cover is displayed by selecting Variables under the Options menu, then selecting the Ice Cover option. The ice thicknesses in the right overbank, main channel, and left overbank are displayed. The default color and fill pattern can be changed by the user by selecting Lines and Symbols under the Options menu. Note that multiple profiles and multiple plans can be displayed on the same plot.
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Figure 9-21 Profile Plot with Ice Cover
Ice information can also be displayed in profile plots by selecting the General Profile option and then selecting Variables under the Options menu. This provides a number of ice variables, including ice volume in the channel, left, and right overbanks; ice thickness in the channel, left, and right overbanks; top of ice elevation in the channel, left, and right overbanks; and bottom of ice elevations in the channel, left, and right overbanks. These plots can all be viewed in different widow sizes and printed. X-Y-Z Perspective Plot. As with the Cross Section plot, the ice cover is displayed by selecting Variables under the Options menu, then selecting the Ice Cover option. The ice thicknesses in the right overbank, main channel, and left overbank are displayed. The default color and fill pattern can be changed by the user by selecting Lines and Symbols under the Options menu.
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Viewing Tabular Ice Information Tabular information describing the results of the ice calculations can be displayed by selecting Profile Summary Table under the View menu on the HEC-RAS main window. Ice information is available directly by selecting the Ice Cover option under the Std. Tables menu of the Profile Table window. The Ice Cover option provides a table that includes the ice volume, ice thickness, and composite Manning’s n value for the main channel, left overbank, and right overbank. In addition, the Ice Cover Table includes the water surface elevation and the cumulative ice volume starting from the downstream end of the channel. An example table of ice information is shown in Figure 9-22. Tables of ice information can also be created using the Define Table option under the Options menu of the Profile Table window.
Figure 9-22 Ice Cover Table
Viewing Data Contained in an HEC-DSS File The HEC-RAS software can write and read data to and from the HECDSS (Data Storage System) database. The steady flow portion of HEC-RAS can read flow data to be used as profile information, and can write water surface profiles, storage-outflow information, and rating curves. The unsteady flow portion of HEC-RAS can read complete hydrographs (stage and flow), as well as gate settings to be used during a simulation. Observed data contained in a DSS file can be attached to specific cross sections for comparison with computed 9-30
Chapter 9 Viewing Results results at those locations, and computed profiles and hydrographs are written to the DSS file during an unsteady flow simulation. Because a DSS file can be used to share information between different HEC programs (such as HEC-HMS and HEC-RAS), it is often necessary to be able to view data contained within a DSS file. A DSS viewer is available from within the HEC-RAS software. To bring up the DSS viewer select DSS Data from the View menu of the main HEC-RAS window (Or press the button labeled DSS on the main window). When this option is selected a window will appear as shown in Figure 9-23.
Figure 9-23 HEC-DSS Viewer Window
As shown in Figure 9-23, the user selects a DSS file by pressing the open file button located next to the DSS Filename field. When a DSS file is selected, a list of the available pathnames within that file will show up in the table. Each DSS pathname represents a record of data stored within the DSS file. The user can select one or more DSS pathnames to be plotted and/or tabulated. A pathname is selected by using the left mouse button to select a row(s) in the table, then the button labeled Select highlighted DSS Pathnames is pressed and the pathname shows up in the lower box. The final step is to hit the Plot/Tabulate Selected Pathnames button, and the data will be plotted. An example plot is shown in Figure 9-24. As shown in Figure 9-24 there are two tabs on the window, one says Plot and the other says Table. By default the window comes up plotting the data. To view the data as a table, simply press the table tab. 9-31
Chapter 9 Viewing Results
Figure 9-24 Example Plot from the HEC-RAS DSS Viewer
Data can be viewed from one or more DSS files simultaneously. The user simple opens one DSS file and picks the desired pathnames, then opens another DSS file and selects additional pathnames. When the Plot/Tabulate button is pressed, the data from both DSS files will be plotted and/or tabulated. A few utilities are also available from the DSS viewer. These utilities include: Time Series Importer; Delete Selected Pathnames; and Squeeze the DSS file. The time series importer allows the user to enter regular interval time series data into a table, which can then be imported into a DSS file. To use this option select Time Series Import from the Utilities menu of the DSS Data Viewer. When this option is selected a window will appear as shown in Figure 9-25.
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Chapter 9 Viewing Results
Figure 9-25 DSS Time Series Data Import Utility
As shown in Figure 9-25, the user first selects a DSS file to import data into. Next a DSS Pathname must be entered for the data to be written to the DSS file. The pathname parts are separated with a “/” between each pathname part. Some parts can be left blank, but the B and C part must be entered at a minimum. Next the user enters the date and time of the first data point, as well as the interval of the data (the interval is selected from the available DSS intervals). Next the data units and data type are selected from the drop down lists. If the lists do not contain the units of your data you can enter them directly into the field. The data is then entered into the table at the bottom. You can cut and paste information into this table, using the standard windows keys of Ctrl-C for cut, and Ctrl-V for paste. There are buttons available to perform the following tasks: set the number of rows in the table (the default is 99); linearly interpolate missing values; delete a row; insert a row; add a constant to a highlighted section of the table; multiply the highlighted section by a factor; and set a highlighted section to a specific value. The utility labeled Delete Selected Paths is used to delete data from the DSS file. The user simply selects the pathnames they want to 9-33
Chapter 9 Viewing Results delete, then selects this option from the Utilities menu. A window will appear to asking if you are sure you want to delete the selected pathnames. If you answer OK, then the data will be deleted from the DSS file. The utility labeled Squeeze DSS File is used to compress the DSS file, such that it takes significantly less hard disk space. This is a convenient function if you are working with very large DSS files. To use this option just select Squeeze DSS File from the Utilities menu. A window will come up asking you if you want to squeeze the currently opened DSS file. If you answer OK then the file will be compressed.
Exporting Results to HEC-DSS The HEC-RAS software has the ability to export a limited set of results to a HEC-DSS file for both steady and unsteady flow simulations. When performing an Unsteady flow simulation, the program automatically writes stage and flow hydrographs to the DSS file, but only for the user-selected hydrograph output locations. Water surface profiles are also automatically written to the DSS file. The profiles are written for the user selected detailed output interval, as well as the overall maximum water surface profile (profile of the maximum stage at every cross section). Once a steady flow or unsteady flow simulation is performed, the user can write the following information to a DSS file: water surface profiles; computed rating curves; and storage-outflow information. To export computed results to a DSS file the user selects Export To HEC-DSS from the File menu of the main HEC-RAS window. When this option is selected a window will appear as shown in Figure 9-26.
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Figure 9-26 Export Computed Results to DSS Window
As shown in Figure 9-26, there are three tabs on the window; one for profiles, rating curves, and storage outflow. To export computed water surface profiles, select the Profiles tab from the window. Select the type of profiles that you want to export (water surface elevations or flow). Next select the specific profiles to be exported, as well as the reaches that you want to have profiles for. Select how you want the stationing to be labeled. This is accomplished by selecting one of the options under the field labeled Reach Starting Station. The user can have the river stationing labeled in feet or miles, and have it start at zero or whatever the magnitude is of the most downstream cross section. The final option is to press the Export Profile Data button, and the data will then be written to the DSS file. To write computed rating curves to the DSS file select the Rating Curve tab. When the rating curve tab is selected, the window will change to what is shown in Figure 9-27.
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Figure 9-27 Exporting Computed Rating Curves to HEC-DSS
As shown in Figure 9-27, to export a computed rating curve to DSS, select the river, reach, and river stations that you want to have exported to the DSS file. Then simply press the Export Rating Curves button to have the program write the data to the DSS file. If your profiles are not in the order from lowest flow to highest flow, turn on the option that says Sort flows in rating curve. This option will ensure that the curve is written in the order of increasing flow rate. The HEC-RAS program computes cumulative storage volumes for each of the water surface profiles. This information can be used for hydrologic routing in a hydrology model such as HEC-HMS or HEC-1. The HEC-RAS program allows the user to write out storage versus volume information to a DSS file. To use this option select the Storage Outflow tab from the Export to DSS window. When this option is selected a window will appear as shown in Figure 9-28.
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Figure 9-28 Exporting Storage-Outflow Information to HEC-DSS
As shown in Figure 9-28, the user selects the River, upstream reach, upstream river station, downstream reach, and downstream river station to completely define a routing reach in which they want to have storage-outflow information written to the DSS file. This can be done for as many reaches as you want within the model. After all of the reaches are defined, simply press the button labeled Export Volume Outflow Data to write the information to the DSS file.
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Chapter 10 Performing a Floodway Encroachment Analysis
CHAPTER
10
Performing a Floodplain Encroachment Analysis The evaluation of the impact of floodplain encroachments on water surface profiles can be of substantial interest to planners, land developers, and engineers. Floodplain and floodway evaluations are the basis for floodplain management programs. Most of the studies are conducted under the National Flood Insurance Program and follow the procedures in the "Flood Insurance Study Guidelines and Specifications for Study Contractors," FEMA 37 (Federal Emergency Management Agency, 11085). FEMA 37 defines a floodway "...as the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water-surface elevation by more than a designated height." Normally, the base flood is the one-percent chance event (100-year recurrence interval), and the designated height is one foot, unless the state has established a more stringent regulation for maximum rise. The floodway is usually determined by an encroachment analysis, using an equal loss of conveyance on opposite sides of the stream. For purposes of floodway analysis, the floodplain fringe removed by the encroachments is assumed to be completely blocked. HEC-RAS contains five optional methods for specifying floodplain encroachments. For information on the computational details of each of the five encroachment methods, as well as special considerations for encroachments at bridges, culverts, and multiple openings, see Chapter 10 of the HEC-RAS hydraulics reference manual. This chapter describes how to enter floodplain encroachment data, how to perform the encroachment calculations, viewing the floodplain encroachment results, and how to perform a floodplain encroachment analysis within the unsteady flow computations module.
Contents
General
Entering Floodplain Encroachment Data
Performing the Floodplain Encroachment Analysis
Viewing the Floodplain Encroachment Results
Floodplain Encroachments With Unsteady Flow 10-1
Chapter 10 Performing a Floodway Encroachment Analysis
General The HEC-RAS floodplain encroachment procedure is based on calculating a natural profile (existing conditions geometry) as the first profile in a multiple profile run. Other profiles, in a run, are calculated using various encroachment options, as desired. Before performing an encroachment analysis, the user should have developed a model of the existing river system. This model should be calibrated to the fullest extent that is possible. Verification that the model is adequately modeling the river system is an extremely important step before attempting to perform an encroachment analysis. Currently, the HEC-RAS steady flow program has 5 methods to determine floodplain encroachments. These methods are: Method 1 -
User enters right and left encroachment stations
Method 2 -
User enters fixed top width
Method 3 -
User specifies the percent reduction in conveyance
Method 4 -
User specifies a target water surface increase
Method 5 -
User specifies a target water surface increase and maximum change in energy
For unsteady flow analysis, only method one has been implemented so far in HEC-RAS. For a detailed discussion on each of these methods, the user is referred to Chapter 10 of the Hydraulic Reference Manual. The goal of performing a floodplain encroachment analysis is to determine the limits of encroachment that will cause a specified change in water surface elevation. To determine the change in water surface elevation, the program must first determine a natural profile with no encroachments. This base profile is typically computed using the one percent chance discharge. The computed profile will define the floodplain, as shown in Figure 10-1. Then, by using one of the 5 encroachment methods, the floodplain will be divided into two zones: the floodway fringe and the floodway. The floodway fringe is the area blocked by the encroachment. The floodway is the remaining portion of the floodplain in which the one-percent chance event must flow without raising the water surface more than the target amount.
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Chapter 10 Performing a Floodway Encroachment Analysis
Δ
Water Surface
Natural Water Surface Encroached Water Surface Cross Section View
Plan View Main Channel
Floodway Fringe
Floodway
Floodway Fringe
100 - Year Floodplain
Figure 10-1 Floodway Definition Sketch
Entering Floodplain Encroachment Data Within HEC-RAS, the data for performing a steady flow floodplain encroachment analysis are entered from the Steady Flow Analysis window. Encroachment information is not considered as permanent geometry or flow data, and is therefore not entered as such. The encroachment information is saved as part of the existing Plan data. To bring up the floodplain encroachment data window, select the Encroachments option from the Options menu of the Steady Flow Analysis window. When this option is selected an Encroachment window will appear as shown in Figure 10-2 (except yours will be blank when you first open it). As shown in Figure 10-2, there are several pieces of data that the user must supply for an encroachment analysis. The encroachment analysis can only be performed for profiles 2 through 15 (or what ever number has been set by the user in the flow data editor). Encroachments are not performed on profile one because most of the encroachment methods rely on having a base profile for comparison.
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Chapter 10 Performing a Floodway Encroachment Analysis
Figure 10-2 Floodplain Encroachment Data Editor
The data for an encroachment analysis should be entered in the following manner: Global Information. Global information is data that will be applied at every cross section for every profile computed. The first piece of global information is the Equal Conveyance Reduction selection box at the top of the Encroachment data editor window. Equal conveyance reduction applies to encroachment methods 3, 4, and 5. When this is turned on, the program will attempt to encroach, such that an equal loss of conveyance is provided on both sides of the stream. If this option is turned off, the program will encroach by trying to maintain a loss in conveyance in proportion to the distribution of natural overbank conveyance. The default is to have equal conveyance reduction turned on. The second item under global information is the Left bank offset and the Right bank offset. The left and right offsets are used to establish a buffer zone around the main channel for further limiting the amount of the encroachments. For example, if a user established a right offset of 5 feet and a left offset of 10 feet, the model will limit all encroachments to 5 feet from the right bank station and 10 feet from the left bank station. The default is to have no right or left offset, this will allow the encroachments to go up to the main channel bank stations, if necessary. 10-4
Chapter 10 Performing a Floodway Encroachment Analysis River, Reach and River Station Selection Boxes. The next piece of data for the user to select is the river and reach in which to enter encroachment data. The user is limited to seeing one reach at a time on the encroachment data editor. Once a reach is selected, the user can then enter a Starting and Ending River Station to work on. By default, the program selects all the sections in the reach. The user can change this to any range of cross sections within the reach. Profile. Next, the user should select a profile number to work on. Profiles are limited to 2 through the maximum number set in the currently opened flow data (e.g., 2 through 4, if the user has set 4 profiles in the flow data editor). The user can not set encroachments for profile 1. Method and Target Values. The next step is to enter the desired encroachment method to be used for the currently selected profile. Once a method is selected, the data entry boxes that corresponds to that method will show up below the method selection box. Some of the methods require only one piece of data, while others require two. The user should then enter the required information that corresponds to the method that they have selected. For example, if the user selects encroachment method 4, only one piece of information is required, the target change in water surface elevation. The available encroachment methods in HEC-RAS are: Method 1 -
User enters right and left encroachment station
Method 2 -
User enters a fixed top width
Method 3 -
User specifies the percent reduction in conveyance
Method 4 -
User specifies a target water surface increase
Method 5 -
User specifies target water surface increase and maximum change in energy
Set Selected Range. Once the encroachment method is selected, and its corresponding data are entered, the user should press the Set Selected Range button. Pressing this button will fill in the table below with the selected range of river stations; the selected method; and the corresponding data for the method. Note that, if the selected method only has one data item, that method’s data will go under the Value 1 column of the table. If the selected method has two data items, the first goes into the Value 1 column and the second goes into the Value 2 column. Once the data is put into the table, the user can change the method and corresponding data values directly from the table. At this point the user should repeat these tasks until all of the encroachment data are entered (i.e., for all the reaches and locations in the model, as well as all of the profiles for which the user wants to 10-5
Chapter 10 Performing a Floodway Encroachment Analysis perform the encroachment analysis). Once all of the encroachment data are entered, the user presses the OK button and the data will be applied and the window will close. The user can return to the encroachment window and edit the data at any time. The encroachment data are not saved to the hard disk at this time, they are only saved in memory. To save the data to the hard disk, the user should either select Save Project from the File menu of the main HEC-RAS window, or select Save Plan from the File menu of the Steady Flow Analysis window. The Import Method 1 option, allows the user to transfer the computed encroachment stations from a previous run (output file) to the input data for a future run. For example, if the user performs a preliminary encroachment analysis using any of the methods 2 through 5, they may want to convert the results from one of the runs to a method 1 encroachment method. This will allow the user to further define the floodway, using method 1, without having to enter all of the encroachment stations. The import of encroachment stations, in this manner, is limited to the results of a single encroachment profile for each reach.
Performing the Floodplain Encroachment Analysis The HEC-RAS floodway procedure is based on calculating a natural profile (no encroachments) as the first profile of a multiple profile run. Subsequent profiles are calculated with the various encroachment options available in the program. In general, when performing a floodway analysis, encroachment methods 4 and 5 are normally used to get a first cut at the encroachment stations. Recognizing that the initial floodway computations may provide changes in water surface elevations greater, or less, than the "target" increase, initial computer runs are usually made with several "target" values. The initial computer results should then be analyzed for increases in water surface elevations, changes in velocities, changes in top width, and other parameters. Also, plotting the results with the X-Y-Z perspective plot, or onto a topo map, is recommended. From these initial results, new estimates can be made and tested. After a few initial runs, the encroachment stations should become more defined. Because portions of several computed profiles may be used, the final computer runs are usually made with encroachment Method 1 defining the specific encroachment stations at each cross section. Additional runs are often made with Method 1, allowing the user to adjust encroachment stations at specific cross sections to further define the floodway. While the floodway analysis generally focuses on the change in water surface elevation, it is important to remember that the floodway must be consistent with local development plans and provide reasonable 10-6
Chapter 10 Performing a Floodway Encroachment Analysis hydraulic transitions through the study reach. Sometimes the computed floodway solution, that provides computed water surfaces at or near the target maximum, may be unreasonable when transferred to the map of the actual study reach. If this occurs, the user may need to change some of the encroachment stations, based on the visual inspection of the topo map. The floodway computations should be re-run with the new encroachment stations to ensure that the target maximum is not exceeded.
Viewing the Floodplain Encroachment Results Floodplain encroachment results can be viewed in both graphical and tabular modes. Graphically, the encroachment results show up on the cross section plots as well as the X-Y-Z Perspective plot. An example cross-section plot is shown in Figure 10-3.
Figure 10-3 Example Cross Section Plot with Encroachments
As shown in Figure 10-3, the encroachments are plotted as outlined blocks. In this example, the water surface profile for the base run (first profile) is plotted along with one of the encroached profiles. The user can plot as many profiles as they wish, but it may become a little confusing with several sets of encroachments plotted at they same time.
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Chapter 10 Performing a Floodway Encroachment Analysis Another type of graphic that can be used to view the encroachments is the X-Y-Z perspective plot, an example is shown in Figure 10-4. In this example, the base profile (profile 1) as well as one of the encroached profiles is plotted at the same time over a range of cross sections. This type of plot allows the user to get a reach view of the floodplain encroachment. The user can quickly see if the encroachments transition smoothly or if they are erratic. In general, the final encroachments should have a consistent and smooth transition from one cross section to the next. With the assistance of this type of plot, the user may want to further refine the final encroachment stations and re-run the model.
Figure 10-4 Example X-Y-Z Perspective Plot with Base and Encroached Profiles
Encroachment results can also be viewed in a tabular mode from the Profile Output Tables. Select Profile Table from the View menu of the main HEC-RAS window. When the table comes up, the user can select from three different pre-defined encroachment tables. To bring up one of the encroachment tables, select Encroachment 1 from the Std. Tables menu on the Profile table window. An example of 10-8
Chapter 10 Performing a Floodway Encroachment Analysis Encroachment 1 table is shown in Figure 10-5. The table shows the basic encroachment results of: computed water surface elevation; change in water surface from the base profile; the computed energy; top width of the active flow area; the flow in the left overbank, main channel, and right overbank; the left encroachment station; the station of the left bank of the main channel; the station of the right bank of the main channel; and the right encroachment station.
Figure 10-5 Example of the Encroachment 1 Standard Table
Encroachment 2 table provides some additional information that is often used when plotting the encroachments onto a map. This table includes: the change in water surface elevations from the first profile; the top width of the active flow area; the percentage of conveyance reduction in the left overbank; the left encroachment station; the distance from the center of the main channel to the left encroachment station; the station of the center of the main channel; the distance from the center of the main channel to the right encroachment station; the right encroachment station; and the percentage of conveyance reduction in the right overbank. An example of the Encroachment 2 standard table is shown in Figure 10-6.
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Chapter 10 Performing a Floodway Encroachment Analysis
Figure 10-6 Example of the Encroachment 2 Standard Table
The last encroachment table, Encroachment 3, provides the minimum floodway data for reporting. This table includes: the active flow top width; the flow area (including any ineffective flow area); the average velocity of the entire cross section; the computed water surface elevation; the base water surface elevation (profile 1); and the change in water surface from the first profile. An example of this table is shown in Figure 10-7.
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Chapter 10 Performing a Floodway Encroachment Analysis
Figure 10-7 Example of the Encroachment 3 Standard Table
Floodway Encroachments with Unsteady Flow Encroachment analyses can also be performed with the unsteady flow computations module within HEC-RAS. However, only method one (user placed encroachments) has been added to the unsteady flow computations. A suggested methodology for performing an encroachment analysis with an unsteady flow model is the following: 1.
First, develop the unsteady flow model of the river system and calibrate it to the extent possible.
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Chapter 10 Performing a Floodway Encroachment Analysis 4. Develop an unsteady flow plan of the 100 yr event in order to establish the base floodplain. 5. Develop a steady flow plan that incorporates the peak flows from the unsteady flow run as the 100 yr event for the model. Set up the model for two profiles with the same flows. 6. Perform a steady flow encroachment analysis using the available steady flow encroachment methods to calculate an approximate floodway. 7. Copy the unsteady flow plan to a new plan (using the Save As option), and give it a name that represents the encroached plan. 8. Adjust the boundary conditions file to reflect an increased water surface elevation at the downstream boundary for the range of possible flows. If using a rating curve, you will need to develop a new rating to reflect the encroached condition at the downstream boundary. If you are using normal depth or critical depth, no change is necessary, since the program will calculate a new water surface with the encroachments. 9. Go to the Options menu of the unsteady flow analysis window and select Unsteady Encroachments. This will bring up the Unsteady flow Encroachment editor shown in Figure 10-8. 10. Import the final encroachments from the steady flow encroachment run in to the unsteady flow encroachment editor. This is accomplished by pressing the button labeled “Get Encroachments from Steady Flow Plan”, and then selecting the appropriate plan and profile number from the steady flow encroachment analysis. 11. Run the unsteady flow model with the encroachments and compare the output of the encroached unsteady flow plan with the output from the base unsteady flow plan. 12. Adjust the encroachments as necessary to stay within the limits for increased water surface elevations. Re-run the unsteady flow model. Repeat this process until a final floodway is achieved.
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Chapter 10 Performing a Floodway Encroachment Analysis
Figure 10-8 Unsteady Flow Encroachment Editor
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Chapter 10 Performing a Floodway Encroachment Analysis
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Chapter 11 Troubleshooting With HEC-RAS
CHAPTER
11
Troubleshooting With HEC-RAS For a steady flow analysis, the HEC-RAS software is designed to continue its computations all the way through completion, even when the user has entered poor data. Because of this, the fact that the program executes a complete run does not necessarily mean that the results are good. The user must carefully review the results to ensure that they adequately represent the study reach and that they are reasonable and consistent. The HEC-RAS software is an engineering tool, it is by no means a replacement for sound engineering. The HEC-RAS software contains several features to assist the user in the development of a model; debugging problems; and the review of results. These features include: built in data checking; an Errors, Warnings, and Notes system; and a computational Log Output file. In addition to these features, the user can use the graphical and tabular output to review the results and check the data for reasonableness and consistency. Most of the information contained within this chapter is based on performing a steady flow analysis. Much of the information is also useful when performing an unsteady flow analysis. More assistance for solving unsteady flow stability problems can be found in Chapter 8 of this manual.
Built in Data Checking The HEC-RAS user interface has two types of built in data checking. The first type of data checking is performed as the user enters the data. Each data field of the data entry editors has some form of data checking. The second type of data checking occurs when the user starts the steady flow or unsteady flow computations. When the user presses the compute button, on the steady flow or unsteady flow analysis window, the entire data set is processed through several data checks before the computations begin. A detailed discussion of each of these two data checking features is described below.
Checking the Data as it is Entered This type of data checking occurs whenever the user enters data into a single data field or table. Once the user leaves a particular data entry field or table, the program will automatically check that data for reasonableness. The following is a list of some of the types of data checks that are performed: 11-1
Chapter 11 Troubleshooting With HEC-RAS 1. Minimum and maximum range checking for variables. 2. Alpha and numeric data checks. This is done to ensure that the right type of data is entered in each field. 3. Increasing order of station for cross sections, bridge deck/roadway, and abutments. 4. Data consistency checks (i.e., when the main channel bank stations are entered, the program checks to see if they exist in the cross section station and elevation data). 5. Data deletion warnings. When you delete data the software will give you a warning before it is deleted. 6. File management warnings (i.e., program will give you a chance to save the data to the hard disk before the program is closed, or a different data set is opened). 7. Data geometry checks (i.e., when a bridge deck/roadway is entered, the program checks to ensure that the deck/roadway intersects with the ground data).
Data Checking Before Computations are Performed The second type of data checking is performed to evaluate the completeness and consistency of the data. This type of data checking occurs before the computations take place. When the user presses the Compute button on the Steady Flow or Unsteady Flow Analysis window, the program will perform a series of data checks before the computations are allowed to proceed. If any data errors are found, the program will not perform the computations. The following is a list of some of the types of checks that are made during this time: 1. Data completeness. These data checks insure that all of the required data exists for the entire data set. If any missing data are found, a complete list of all the missing data and their specific locations is displayed on the screen. An example of this is shown in Figure 11-1. 2. Data consistency. This type of data checking is performed to ensure that the data is consistent with the computations that are being requested. For example, if the user asks to perform a mixed flow regime computation, the program checks to ensure that upstream as well as downstream boundary conditions have been specified. Likewise, if an encroachment analysis is requested, the program checks to ensure that the number of profiles lines up with the number specified in the encroachment data. There are several other checks of this type.
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Figure 11-1. Data Completeness Checking Window
Errors, Warnings, and Notes The HEC-RAS software has a system of Errors, Warnings, and Notes that are passed from the computation programs to the user interface. During the computations, the computation programs will set flags at particular nodes (nodes are cross sections, bridges, culverts, or multiple openings) whenever it is necessary. These message flags are written to the standard output file, along with the computed results for that node. When the user interface reads the computed results from the output file, if any errors, warnings, or notes exist, they are interpreted and displayed in various locations from the interface. The user can request a summary of all the errors, warnings, and notes that occurred during the computations. This is accomplished by selecting Summary Errors, Warnings, and Notes from the View menu on the main HEC-RAS window. Once this is selected, a window will pop up displaying all of the messages. The user can select a specific River and Reach, as well as which Profile and Plan to view. The user has the options of expanding the window; printing the messages; or sending them to the windows clipboard. An example of the Errors, Warnings, and Notes window is shown in Figure 11-2.
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Figure 11-2. Summary of Errors, Warnings, and Notes Window
Besides the summary window, messages will automatically appear on the cross section specific tables. When a cross section or hydraulic structure is being displayed, any errors, warnings, or notes for that location and profile will show up in the Errors, Warnings, and Notes message box at the bottom of the table. An example of this table is shown in Figure 11-3. In general, the errors, warnings, and notes messages should be self explanatory. The three categories of messages are the following: ERRORS: Error messages are only sent when there are problems that prevent the program from being able to complete the run. WARNINGS: Warning messages provide information to the user that may or may not require action on the user’s part. In general, whenever a warning is set at a location, the user should review the hydraulic results at that location to ensure that the results are reasonable. If the hydraulic results are found to be reasonable, then the message can be ignored. However, in many instances, a warning level message may require the user to take some action that will cause the message to disappear on future runs. Many of the warning messages are caused by either inadequate or bad data. Some common problems that cause warning messages to occur are the following: Cross sections spaced to far apart. This can cause several warning messages to be set. Cross sections starting and ending stations not high enough. If a computed water surface is higher than either end point of the cross section, a warning message will appear.
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Chapter 11 Troubleshooting With HEC-RAS Bad Starting Water Surface Elevation. If the user specifies a boundary condition that is not possible for the specified flow regime, the program will take action an set an appropriate warning message. Bad Cross Section Data. This can cause several problems, but most often the program will not be able to balance the energy equation and will default to critical depth. NOTES: Note level messages are set to provide information to the user about how the program is performing the computations.
Figure 11-3. Cross Section Table with Errors, Warnings, and Notes
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Log Output
Steady Flow Log Output This option allows the user to set the level of the Log file for a steady flow analysis. This file contains information tracing the program process. Log levels can range between 0 and 10, with 0 resulting in no Log output and 10 resulting in the maximum Log output. In general, the Log file output level should not be set unless the user gets an error during the computations. If an error occurs in the computations, set the log file level to an appropriate value. Re-run the computations and then review the log output, try to determine why the program got an error. When the user selects Set Log File Output Level from the Options menu, a window will appear as shown in Figure 11-4. The user can set a "Global Log Level," which will be used for all cross sections and every profile. The user can also set log levels at specific locations for specific profiles. In general, it is better to only set the log level at the locations where problems are occurring in the computations. To set the specific location log level, first select the desired reach and river station. Next select the log level and the profile number (the log level can be turned on for all profiles). Once you have everything set, press the Set button and the log level will show up in the window below. Log levels can be set at several locations individually. Once all of the Log Levels are set, press the OK button to close the window. Warning !!! - setting the global log output level to 4 or 5 can result in very large log file output. Global log level values of 6 or larger can result in extremely large log files.
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Figure 11-4. Log File Output Level Window
Unsteady Flow Log Output The unsteady flow computation program can write out a detailed log file of its computations. This file is very different from the steady flow program, but serves the purpose of debugging computational problems. This option is turned on by selecting Output Options from the Options menu on the Unsteady Flow Analysis window. When this option is selected a window will appear as shown in Figure 11-5.
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Figure 11-5. Unsteady Flow Output Control Window
As shown in Figure 11-5, this option controls various types of output. To turn on the detailed log output, the user must check the box labeled Write Detailed Log Output for Debugging. The user has the option for setting a time window, which will limit the output to only within this time frame. After this option is selected, the computations must be rerun in order for the output to be produced.
Viewing the Log File This option allows the user to view the contents of the log file. For steady flow analyses, the user brings up the log output by selecting View Log File from the Options menu of the Steady Flow Analysis window. For unsteady flow analyses, the user brings up the log output by selecting View Computation Log File from the Options menu of the Unsteady Flow Analysis window. The interface uses the Windows Write program to accomplish viewing the output (unless the user has set a different program to be used as the default file viewer). It is up to the user to set an appropriate font in the Write program. If the user sets a font that uses proportional spacing, the information in the log file will not line up correctly. Some fonts that work well are: Line Printer; Courier (8 pt.); and Helvetica (8 pt.). Consult your Windows user's manual for information on how to use the Write program.
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Reviewing and Debugging the Normal Output After the user has successfully completed a run, and reviewed all the errors, warnings, and notes, the normal output should be reviewed for consistency and reasonableness.
Viewing Graphics In general, the graphical output should be used as much as possible to get a quick view of the results. The user should look at all of the cross sections with the cross section plotting capability. The cross section plots will assist the user in finding data mistakes, as well as possible modeling mistakes (mistakes in ineffective flow areas, levees, n values, etc.). The profile plotting capability is a good way to get a quick overview of the entire study area. The user should look for sudden changes to the energy grade line and the water surface. In general, these two variables should transition smoothly along the channel. If the user finds rapid changes in the energy or the water surface, the results at those locations should be reviewed closely to ensure that they are correct. The X-Y-Z Perspective Plot can also be used to get a quick view of an entire reach. This plot is very helpful for viewing the top width of the flow area. If the user finds dramatic changes in the top width from one cross section to the next, then the results at those locations should be reviewed closely. Dramatic changes in top width may indicate the need for additional cross sections.
Viewing Tabular Output There are several types of tabular output. The user should try to make use of all of them when viewing tabular results. In general, the profile summary types of tables should be used to get an overview of some of the key variables at several locations. If any problems are found, or any results that seem suspect, the user should use the detailed output specific tables to get detailed results at a single location.
The Occurrence of Critical Depth During the steady flow water surface profile calculations, the program may default to critical depth at a cross section in order to continue the calculations. Critical depth can occur for the following reasons:
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Chapter 11 Troubleshooting With HEC-RAS 1. Bad cross section data: If the energy equation can not balance because of bad cross section data, the program defaults to critical depth. 2. Program can not balance the energy equation above or below the top of a levee or ineffective flow area: On occasion, when the program is balancing a water surface that is very close to the top of a levee, or an ineffective flow area, the program may go back and forth (above and below the levee) without being able to balance the energy equation. When this occurs, the program will default to critical depth. 3. Cross sections spaced too far apart: If the cross sections are spaced to far apart, the program may not be able to calculate enough energy losses to obtain a subcritical water surface at the upstream section. 4. Wrong flow regime: When calculating a subcritical profile, and the program comes to a reach that is truly supercritical, the program will default to critical depth. Likewise, when calculating a supercritical profile, if the reach is truly subcritical, the program will default to critical depth.
Computational Program Does Not Run To Completion While running the computational part of the software, when the steady flow program is finished you should get the message "Finished Steady Flow Simulation" or “Finished Post Processing,” for an unsteady flow run. If the user has entered bad data, the computational program may not be able to run to completion. When this happens the program will stop and write an error message to the screen. This message may be a trapped error by the program, or it could be just a generic Fortran error message. Fortran error messages come from the Fortran compiler that was used to develop the computational program. The message basically says that a math error occurred and therefore the program could not continue. When this type of error occurs, it is most often a data input problem. There is a possibility that it could be a bug in the program, but the user should exhaust all the possible data input errors before assuming that the program has a "Bug." The first step in finding the problem is to realize where the error is occurring. For a steady flow analysis, the program will display which cross section it is working on, and for which profile. This means that the error occurred at that cross section (or hydraulic structure, such as a bridge). Go to the Geometric Data editor and review the input data closely at the problem location. During an unsteady flow analysis, the program displays the time step that it is working on and the number of iterations it took to solve the equations. As the program is running, if it consistently goes to the maximum number of iterations (20 is the default), the user should 11-10
Chapter 11 Troubleshooting With HEC-RAS take not of the time step that this started to occur. The user must turn on the detailed log output, and then review that output in the vicinity of that particular time step, in order to figure out what is going wrong. Computational errors often occur at bridges. Check your data closely for any inconsistencies in the bridge geometry. Many of the problems that occur at bridges are due to bad Deck/Roadway data. Go to the Bridge/Culvert Data editor and turn on the option Highlight Weir, Opening Lid and Ground from the View menu. This option will assist you in finding any geometric mistakes in the bridge data.
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Chapter 12 Computing Scour at Bridges
CHAPTER
12
Computing Scour at Bridges The computation of scour at bridges within HEC-RAS is based upon the methods outlined in Hydraulic Engineering Circular No. 18 (FHWA, 2001). Before performing a scour analysis with the HEC-RAS software, the engineer should thoroughly review the procedures outlined in the Hydraulic Engineering Circular No. 18 (HEC 18) report. This chapter presents the data input required for computing contraction scour and local scour at piers and abutments. For information on the bridge scour equations, please see Chapter 10 of the HEC-RAS Hydraulic Reference Manual.
Contents
General Modeling Guidelines
Entering Bridge Scour Data
Computing Total Bridge Scour
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General Modeling Guidelines In order to perform a bridge scour analysis, the user must first develop a hydraulic model of the river reach containing the bridge to be analyzed. This model should include several cross sections downstream from the bridge, such that any user defined downstream boundary condition does not affect the hydraulic results inside and just upstream of the bridge. The model should also include several cross sections upstream of the bridge, in order to evaluate the long term effects of the bridge on the water surface profile upstream. The hydraulic modeling of the bridge should be based on the procedures outlined in Chapter 5 of the Hydraulic Reference Manual. If observed data are available, the model should be calibrated to the fullest extent possible. Once the hydraulic model has been calibrated (if observed data are available), the modeler can enter the design events to be used for the scour analysis. In general, the design event for a scour analysis is usually the 100 year (1 percent chance) event. In addition to this event, it is recommended that a 500 year (0.2 percent chance) event also be used in order to evaluate the bridge foundation under a super-flood condition. The next step is to turn on the flow distribution option in the HEC-RAS software. This option allows for additional output showing the distribution of flow for multiple subdivisions of the left and right overbanks, as well as the main channel. The output of the flow distribution option includes the following items for each flow slice: percentage of flow; flow area; wetted perimeter; conveyance; hydraulic depth; and average velocity. The user can control the number of slices in each flow element (left overbank, main channel, and right overbank), up to a maximum of 45 total slices. The flow distribution output is controlled from the Options menu of the Steady Flow Analysis window (see Chapter 7, Simulation Options). The user must request the flow distribution output for the cross sections inside the bridge, the cross section just upstream of the bridge, and the approach section (cross section upstream of the bridge at a distance such that the flow lines are parallel and the flow has not yet begun to contract due to the bridge constriction). Flow distribution output can be requested at additional cross sections, but these are the only cross sections that will be used in the bridge scour computations. The flow distribution option must be turned on in order to get more detailed estimates of the depth and velocity at various locations within the cross section. Once the user has turned this option on, the profile computations must be performed again in order for the flow distribution output to be computed and included in the output file. After performing the water surface profile calculations for the design events, and computing the flow distribution output, the bridge scour can then be evaluated. The total scour at a highway crossing is comprised of three components: long-term aggradation and degradation; contraction scour; and local scour at piers and abutments. The scour computations in the HEC-RAS software allow the user to compute contraction scour and local scour at piers and abutments. The current version of the HEC-RAS software does not allow 12-2
Chapter 12 Computing Scour at Bridges the user to evaluate long-term aggradation and degradation. Long term aggradation and degradation should be evaluated before performing the bridge scour analysis. Procedures for performing this type of analyses are outlined in the HEC No. 18 report.
Entering Bridge Scour Data The bridge scour computations are performed by opening the Hydraulic Design Functions window and selecting the Scour at Bridges function. Once this option is selected the program will automatically go to the output file and get the computed output for the approach section, the section just upstream of the bridge, and the sections inside of the bridge. The Hydraulic Design window for Scour at Bridges will appear as shown in Figure 12-1.
Figure 12-1. Hydraulic Design Window for Scour at Bridges
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Chapter 12 Computing Scour at Bridges
As shown in Figure 12-1, the Scour at Bridges window contains the input data, a graphic, and a window for summary results. Input data tabs are available for contraction scour, pier scour, and abutment scour. The user is required to enter only a minimal amount of input and the computations can be performed. If the user does not agree with any of the data that the program has selected from the output file, the user can override it by entering their own values. This provides maximum flexibility in using the software.
Entering Contraction Scour Data Contraction scour can be computed in HEC-RAS by either Laursen’s clearwater (Laursen, 1963) or live-bed (Laursen, 1960) contraction scour equations. Figure 12-2 shows all of the data for the contraction scour computations. All of the variables except K1 and D50 are obtained automatically from the HEC-RAS output file. The user can change any variable to whatever value they think is appropriate. To compute contraction scour, the user is only required to enter the D50 (mean size fraction of the bed material) and a water temperature to compute the K1 factor.
Figure 12-2. Example Contraction Scour Calculation
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Chapter 12 Computing Scour at Bridges Each of the variables that are used in the computation of contraction scour are defined below, as well as a description of where each variable is obtained from the output file. Y1: The average depth (hydraulic depth) in the left overbank, main channel, and the right overbank, at the approach cross-section. V1: The average velocity of flow in the left overbank, main channel, and right overbank, at the approach section. Y0: The average depth in the left overbank, main channel, and right overbank, at the contracted section. The contracted section is taken as the cross section inside the bridge at the upstream end of the bridge (section BU). Q2: The flow in the left overbank, main channel, and right overbank, at the contracted section (section BU). W2: The top width of the active flow area (not including ineffective flow area), taken at the contracted section (section BU). D50: The bed material particle size of which 50% are smaller, for the left overbank, main channel, and the right overbank. These particle sizes must be entered in millimeters by the user. Equation: The user has the option to allow the program to decide whether to use the live-bed or clear-water contraction scour equations, or to select a specific equation. If the user selects the Default option (program selects which equation is most appropriate), the program must compute Vc, the critical velocity that will transport bed material finer than D50. If the average velocity at the approach cross section is greater than Vc, the program uses the live-bed contraction scour equation. Otherwise, the clear-water contraction scour equation will be used. Q1: The flow in the left overbank, main channel, and right overbank at the approach cross-section. W1: The top width of the active flow area (not including ineffective flow area), taken at the approach cross section. K1: An exponent for the live-bed contraction scour equation that accounts for the mode of bed material transport. The program can compute a value for K1 or the user can enter one. To have the program compute a value, the K1 button must be pressed. Figure 12-3 shows the window that comes up when the K1 button is pressed. Once a water temperature is entered, and the user presses the OK button, the K1 factor will be displayed on the main contraction scour window. K1 is a function of the energy slope (S1) at the approach section, the shear velocity (V* ) at the approach section, water temperature, and the fall velocity (w) of the D50 bed material.
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Figure 12-3. Computation of the K1 Factor
Approach XS River Sta.: The river station of what is being used as the approach cross section. The approach cross section should be located at a point upstream of the bridge just before the flow begins to contract do to the constriction of the bridge opening. The program assumes that the second cross section upstream of the bridge is the approach cross section. If this is not the case, the user can select a different river station to be used as the approach cross section. As shown in Figure 12-2, the computation of contraction scour is performed separately for the left overbank, main channel, and right overbank. For this example, since there is no right overbank flow inside of the bridge, there is no contraction scour for the right overbank. The summary results show that the computed contraction scour, Ys, was 2.26 feet (0.69 m) for the left overbank, and 6.67 feet (2.03 m) for the main channel. Also note that the graphic was updated to show how far the bed would be scoured due to the contraction scour.
Entering Pier Scour Data Pier scour can be computed by either the Colorado State University (CSU) equation (Richardson, et al, 1990) or the Froehlich (1988) equation (the Froehlich equation is not included in the HEC No.18 report). The CSU equation is the default. As shown in Figure 12-4, the user is only required to enter the pier nose shape (K1), the angle of attack for flow hitting the piers, the condition of the bed (K3), and a D95 size fraction for the bed material. All other values are automatically obtained from the HEC-RAS output file.
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As shown in Figure 12-4, the user has the option to use the maximum velocity and depth in the main channel, or the local velocity and depth at each pier for the calculation of the pier scour. In general, the maximum velocity and depth are used in order to account for the potential of the main channel thalweg to migrate back and forth within the bridge opening. The migration of the main channel thalweg could cause the maximum potential scour to occur at any one of the bridge piers. Each of the variables that are used in the computation of pier scour are defined below, as well as a description of where each variable is obtained from the output file. Maximum V1 Y1: If the user selects this option, the program will find the maximum velocity and depth located in the cross section just upstream and outside of the bridge. The program uses the flow distribution output to obtain these values. The maximum V1 and Y1 will then be used for all of the piers.
Figure 12-4. Example Pier Scour Computation
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Local V1 Y1: If the user selects this option, the program will find the velocity (V1) and depth (Y1) at the cross section just upstream and outside of the bridge that corresponds to the centerline stationing of each of the piers. Method: The method option allows the user to choose between the CSU equation and the Froehlich equation for the computation of local scour at bridge piers. The CSU equation is the default method. Pier #: This selection box controls how the data can be entered. When the option "Apply to All Piers" is selected, any of the pier data entered by the user will be applied to all of the piers. The user does not have to enter all of the data in this mode, only the portion of the data that should be applied to all of the piers. Optionally, the user can select a specific pier from this selection box. When a specific pier is selected, any data that has already been entered, or is applicable to that pier, will show up in each of the data fields. The user can then enter any missing information for that pier, or change any data that was already set. Shape: This selection box is used to establish the pier nose (upstream end) shape. The user can select between square nose, round nose, circular cylinder, group of cylinders, or sharp nose (triangular) pier shapes. When the user selects a shape, the K1 factor for the CSU equation and the Phi factor for the Froehlich equation are automatically set. The user can set the pier nose shape for all piers, or a different shape can be entered for each pier. a: This field is used to enter the width of the pier. The program automatically puts a value in this field based on the bridge input data. The user can change the value. D50: Median diameter of the bed material of which 50 percent are smaller. This value is automatically filled in for each pier, based on what was entered for the left overbank, main channel, and right overbank, under the contraction scour data. The user can change the value for all piers or any individual pier. This value must be entered in millimeters. Y1: This field is used to display the depth of water just upstream of each pier. The value is taken from the flow distribution output at the cross section just upstream and outside of the bridge. If the user has selected to use the maximum Y1 and V1 for the pier scour calculations, then this field will show the maximum depth of water in the cross section for each pier. The user can change this value directly for each or all piers. V1: This field is used to display the average velocity just upstream of each individual pier. The value is taken from the flow distribution output at the cross section just upstream and outside of the bridge. If the user has selected to use the maximum Y1 and V1 for the pier scour calculations, then this field will show the maximum velocity of water in the cross section for all piers. The user can change this value directly for each or all piers.
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Angle: This field is used to enter the angle of attack of the flow approaching the pier. If the flow direction upstream of the pier is perpendicular to the pier nose, then the angle would be entered as zero. If the flow is approaching the pier nose at an angle, then that angle should be entered as a positive value in degrees. When an angle is entered, the program automatically sets a value for the K2 coefficient. When the angle is > 5 degrees, K1 is set to 1.0. L: This field represents the length of the pier through the bridge. The field is automatically set by the program to equal the width of the bridge. The user can change the length for all piers or each individual pier. This length is used in determining the magnitude of the K2 factor. K1: Correction factor for pier nose shape, used in the CSU equation. This factor is automatically set when the user selects a pier nose shape. The user can override the selected value and enter their own value. K2: Correction factor for angle of attack of the flow on the pier, used in the CSU equation. This factor is automatically calculated once the user enters the pier width (a), the pier length (L), and the angle of attack (angle). K3: Correction factor for bed condition, used in the CSU equation. The user can select from: clear-water scour; plane bed and antidune flow; small dunes; medium dunes; and large dunes. D95: The median size of the bed material of which 95 percent is finer. The D95 size fraction is used in the computation of the K4 factor, and must be entered in millimeters directly by the user. K4: The K4 factor is used to decrease scour depths in order to account for armoring of the scour hole. This factor is only applied when the D50 of the bed material is greater than 0.006 feet (0.2 mm) and the D95 is greater than 0.06 feet (2.0 mm). This factor is automatically calculated by the program, and is a function of D50; D95; a; and the depth of water just upstream of the pier. The K4 factor is used in the CSU equation. a: The projected pier width with respect to the direction of the flow. This factor should be calculated by the user and is based on the pier width, shape, angle, and length. This factor is specific to Froehlich’s equation. Phi: Correction factor for pier nose shape, used in the Froehlich equation. This factor is automatically set when the user selects a pier nose shape. The user can override the selected value and enter their own value. For the example shown in Figure 12-4 the CSU equation was used, resulting in a computed pier scour of 10.85 feet (3.31 m) at each pier (shown under summary results in Figure 12-4). Also shown in Figure 12-4 is an updated graphic with both contraction and pier scour shown.
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Entering Abutment Scour Data Abutment scour can be computed by either the HIRE equation (Richardson, 1990) or Froehlich’s equation (Froehlich, 1989). The input data and results for abutment scour computations are shown in Figure 12-5.
Figure 12-5. Example Abutment Scour Computations
As shown in Figure 12-5, abutment scour is computed separately for the left and right abutment. The user is only required to enter the abutment type (spill-through, vertical, vertical with wing walls). The program automatically selects values for all of the other variables based on the hydraulic output and default settings. However, the user can change any variable. The location of the toe of the abutment is based on where the roadway embankment intersects the natural ground. This stationing is very important because the hydraulic variables used in the abutment scour computations will be obtained from the flow distribution output at this cross section stationing. If the user does not like the stationing that the model picks, they can override it by entering their own value.
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Chapter 12 Computing Scour at Bridges
Each of the variables that are used in the computation of abutment scour are defined below, as well as a description of where each variable is obtained from the output file. Toe Sta at Bridge: This field is used to define the stationing in the upstream bridge cross section (section BU), where the toe of the abutment intersects the natural ground. The program automatically selects a value for this stationing at the point where the road embankment and/or abutment data intersects the natural ground cross-section data. The location for the abutment toe stationing can be changed directly in this field. Toe Sta at App.: This field is used to define the stationing in the approach cross section (section 4), based on projecting the abutment toe station onto the approach cross section. The location for this stationing can be changed directly in this field. Length: Length of the abutment and road embankment that is obstructing the flow. The program automatically computes this value for both the left and right embankments. The left embankment length is computed as the stationing of the left abutment toe (projected up to the approach cross section) minus the station of the left extent of the active water surface in the approach cross section. The right embankment length is computed as the stationing of the right extent of the active water surface minus the stationing of the toe of the right abutment (projected up to the approach cross section), at the approach cross section. These lengths can be changed directly. Y1: This value is the computed depth of water at the station of the toe of the embankment, at the cross section just upstream of the bridge. The value is computed by the program as the elevation of the water surface minus the elevation of the ground at the abutment toe stationing. This value can also be changed by the user. This value is used in the HIRE equation. K1: This value represents a correction factor accounting for abutment shape. The user can choose among: vertical abutments; vertical with wing walls; and spill-through abutments. Skew: This field is used to enter the angle of attack of the flow against the abutment. A value of 90 degrees should be entered for abutments that are perpendicular to the flow (normal situation). A value less than 90 degrees should be entered if the abutment is pointing in the downstream direction. A value greater than 90 degrees should be entered if the abutments are pointing in the upstream direction. The skew angle is used in computing the K2 factor. K2: Correction factor for angle of attack of the flow on the abutments. This factor is automatically computed by the program. As the skew angle becomes greater than 90 degrees, this factor increases from a value of one. As the skew angle becomes less than 90 degrees, this value becomes less than one. Equation: This field allows the user to select a specific equation (either the HIRE or Froehlich equation), or select the default mode. When the default mode is selected, the program will choose the equation that is the most applicable to the situation. The selection is based on computing a factor of the embankment length divided by the approach depth. If this factor is 12-11
Chapter 12 Computing Scour at Bridges greater that 25, the program will automatically use the HIRE equation. If the factor is equal to or less than 25, the program will automatically use the Froehlich equation. L: The length of the abutment (embankment) projected normal to the flow (projected up to the approach cross section). This value is automatically computed by the program once the user enters an abutment length and a skew angle. This value can be changed by the user. Ya: The average depth of flow (hydraulic depth) that is blocked by the embankment at the approach cross section. This value is computed by projecting the stationing of the abutment toe’s up to the approach cross section. From the flow distribution output, the program calculates the area and top width left of the left abutment toe and right of the right abutment toe. Ya is then computed as the area divided by the top width. This value can be changed by the user directly. Qe: The flow obstructed by the abutment and embankment at the approach cross section. This value is computed by projecting the stationing of the abutment toes onto the approach cross-section. From the flow distribution output, the program calculates the percentage of flow left of the left abutment toe and right of the right abutment toe. These percentages are multiplied by the total flow to obtain the discharge blocked by each embankment. These values can be changed by the user directly. Ae: The flow area that is obstructed by the abutment and embankment at the approach cross section. This value is computed by projecting the stationing of the abutment toes onto the approach cross-section. From the flow distribution output, the program calculates the area left of the left abutment toe and right of the right abutment toe. These values can be changed by the user directly. V1: The velocity at the toe of the abutment, taken from the cross section just upstream and outside of the bridge. This velocity is obtained by finding the velocity in the flow distribution output at the corresponding cross section stationing of the abutment toe. These values can be changed by the user directly.
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Chapter 12 Computing Scour at Bridges
In addition to the abutment input data, once the compute button is pressed, the bridge scour graphic is updated to include the abutment scour and the summary results window displays the computed abutment results. For the example shown in Figure 12-5, the program selected the HIRE equation and computed 10.92 feet (3.33 m) of local scour for the left abutment and 14.88 feet (4.54 m) of local scour for the right abutment.
Computing Total Bridge Scour The total scour is a combination of the contraction scour and the individual pier and abutment scour at each location. Table 12.1 shows a summary of the computed results, including the total scour.
Table 12.1
Summary of Scour Computations Contraction Scour Left O.B.
Main Channel
Right O.B.
Ys = 2.07 ft (0.63 m)
6.67 ft (2.03 m)
0.00 ft (0.0 m)
Eqn = Clear-Water
Live-Bed Pier Scour
Piers 1-6
Ys = 10.85 ft (3.31 m)
Eqn. =
CSU equation Abutment Scour
Left
Right
Ys = 10.92 ft (3.33 m)
14.88 ft (4.54 m)
Eqn = HIRE equation
HIRE equation Total Scour
Left Abutment
=
12.99 ft (3.96 m)
Right abutment
=
21.55 ft (6.57 m)
Piers 1-2 (left O.B.) =
12.92 ft (3.94 m)
Piers 3-6 (main ch.) =
17.52 ft (5.34 m)
Once all three types of scour data are entered, and the compute button is pressed, the bridge scour graphic is updated to reflect the total computed scour. Shown in Figure 12-6 is the graphic of the final results (the graphic has been zoomed in to see more detail). The graphic and the tabular results can be sent directly to the default printer, or they can be sent to the Windows Clipboard in order to be pasted into a report. A detailed report can be generated, which shows all of the input data, computations, and final results.
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Chapter 12 Computing Scour at Bridges
The bridge scour input data can be saved by selecting Save Hydraulic Design Data As from the File menu of the Hydraulic Design Function window. The user is only required to enter a title for the data. The computed bridge scour results are never saved to the hard disk. The computations can be performed in a fraction of a second by simply pressing the compute button. Therefore, when the Hydraulic Design Function window is closed, and later re-opened, the user must press the compute button to get the results.
Bridge Scour RS = 10.36 Legend 20
WS PF#1 Ground Ineff
15
Bank Sta Contr Scour
Elevation (ft)
10
Total Scour
5
-5
-10 1000
1200
1400
Station (ft)
Figure 12-6. Total Scour Depicted in Graphical Form
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1600
Chapter 13 Performing Channel Design/Modifications
CHAPTER
13
Performing Channel Design/Modifications The channel design/modification tools in HEC-RAS allow the user to perform a series of trapezoidal cuts into the existing channel geometry or to create new channel geometry. The current version of HEC-RAS has two tools for performing channel modifications. These tools are available from the Tools menu of the Geometric Data editor and are labeled Channel Design/Modification and Channel Modification (original). The tool labeled Channel Design/Modification is a new tool for HEC-RAS version 4.0. The tool labeled Channel Modification (original) is the original channel modification tool developed for HECRAS. The original channel modification tool has been left in HEC-RAS for those user’s who may prefer this tool to the new one. Both channel modification tools will be described in this chapter. In general, these tools are used for planning studies, but it can also be used for hydraulic design of flood control channels. This chapter does not cover the concepts of stable channel design. This software is designed to evaluate the hydraulics of various channel modifications. It is up to the user to ensure that any channel modification will not cause further scour of the channel bed and banks. Discussions on stable channel design can be found in many hydraulic text books, as well the Corps Engineering Manual "Hydraulic Design of Flood Control Channels" (USACE, 1991). This chapter discusses: general modeling guidelines for using the channel modification option; how to enter the necessary input data; performing the channel modifications; and how to compare existing condition and modified condition results.
Contents
General Modeling Guidelines
Using the Original Channel Modifications Tool
Using the New Channel Design/Modifications Tool
Comparing Existing and Modified Conditions
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Chapter 13 Performing Channel Design/Modifications
General Modeling Guidelines In order to perform a channel modification analysis, the user should first develop a hydraulic model of the existing river reach containing the area in which the channel modification will be analyzed. This model should include several cross sections downstream from the study reach, such that any user defined downstream boundary condition does not affect the hydraulic results inside the channel modification region. The model should also include several cross sections upstream of the study reach, in order to evaluate the effects of the channel modification on the water surface profile upstream. Once a model of the existing river system is completed, the user can use the Channel Modification (old tool) or Channel Design/Modification tools to perform trapezoidal cuts and fills into the existing geometry. Once the user has performed all of the desired channel modifications, then the modified geometry data is saved into a new geometry file. The user can then create a new plan, which contains the modified geometry and the original flow data that was used under the existing conditions plan. Computations can then be performed for the modified condition, and the user can compare the water surface profiles for both existing and modified conditions.
The Channel Modification (original) option in HEC-RAS allows for:
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Multiple trapezoidal cuts (up to three)
Independent specification of left and right trapezoidal side slopes
Ability to change the Manning’s n value for the trapezoidal cut
Separate bottom widths for each trapezoidal cut
Ability to set new channel reach lengths
Multiple ways of locating the main channel centerline
User can explicitly define the elevation of the new channel invert, or it can be based on the original channel invert, or it can be based on projecting a slope from a downstream cross section or an upstream cross section
The centerline of the trapezoidal cut can be entered directly, or it can be located midway between the original main channel bank stations
Option to fill the existing channel before performing cuts
Cut and fill areas and volumes are computed
Chapter 13 Performing Channel Design/Modifications
The Channel Design/Modification tool in HEC-RAS allows for:
Identifying river reach channel designs by Alternative
Independent specification of cuts for the left and right overbank (width, depth, side slopes, and Manning’s n values)
Ability to set new channel reach lengths
Identifying separate channel cut data in a Template
User can explicitly define the elevation of the new channel invert, or it can be based on the original channel invert, or it can be based on projecting a slope from a downstream cross section or an upstream cross section
The centerline of the trapezoidal cut can be entered directly, or it can be located midway between the original main channel bank stations
Option to fill the existing channel before performing cuts
Cut and fill volumes are computed
The general concept behind the Channel Design/Modification tool is that a user develops a cross-section Template. The Template may then be applied to existing cross-sectional data (as performed historically in HEC-RAS) to perform Channel Modifications. The Template may be also be used in Channel Design to create new cross sections on a river reach.
Using the Original Channel Modifications Tool
Entering Channel Modification Data Within HEC-RAS, the data for performing a channel modification analysis are entered from the Geometric Data window. The channel modification data are stored within the geometry file of the base geometric data (the geometric data set in which the channel modification is being performed on).
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Chapter 13 Performing Channel Design/Modifications
To bring up the Channel Modification Data window, select Channel Modification (original) from the Tools menu of the Geometric Data window. When this option is selected, a Channel Modification window will appear as shown in Figure 13-1 (except yours will not have any data in it the first time you bring it up).
Figure 13-1. Channel Modification Data Editor
As shown in Figure 13-1, there are several pieces of data that the user must enter in order to perform a channel modification analysis. The editor is divided into three separate areas. The top portion of the window contains selection boxes for the River and Reach; titles for the base geometry file and the modified geometry file; buttons for performing the cuts and viewing cut and fill volumes; and controls for rotating the graphic. The middle portion of the window contains a data input area for entering channel modification information over a range of cross sections, as well as a graphic of the cross sections that are being modified. The bottom portion of the window contains a table that lists the channel modification data for all of the cross sections in the selected Reach of a particular River.
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Chapter 13 Performing Channel Design/Modifications
The first step in performing a channel modification is to select the River and Reach in which you want to perform the analysis. This is accomplished from the River and Reach selection boxes in the upper left corner of the window. The next step is to select a range of cross sections in which you would like to perform a channel modification. This is accomplished by first selecting a cross section from the Starting Riv Sta box and then from the Ending Riv Sta box. Once this is done, all of the cross sections within the range of the specified starting and ending river stations will appear in the graphic on the right. The next step is to specify the channel modification data that you would like to apply to this range of cross sections. This is accomplished by entering information into the table contained in the "Set Range of Values" area of the window. This table allows the user to enter information for up to three cuts, which can then be applied to the selected range of cross sections. The information contained in this table is as follows: Center Cuts (y/n): This column in the table is used to define how the trapezoidal cuts will be centered within the existing cross section data. If the user enters a "y" in this column, then that particular cut will be centered between the existing cross-section main channel bank stations. When all of the cut information is entered, and the Apply Cuts to Selected Range button is pressed, the program will automatically fill in the center stationing of the trapezoidal cuts in the lower table. If an "n" is entered, then it is up to the user to specify the center stationing for each cross section, and each cut, in the table at the bottom of the window. Bottom Width: This column is used for entering the bottom width of the trapezoidal cuts. If this column is left blank, it is assumed that the bottom width will be zero. The user always has the option of directly entering the bottom width for each cross section in the table at the bottom of the window. Invert Elevation: This column is used to specify the invert elevation of the trapezoidal cuts. If this column is left blank for a particular cut, then it is assumed that the invert elevation of that trapezoidal cut will be set equal to the invert elevation of the existing channel. If the user wants to have invert elevations that are not equal to the existing channel inverts, then they must enter elevations into this column and select one of the slope projection options below this table. The user has the option to use the specified invert elevations for each of the cross sections in the selected range; or they can enter elevations for the most upstream cross section and have the other invert elevations computed by projecting the cuts on a constant slope; or the elevations entered can be applied to the most downstream cross section of the range, and all others will be computed by projecting a user specified slope upstream.
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Chapter 13 Performing Channel Design/Modifications
Left Slope: This column is used to specify the slope of the left bank for each of the trapezoidal cuts. The slope is entered in units of horizontal distance to one unit in the vertical. (e.g., a value of 2 means the left bank slope will project 2 feet horizontally for every 1 foot vertically). Right Slope: This column is used to specify the slope of the right bank for each of the trapezoidal cuts. The slope is entered in units of horizontal distance to one unit in the vertical. (e.g., a value of 2 means the right bank slope will project 2 feet horizontally for every 1 foot vertically). Cut n Val: This column is used to specify the new Manning’s n value to be applied to each of the trapezoidal cuts. If this column is left blank for any cut, then the existing n values will be used for that cut. Once this table has been filled out, the user must select one of the three slope projection options listed below the table. The three options are: Same Cut to all sections: If this option is selected, then the channel modification data entered into the table will be applied to all of the cross sections in the selected range. Project cut from upper RS at slope: When this option is selected, the invert elevations that were entered into the table will be applied to the most upstream cross section in the selected range. The invert elevation of all of the other cross sections will be based on projecting a user entered slope from the most upstream cross section to each cross section downstream. The user must enter a slope when this option is selected. The elevations of each cross sections trapezoidal cuts are based on the user entered slope times the distance that each cross section is from the most upstream cross section. The distance is the cumulative main channel reach length for each of the individual cross sections. Project cut from lower RS at slope: When this option is selected, the invert elevations that were entered into the table will be applied to the most downstream cross section in the selected range. The invert elevation of all of the other cross sections will be based on projecting a user entered slope from the most downstream cross section to each cross section upstream. The user must enter a slope when this option is selected. The elevations of each cross section’s trapezoidal cuts are based on the user entered slope times the distance that each cross section is from the most downstream cross section. The distance is the cumulative main channel reach length for each of the individual cross sections. A final option that can be applied to the selected range of cross sections is the Fill Channel option. When this option is turned on, the main channel of the base cross-section data will be filled before any of the trapezoidal cuts are applied. The main channel is filled to an elevation equal to the elevation of the lower of the two main channel bank stations. 13-6
Chapter 13 Performing Channel Design/Modifications Once the user has filled in all of the desired data in the "Set Range of Values" data area, then the Apply Cuts to Selected Range button should be pressed. When this button is pressed, the lower table is filled with the specific information that will be applied to each of the cross sections in the selected range. The cut information is then applied to each of the cross sections, and the graphic is updated to show both the existing cross section and the modified cross sections. The user has the option of entering and modifying the channel modification data directly in the table at the bottom of the window, or they can use the "Set Range of Values" data area to apply a set of channel cut properties to a range of cross sections (this can be done several times for different ranges of cross sections within the reach). A final option available to the user is Cut cross section until cut daylights once. This is a global option that will be applied to all of the channel modification data. When this option is selected, as the program performs the cutting of the trapezoidal channel, the left and right banks of the channel will start at the bottom of the trapezoid and cut through the ground until they reach open air, then the cutting will stop. If this option is turned off, the left and right banks of the trapezoid will be projected to infinity, continually cutting any ground that lies above them.
Performing the Channel Modifications Once all of the desired channel modification data are entered for a reach, the user should press the Compute Cuts button at the top of the graphic. When this button is pressed, all of the channel modification data from the lower table is applied and the graphic is updated to reflect the new cut information. The user can continue to modify the data and press the Compute Cuts button as many times as is necessary to get the desired cuts. The cut information is always applied to the base geometry data. Once the user has completed the desired channel modifications for the reach, they can view the cut and fill quantities by pressing the Cut and Fill Areas button. When this button is pressed, a window will appear as shown in Figure 13-2.
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Chapter 13 Performing Channel Design/Modifications
Figure 13-2. Channel Modification Cut and Fill Quantities
The cut and fill quantities table shows the cut, fill, and net areas and volumes for each of the individual cross sections, as well as the totals for the reach. The table shows the cut and fill quantities that were necessary in order to transform the existing cross-section data into the modified cross-section data. The areas and volumes are provided in the categories of left overbank, main channel, right overbank, and total. These categories are based on the main channel bank stations of the base geometry data. The volumes listed at a particular cross section, represent the volume between that cross section and the next downstream cross section. The total volume and area at a particular cross section is the sum of the left overbank, main channel, and right overbank quantities for that individual cross section only. Total volumes for the entire reach are listed at the bottom of the table. The Cut and Fill Quantities table can be printed, sent to a file, or copied to the clipboard, by pressing the desired button at the bottom of the window. The channel modification option has been set up to work with one Reach of the model at a time. If the user needs to perform channel modifications to more than one reach of a multiple reach model, they can simply select a new reach at any time. While the information in the tables and the graphic only show a single reach, the channel modification information is stored for all of the reaches.
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Chapter 13 Performing Channel Design/Modifications
Once the user has finished all of the desired channel modifications, for all of the desired reaches, a new geometry file should be created for the modified geometry. To create a modified geometry file, the user must enter a title for the modified geometry file in the upper right hand side of the window. Once the new geometry file title is entered, the file can be created by pressing the Create Modified Geometry button at the bottom of the window. When this button is pressed, a Save Geometry Data As window will appear. The user has the options to change the directory in which the geometry file will be stored, change the name of the geometry file title, or select an existing geometry file to over write. Once the user has decided on a title and a directory, the OK button can be pressed to save the modified geometry to the hard disk. However, the original geometry file is still the one that is in memory. If the user wants to work with the new modified geometry file, they will need to open it from the Geometric Data Editor window. Note: the data entered into the channel modification editor is saved as part of the base geometry file (i.e., it is not saved with the modified geometry file). This allows the user to open the base geometry file and recreate the modified geometry. In order for this data to be saved, the user must select Save Geometry Data from the file menu of the geometric data editor, after they have entered the channel modification data.
Using the New Channel Design/Modifications Tool
Entering Channel Modification Data Within HEC-RAS, the data for performing channel modifications are entered from the Geometric Data window. The channel modification data are stored within the geometry file of base geometric data (the geometric data set on which the channel modifications are being performed). To bring up the Channel Design/Modification data window, select Channel Design/Modification from the Tools menu of the Geometric Data window. When this option is selected, a Channel Design/Modification window will appear, as shown in Figure 13-3.
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Chapter 13 Performing Channel Design/Modifications
Figure 13-3. Channel Design/Modification data editor.
As shown in Figure 13-3, there are several pieces of data that the user must enter to perform channel modifications or create new cross sections. The editor is comprised of two main components: data entry and data visualization. The table at the bottom of the window displays a summary of all modifications (by “Alternative”) that will be made to the selected River and Reach when new geometry is created. Several buttons are provided to allow the user to quickly enter data into the summary table. As data is entered into the editor the graphics plot in the upper portion of the window will be updated. Note that there are different graphic plots and each graphic plot may have more than one way to display data. Once data has been specified for the cross sections to be modified, the user may create a new geometry file by pressing the Create a Geometry File with these Modifications … button. Identify the River and Reach to perform the channel modification and select or create a new Channel Design/Modification Alternative. This will update the channel modifications summary table, displaying the all of the cross sections for the selected River and Reach and cut data for the specified Alternative. You then have access to directly change the data in the table provided.
Alternatives The Channel Design/Modification Alternative is used to group a set of cross section cut data. The default name for an Alternative is “Alternative #1”, but 13-10
Chapter 13 Performing Channel Design/Modifications may be renamed as desired. To the right of the Alternative select list box are buttons that allow you to Create, Rename, Delete, or Copy an Alternative. These functions are also available from the Options menu. All of the data provided in the Channel Design/Modification Alternative data table is associated with the Alternative. Data that is displayed in black has been saved to the Alternative while that data shown in red has not. Switching between multiple alternatives will automatically save the data currently displayed in the Alternative. When the summary data table showing the cross section cut data is loaded, some of the data comes from the existing cross sections and some comes from the modification data. Some of the data may be edited while the grayed out fields are set or computed internally. Manual entry of data may be performed on a cell-by-cell basis or over a selected range. Further, the Selected Area Edit Options may be used to automate data entry. Tool tips are provided for each of the buttons: Add, Multiply, Set, Replace, Interpolate, Copy Invert, Reset Lengths, and Reset Stations. The table fields and their use are described below. River – This column identifies the River that the cross section is location on. It is not editable by the user. Reach – This column identifies the Reach that the cross section is location on. It is not editable by the user. RS – This column identifies the River Station of the cross section and is not editable. Invert Elev. – This column displays the computed lowest point in the existing cross section. It is not editable by the user. Template Elev. – This is the elevation at which the cross section Template will be applied. While this column is not directly editable, it is set based on the values applied in the Fixed Elev., Slope US, or Slope DS columns. By default the values in this column are copied from the Invert Elev. column. LOB Length – This is the Reach Length for the Left Overbank. The default value displayed comes from the cross-sectional data. Channel Length – This is the Reach Length for the main Channel. The default value displayed comes from the cross-sectional data. ROB Length – This is the Reach Length for the Right Overbank. The default value displayed comes from the cross-sectional data. Center Station – This is the station at which the center of the cross-section Template will be applied. The default value is computed as the center of the existing cross section between the main channel bank stations. Template – This is name of the cross-section Template. The Template name is selected from a drop down list in the table. The user must create a Template using the Template Design editor prior to selecting one in the table.
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Chapter 13 Performing Channel Design/Modifications The Template contains several parameters not displayed in the summary table. Fixed Elev. – This is the elevation at which the bottom of the Template will be applied to the cross section as fixed by the user. This value is copied to the Template Elev. column. Slope US – The Template Elev. is computed using the Slope projected from the Upstream XS and the upstream Channel Length. Slope DS – The Template Elev. is computed using the Slope projected from the Downstream XS and the current Channel Length. Interp. Dist. – The maximum interpolation distance between the current cross section and the next cross section downstream. Leaving the Interp. Dist. field blank means that no interpolation is desired. Transition – The name of the Transition type that will be applied between cross sections. Default is linear. Cut Area – This column displays the computed Cut Area for the cross section. Fill Area – This column displays the computed Fill Area for the cross section.
Template Design The Cross Section Template Design editor shown in Figure 13-4 is used to create a cross-section Template to modify existing cross sections. Template data are saved with the geometry file, so that it may be used repetitively to perform channel modifications on various cross sections. The Template Design editor provides the user with tools to manage templates by name, edit the template cuts and properties, and visualize the resulting template. The user has two methods available for creating a template. One template option is a User Entered Table of DX, DY, Slope, and Mannings n values for the left and right side of the template. The other template option is a Simple Trapazoid. The simple trapezoid template option allows the user to enter a channel depth, bottom width, side slope, and Manning’s n value to create the trapezoid.
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Chapter 13 Performing Channel Design/Modifications
Figure 13-4. -section Template Design editor with User Entered Table option selected.
Prior to entering Template information, the user must create a new Template. Templates are managed from the Options menu or using the buttons shown below the menu bar at the top of the editor. The Options available from the Template Design editor include New, Rename, Delete, and Copy. New, Rename, and Copy will prompt the user for a new Template name. Delete will remove the currently active Template from the geometric data. If the user has selected the User Entered Table option for creating a template, cut data are entered into the table for the Left and Right side of the template separately. Data is entered starting from the centerline of the template and moving out towards the overbanks (i.e. the first line in the table is information that starts at the centerline of the template and goes to the left and right). Data that defines the shape of the template includes DX, DY, Slope, N Val, and main channel Bank location for the left and right sides of the template. Other data to consider is whether to Fill Channel Below Template and the value to Cut to Daylight Side Slope (horizontal to 1 vertical). 13-13
Chapter 13 Performing Channel Design/Modifications DX – This column specifies the length of the cut in the horizontal direction over the corresponding elevation change of DY. DY – This column specifies the change of elevation over the length specified in corresponding DX. Slope – This is the slope of DY/DX. It will be calculated if DX and DY are specified. If only DX (or DY) is already specified, the Slope may be entered to calculate DY (or DX). N Val – This is the Manning’s n value to be applied to this portion of the template cross section. Bank? – This column is used to specify the hinge point used to establish the main channel Bank Station for the template. Only one row per side may be selected for the bank station. Fill Channel Below Template – If checked, this option will fill the main channel of the cross section prior to applying the Template. Cut to Daylight Side Slope – This is the slope (horizontal to 1 vertical) to apply a cut above the last point in the template. The default (blank) will result in vertical wall after the last point in the template. (e.g. a value of 2 means that the cuts for the left and right bank beyond the last point of the template will be projected to the existing cross section 2 units horizontally for every 1 foot vertically). Copy Left to Right – This button is used to copy the information specified for the left side of the template to the right side. This is a convenience function that can be used when the right side of the template is a mirror image of the left side. If the user has selected the Simple Trapezoid option for the template type, then the template editor will look like what is shown in Figure 13-5. When this type of template is picked, the user enters a channel Depth; Bottom Width; Side Slope; and Manning’s n Value for the simple trapezoid. Multiple trapezoidal templates can be developed and saved under different names, and then applied within a given Channel Design/Modification Alternative.
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Chapter 13 Performing Channel Design/Modifications
Figure 13-5. Template editor with Simple Trapezoid option selected.
Modify a Range of XS Channel design/modification information may be entered to a range of cross sections using the Modify a Range of XS editor, shown in Figure 13-6, rather that entering the data in the table provided on the main Channel Design/Modification editor. This editor is appropriate when you want to create a channel within one River Reach by applying the same Template to a number of cross sections at a given slope. A Template should have been defined prior to entering the editor, but the Template Design button allows the user access to the Template Design editor. The intended use of this editor is to allow the user to apply a template over an existing set of cross sections; however, the Add New XS button allows 13-15
Chapter 13 Performing Channel Design/Modifications the user to specify a new cross section at a river station along the specified River Reach. The new cross section will be created only when the user presses the Create a New Geometry File with these Modifications button. The user is not provided with the option to specify any crosssectional properties such as station-elevation data, Manning’s n value data, or downstream reach lengths. Therefore, cross sections should be added to the geometry in this manner sparingly.
Figure 13-6. Apply channel modifications to a range of cross sections.
When the Modify a Range of Cross Sections button is pressed, the window will initially open with the current River and Reach selected, but the user may select any River Reach currently in the data set. The user will then select the Template to apply to the specified reach. If a Template has not yet been defined the user can access the template Design editor through the button provided. Cross sections over which the Template information should be applied are selected using the US RS (Upstream River Station) and DS RS (Downstream River Station) list boxes. All cross sections in the exiting geometry, as well as those “new XS” locations that have been added, will be available to select. As a river station is selected, elevation data and reach length data on the form will be updated. Each time a river station is selected from the US RS or DS RS list box the corresponding elevation data will be updated from the information on the main Channel Design/Modification editor. After selecting the US and DS river stations fix the elevations for the Template invert at the river stations. This may be done by entering an elevation value in the Elev. field or by entering a slope value. If one of the slope fields is chosen, the invert elevation for the river station will be computed using the reach length to the cross section (US or DS) and the cross section’s invert elevation. If both elevations are specified for the US and DS river station the Template will be applied to the range of cross sections on constant slope between the river stations. 13-16
Chapter 13 Performing Channel Design/Modifications
Performing the Channel Modifications Once a design Template has been specified for a given cross section, a preview of the channel as modified is displayed in the XS Plot graphic area. The XS Plot will display the cross section of the row that is currently active in the summary cut table at the bottom of the window. As shown in Figure 13-7, the original cross section is displayed in black, while the new cross section is shown in magenta. Then Manning’s n value data that is furnished at the top of the plot are the values associated with the new, modified cross section. The new bank stations will also be shown in magenta. The Plot Template option will toggle on/off the entire channel design Template, displaying the Template as a dashed line.
Figure 13-7. Preview of a modified cross section in the Channel Design/Modification editor.
New (interpolated) cross sections may also be inserted into the channel modification editor. They will appear as if they are part of the original geometry (in the design/modification editor) to allow you to apply templates; however, as shown in Figure 13-8, the row will be grayed out to indicate the cross section doesn’t actually exist.
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Chapter 13 Performing Channel Design/Modifications
Figure 13-8. A new cross section is added at River Station 10.5 into the Channel Design/Modification editor.
To insert a cross section, select the (Add new design cross section) button. You will be asked to enter a river station for the new cross section, just like adding a cross section in the geometric editor. You are then required to input reach length data to allow HEC-RAS to interpolate the cross section from existing data. (You will also need to adjust the downstream reach lengths for the upstream cross section.) Entry of the Template Elevation into the Fixed Elev. field, inspection of the Center Station for placing the template, and selecting a Template will also be required. To remove and inserted cross (Remove design cross section) button. This section, select the button is only available when an inserted section is selected in the table, as shown in Figure 13-9.
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Chapter 13 Performing Channel Design/Modifications
Figure 13-9. Channel design/modification editor with an inserted cross section with completed data.
Cut and Fill areas are also computed for each cross section. Detailed cut and fill data are available from the Summary Cut and Fill data table. The Cut and button. There are Fill Summary Table is accessible by pressing the several options for displaying the Cut and Fill Volume and Area information. A summary of Volumes is shown in Figure 13-10. If any of the Area options are selected, information for the Upstream (U/S) and Downstream (D/S) cross sections will be displayed. If a section is modified back in the Channel Design/Modification Editor, the Refresh Table button can be used to recomputed the Cut/Fill information and update the summary table.
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Chapter 13 Performing Channel Design/Modifications
Figure 13-10. Summary Cut and Fill information for the channel configuration.
Once the desired channel modification data have been entered for all of the desired reaches, a new geometry file must be created. A new geometry file with the Template data applied is created by pressing the Create a Geometry File with these Modifications button. An intermediate window will appear to allow the current geometric data set to be saved.
Figure 13-11. The new geometry will not contain the channel modification data.
The user will then be prompted to enter a new title for the geometry file that is going to be created. If a title is entered that has already been used for a geometric data set, you will be prompted to overwrite the existing data file. Note that the data entered into the Channel Design/Modification editor is saved with the base geometry file and is not saved with the modified geometry file. This allows the user to open the base geometry file and recreate the modified geometry.
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Chapter 13 Performing Channel Design/Modifications
Comparing Existing and Modified Conditions Once a modified geometry file is created, the user can create a new plan that will incorporate the modified geometry and the previously defined flow data. This is accomplished by first opening the modified geometry file from the Geometric Data window. The next step is to open the Steady Flow Analysis window and create a new Plan. Creating a plan is accomplished by selecting New Plan from the File menu of the Steady Flow Analysis window. Once a new plan is created, the computations can be performed. After the water surface profile computations have been performed for the modified channel conditions, the user can compare the results of the existing and modified conditions on any of the graphics and tables. An example crosssection plot of the two plans is shown in Figure 13-12. Figure 13-12 shows the geometry of the modified and existing conditions, along with the computed water surface elevations from both the existing and modified plans. To display the geometry and results from more than one plan on a graphic, the user can select Plan from the Options menu on any of the graphics. At the top of the plan selection window, turn on the option that says “Compare Geometry As Well As Output.” Select the two plans to be viewed and hit the OK button. The geometry and output for both plans will be displayed. In addition to graphical output, the user can review the computed results from both plans in a tabular form. Figure 13-13 shows the computed results for both plans in Standard Table 1 of the Profile Output table.
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Chapter 13 Performing Channel Design/Modifications
Figure 13-112. and Modified Geometry and Water Surface Elevations.
Figure 13-13. and Modified Geometry and Water Surface Elevations.
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Chapter 14 Using GIS Data With HEC-RAS
CHAPTER
14
Using GIS Data with HEC-RAS HEC-RAS has the ability to import three-dimensional (3D) river schematic and cross section data created in a GIS or CADD system. While the HEC-RAS software only utilizes two-dimensional data during the computations, the three-dimensional information is used in the program for display purposes. After the user has completed a hydraulic analysis, the computed water surface profiles can be exported back to the GIS or CADD system for development and display of a flood inundation map. The importing and exporting of GIS or CADD data is accomplished through the use of formatted ASCII text files. The text files provide a generic way of exchanging data between GIS/CADD systems and HEC-RAS, without adopting any single GIS/CADD system. Appendix B of this manual provides a detailed description and examples of the file formats used for importing and exporting GIS or CADD data. The HEC has developed an ArcGIS extension called HEC-GeoRAS, that was specifically designed to process geospatial data for use with HEC-RAS. The HEC-GeoRAS software allows a user to write geometric data to a file in the required format for HEC-RAS. Additionally, the users can read the HEC-RAS results into HEC-GeoRAS and perform the flood inundation mapping. This software is not part of the HEC-RAS program. The software and a user’s manual are provided as a separate program to be used with ArcGIS. Also, the Intergraph Corporation has adding the capability to exchange data with HEC-RAS in their Software package called Storm and Sewer Works (Intergraph, 1999) This chapter discusses how to import GIS or CADD data into HEC-RAS; what additional information will need to be added to complete the data; and how to export the results back to the GIS or CADD system.
Contents
General Modeling Guidelines
Importing GIS or CADD Data Into HEC-RAS
Completing The Data and Performing The Computations
Exporting Computed Results To The GIS or CADD
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Chapter 14 Using GIS Data With HEC-RAS
General Modeling Guidelines The current version of HEC-RAS has the ability to import the following geometric data from a GIS/CADD system: River System Schematic. The structure of the stream network as represented by a series of interconnected reaches. Each reach is represented as a multi-point line, which is assumed to follow the invert of the main channel. The River and Reach labels, as well as the Junction labels, are also imported from the GIS/CADD. Cross Section Data. The following cross section data can be imported from a GIS/CADD: 1. River, Reach, and River Station identifiers. 2. Cross Section Cut Lines (X and Y coordinates of the plan-view line that represents the cross section). This is a multi-point line that can have two or more points. 3. The cross section surface line. This line is sent to HEC-RAS as a series of X, Y, Z coordinates for each point in the cross section. HEC-RAS transforms these coordinates into station and elevation points (X and Y) for computational purposes. The first station of the cross section is always set to zero. The true (real world) coordinates of the cross section are recomputed from the cross section cut line for the purposes of displaying the data (3D plot). 4. Cross section main channel bank stations. 5. Downstream reach lengths for the left overbank, main channel, and right overbank. 6. Manning’s n values. 7. Levee locations and elevations. 8. Ineffective flow areas. 9. Top of road station and elevation for bridges, culverts, inline and lateral structures. 10. Storage area boundaries and elevation versus volume relationships. At this time, contraction and expansion coefficients, and optional cross section properties (blocked obstructions, etc…) are not imported from a GIS/CADD system. Many of these variables will be added in future versions of the software. The general procedure for utilizing GIS/CADD data with HEC-RAS is the following: 14-2
Chapter 14 Using GIS Data With HEC-RAS 1. The first step is to start a New Project. This is accomplished from the File menu of the main HEC-RAS window. 2. The next step is to go to the Geometric Data editor and import the GIS/CADD data into HEC-RAS. GIS/CADD data are imported by selecting Import Geomtric Data, and then GIS Format from the File menu on the Geometric Data window. This is assuming that you have already used a GIS system to write the required geometry data into a text file, using the required HEC-RAS format. The format of this file is described in Appendix B of this manual. 3. After the GIS data are imported, the user will need to add any additional geometric data that is needed to represent the physical system. 4. The next step is to perform the water surface profile calculations for the desired flow rates. 5. Once the water surface profiles are calculated, the user can then output the results to a text file using the Export GIS Data option from the File menu of the main HEC-RAS window. 6. The last step is to import the HEC-RAS results file into the GIS/CADD system and develop the floodplain maps for each of the profiles. Once the user has a project that is utilizing GIS data, then additional data can be imported directly into an existing HEC-RAS geometry file without starting a new project. This allows the user to go back to the GIS and extract additional cross sections on an as-needed basis. The HEC-RAS program will automatically place the new cross sections into the appropriate River and Reach, based on the identifiers defined for each cross section in the GIS import file. After the user has performed the hydraulic analyses, the computed water surface profiles information can be written to a text file, which can then be imported into the GIS for development and display of floodplain maps. HECRAS exports the cross section Cut Line coordinates (X and Y), as well as the water surface elevation for each profile. This is done for every cross section in the model. Additionally, the program exports a series of bounding polygons (one per river reach) for each computed profile. For information on the HEC-RAS GIS export file format, review the detailed write up found in Appendix B of this manual.
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Chapter 14 Using GIS Data With HEC-RAS
Importing GIS or CADD Data into HEC-RAS Within HEC-RAS, GIS data are imported from the Geometric Data Window. To import geometric data from a GIS/CADD system into HEC-RAS, the following steps should be followed: 1. The first step is to extract the necessary geometric information from a GIS/CADD system and write it to a text file in the required HEC-RAS format. As mentioned previously, HEC has developed an ArcView GIS extension called GeoRAS to help you do this. Likewise, the Intergraph Corporation has added this capability to their program called Stream and Storm Works. You have the option of obtaining the GeoRAS software from HEC (for use in ArcView); using the software developed by Intergraph; or developing your own routines to extract this data from the GIS/CADD system of your choice. The file formats for the required text file are outlined in Appendix B of this manual. 2. The next step is to start a new project in HEC-RAS. This is accomplished by selecting the New Project option from the File menu of the main HEC-RAS window. This option will allow the user to enter a project title and filename. 3. After a new project is started, the user should open the Geometric Data Editor. Once the editor is opened, the user can import GIS/CADD data into HEC-RAS by selecting the Import Geometry Data - GIS Format option from the File menu of the Geometric Data window (Figure 14-1). When this option is selected, a window will appear in which the user can select the file that contains the geometry data from the GIS.
Figure 14-1. The GIS Data Import option on the HEC-RAS Geometric Data Editor
4. Once the user selects the file containing the GIS data, and then presses the OK button, a window will appear that will show you what is available within the import file, and it will allow you to select what you want to import (Error! Reference source not found.) 14-4
Chapter 14 Using GIS Data With HEC-RAS The Import Options window will guide you through the process of importing all or part of the GIS import file. The initial tab of the Import Options dialog is the Intro tab, shown in Figure 14-2. HEC-RAS will read the import file and look for a “UNITS” tag. Based on the value associated with the tag, you will be offered the option to import the data in the current unit system or to convert the data from one unit system to another. If no unit system is found in the GIS file the import dialog will default to your current RAS project units.
Figure 14-2. Unit system conversion is an import option in HEC-RAS.
River Reach Stream Lines The next tab on the import options is the River Reach Stream Lines (see Figure 6-7014-3). This set of options allows you to specify which river reaches to import, how to import the data, and what to name the river and reach. Import options for the river and reaches are summarized in Table 6-14-1.
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Chapter 14 Using GIS Data With HEC-RAS
Figure 14-3. River and reach import options. Table 14-14-1. Summary of River Reach Import option fields. Column
Description
Import As River
The name of the River once it is imported to RAS.
Import As Reach
The name of the Reach once it is imported to RAS.
Import Status
Identifies whether the river reach exists in the RAS geometry file or is new.
Import Stream Lines
Checkbox to choose what river reaches to import. Use the spacebar to toggle the checkbox. All rows can be selected by clicking on the column header.
Merge Mode
The river reach can replace existing data, append upstream, or append downstream.
Cross Section and IB Nodes The next tab on the Import Options window allows you to import cross sections and internal boundaries (bridges and inline structures). The Cross Sections and IB Nodes screen options are shown in Figure 6-7114-4. 14-6
Chapter 14 Using GIS Data With HEC-RAS
Figure 14-4. Cross section and internal boundary import options.
There are several options for importing cross-sectional data. You must first specify the Import River and Import Reach upon which the cross sections reside. The import dialog will inform you what river and reach name the data will import to (Import As) in the HEC-RAS geometry. (The Import As information was specified on the River Reach Stream Lines tab). You then specify the cross sections to import and the specific property to import. Only those cross-sectional properties available from the import file will be available for import. Properties selected will be imported for each cross section specified during the import process. The properties import option will allow you to update individual pieces of data (such as bank station data) without modifying the other data already specified in HEC-RAS. The cross sections that will be imported and how they will be imported are specified in the import table. Import table options are summarized in Table 14-2.
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Table 14-14-2. Summary of Cross Section and IB Nodes Import option fields. Column
Description
Import File River
The name of the River in the import file. Refer to the associated Import As field to see the name of the river that the cross section will be imported into.
Import File Reach
The name of the Reach in the import File. Refer to the associated Import As field to see the name of the reach that the cross section will be imported into.
Import File RS
The name of the River Station in the import file.
Import As RS
The name of the River Station the cross section will be imported into. This data may be user-specified and changed using the provided tools. The “Reset” button will replace the river station data with the data in the import file.
Import Status
The Import Status will be “New” or “Exists”. New will add the cross section to the data. Exists will update (replace) the existing data with the properties specified.
Import Data
Checkbox to choose what river stations to import. Use the spacebar to toggle the checkbox. All rows can be selected by clicking on the column header. You can also use the buttons provided to select all of the New cross sections (Check New) or those that Exist (Check Existing).
There are also several tools provided to change the river station name. River station identifiers are the link between the GeoRAS generated data and the HEC-RAS data. Cross-sectional river stations must be numbers in HEC-RAS. HEC-RAS will use the river stations (along with River names) for determining the order of cross sections for performing water surface profile calculations. River station numbers must increase in the upstream direction. Import options for river stations allow you to match river stations to the existing geometry, round the river station value for import, and create river stationing.
Match River Stations to Existing Geometry The Match Import File RS to Existing Geometry RS option allows you to specify a numeric tolerance to search for duplicate cross sections in existing geometry files. This tool is useful when you are re-importing cross section data where you may have modified the stream centerline or cross section layout. The newly computed river stations may differ from the original stationing due to small spatial changes made in the GIS. This tool is also convenient if you are updating cross sections that have river stations that were rounded during the initial import of the data.
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Chapter 14 Using GIS Data With HEC-RAS
Round River Stations GeoRAS may export the river stationing to more decimal places than are necessary. You can round the river stations to the precision appropriate for your study.
Create River Stations By default, GeoRAS will compute river stations in the unit system of the digital terrain model and will use a zero station at the most downstream end of each river reach. If you wish to change the river stationing you can do so in the GIS, or you can do so during the import process. It is recommended that you document the method used if you change the river stations. Documenting the method used to compute new river stations will be important if you need to re-import cross-sectional data – the river station identifier is the link between the GeoRAS generated data and the HEC-RAS data.
Storage Areas and Connections The Storage Areas and Connections tab, shown in Figure 6-14-5, allows you to specify storage areas and storage area connections to import and what name to import them with.
Figure 14--14-5. Storage Areas and Connections import options.
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Chapter 14 Using GIS Data With HEC-RAS After making the selections of what to import, The user presses the Finished – Import Data button. The data will be imported and a schematic of the river system will show up in the Geometric Data window (Figure 14-6). Once the importing of the data is completed, the user should save the geometric data by selecting Save Geometry Data As from the File menu of the Geometric Data window.
Figure 14-6. River System Schematic of Imported GIS Data
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Chapter 14 Using GIS Data With HEC-RAS
Completing the Data and Performing The Computations After importing any data into HEC-RAS, you must always verify that the data imported is representative of the data you intended to import. Next, you must verify that that the data does not have any significant errors or gaps. Data that is incomplete must be corrected to properly represent the physical system. The cross section plots, tables, and tools in HEC-RAS will assist you in scrutinizing, entering, and modifying data. The Cross Section Plot and Graphical Cross Section Editor are two good ways to visualize the imported cross section data.
Graphical Cross Section Editor The Graphical Cross Sectional Editor is advantageous because not only can you visualize the cross section, you can add, delete, and modify cross section properties. The editor is accessed from the Geometric Data editor by selecting the Tools | Graphical Cross Sectional Edit menu.
Figure 14-7. Accessing the Graphical Cross Section Editor.
The Graphical Cross Section Editor, shown in Figure 14-8, allows you to visualize the shape of the cross section and all of the properties on the cross section. You can also move, add, or delete objects in the cross section from the editor. To change modes, right click in the editor and select from the context menu the mode you wish to work in. The Move Object mode is the default when entering the editor. There are also tools for moving bank stations and Manning n values in the editor.
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Chapter 14 Using GIS Data With HEC-RAS
Figure 14-8. HEC-RAS Graphical Cross Section Editor.
Manning’s n Values Several tables are also convenient for verifying and entering data. Manning’s n value data may be entered using the Tables | Manning n or k values menu item.
Figure 14-9. Entry of Manning's n values through tables in HEC-RAS.
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Chapter 14 Using GIS Data With HEC-RAS
Bridges and Hydraulic Structures Hydraulic structure data may be imported from the GIS. Bridge data will be the most incomplete. Likely the bridge deck top-of-road information will have been imported, but the bridge opening information, piers, and modeling approach information will need to be completed. Often, only the bridge abutment information is included in the digital terrain model. Bridge data is completed in the Bridge and Culvert Data editor access from the Geometric Schematic.
Figure 14-10. Completion of bridge data imported from GeoRAS.
Lateral structures should be examined to verify that the structure begins and ends in the correct location in the HEC-RAS model. If the river station for the lateral structure was not verified prior to GeoRAS export, it must be verified in HEC-RAS. Inline structure and lateral structure data will need to be completed, much like the bridge data. Top–of-weir profile data, gate geometry and settings, and computation methods will need to be modified and input. 14-13
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Cross Section Points Filter Cross sections in HEC-RAS can only have 500 station-elevation points. If you try to run the simulation with more than 500 points the HEC-RAS interface will stop and inform you of the cross sections that have too many points. To filter cross section points, select the Tools | Graphical Cross Section Points Filter menu item from the Geometric Data Editor. You can filter cross sections at a single cross section or at multiple locations. You also have the choice of filtering based on the slope between each point or based on minimizing the change in area in the cross section.
Figure 14-11. Cross section points filter dialog.
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Chapter 14 Using GIS Data With HEC-RAS
Completing the Flow Data and Boundary Conditions Flow data will not be imported from the RAS GIS Import File. You will have to create a Steady Flow or Unsteady Flow file and enter flow change locations and boundary condition data. It is also important in steady-flow simulations to label the flow profiles with a meaningful name.
Examining Results After performing a steady or unsteady flow simulation, you must verify the hydraulic results using the standard plots and tables available in HEC-RAS. You must also verify that the computed water surface profile(s) will result in an appropriate floodplain. For instance, cross sections should be closely spaced together around bends in the river and extend across the entire floodplain. Cross sections should also be wide enough to allow for nonlinear floodplain delineation between cross section. Prior to exporting the water surface profile results, you should also verify the bounding polygon. The bounding polygon limits the area that will be used for floodplain delineation. This is especially important when the river system has levees that may be overtopped by one of the water surface profiles. If a levee upstream is overtopped, you will need to verify that the levees downstream are turned off, as well. Bounding polygon information for each profile can be verified in the Geometric Data editor. Select the Tools | Plot GIS Reach Profile Bounds menu item. You must then select the Profile and River Reach for which to plot data. a.) Levee contains flow
b.) Levee is overtopped
Figure 14-12. GIS bounding polygon information (thick line) for a leveed system (a) contains flow and (b) is overtopped.
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Chapter 14 Using GIS Data With HEC-RAS
Exporting the HEC-RAS Results After steady or unsteady flow simulation, HEC-RAS results can be exported for processing in the GIS by GeoRAS. Select the File | Export GIS Data menu option from the main RAS interface as shown in Figure 14-13.
Figure 14-13. Access the GIS export options from the main RAS interface.
The dialog shown in Figure 14-14 will allow you to choose the file location to write the GIS information to and select the output options. Be sure to select the water surface profiles of interest. The GIS data will be written to the RAS GIS Export file (.RASExport.sdf).
Figure 14-14. GIS export options in HEC-RAS.
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Chapter 14 Using GIS Data With HEC-RAS
As shown in Figure 14-14, the user first enters a filename for the HEC-RAS Export file. Then, they can select which river reaches and storage areas to export results for (By default the entire model is selected). Next, the user can select what they would like to export. Normally the user would select "Export Water Surfaces," and then select which profiles to export by using the Select Profiles to Export button. Once these options are selected, the information can be exported by pressing the Export Data button. Additional options are available to export geometry data from HEC-RAS to the GIS/CADD system. This option can be very useful for supplementing terrain data with additional surveyed cross sections. It is a common occurrence for terrain models to have good information in the overbank areas, but not as good, if at all, in the main channel. HEC-RAS allows the user to export the entire cross section, or just the main channel portion only. Also, the user can send all cross sections, including interpolated sections, or they can turn off the interpolated cross sections. Additionally, there are options to send reach lengths, bank stations, levees, and ineffective flow areas to the GIS system. In order to use the feature of sending terrain data from HEC-RAS to the GIS, the user must enter geospatial coordinates for all of the cross sections, and the stream centerline before exporting the data. These coordinates are required in order to correctly locate the data spatially within the terrain model.
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Chapter 15 Stable Channel Design Functions
CHAPTER
15
Stable Channel Design Functions The channel design functions within HEC-RAS are based upon the methods available in the SAM Hydraulic Design Package for Channels (USACE, 1998), developed by the U.S. Army Corps of Engineers Waterways Experiment Station. This chapter presents the data input required for computing uniform flow parameters, stable channel dimensions, and sediment transport capacity for a given cross section. For information on the Channel Design Functions equations and theory, please see Chapter 15 of the HEC-RAS Hydraulic Reference Manual.
Contents
General Modeling Guidelines
Uniform Flow Computations
Stable Channel Design
Sediment Transport Capacity
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Chapter 15 Stable Channel Design Functions
General Modeling Guidelines The Stable Channel Design Functions within HEC-RAS are meant to be used as an aid in the design of stable channels. The purpose of this application is to provide the qualitative, easy-to-use methodology of the SAM software package within the HEC-RAS framework. Specifically, the Stable Channel Design Functions will allow the user to easily compute the hydraulic parameters of a given cross section, use that information to design a stable channel with regard to its size and armoring, and determine the sediment transport capacity of that cross section.
General Command Buttons The general command buttons can be seen in the top right-hand corner of the window shown in Figure 15-1. The Defaults button restores the current hydraulic design function’s fields to the default values. The Apply button will store the entries on the current window into memory. These values will remain in memory until a new hydraulic design file is opened or the user exits HEC-RAS. The Compute button initiates the computations for whatever hydraulic design function is currently active. The Report button displays a printable report providing detailed hydraulic design information. Output will be displayed in the report window if the computations have been run.
Uniform Flow Computations The uniform flow computations are performed by opening the Hydraulic Design Functions window and selecting the Uniform Flow from the Type menu item. Once this option is selected the program will automatically go to the geometry file and plot a cross section with the station and elevation data entered into the table. The user can select any cross section from the available rivers and reaches. The Hydraulic Design window for uniform flow will appear as shown in Figure 15-1. As shown in Figure 15-1, the Uniform Flow window contains the input data, a graphic, and a window for summary results. Input data tabs included are the S/Q/y/n tab and the Width tab. The S/Q/y/n tab is used for calculating the normal slope, discharge, depth, or roughness for the current cross section. The Width tab is used to calculate the bottom width for a uniform flow solution of a user-entered compound channel (with up to 3 trapezoidal templates). The station, elevation, and roughness values for both the current cross section and the user-defined cross section can easily be manipulated in the table and applied to the current geometry file. The user is required to enter only a minimal amount of input and the computations can be performed.
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Chapter 15 Stable Channel Design Functions
Figure 15-1. Hydraulic Design Window for Uniform Flow
Solving for Slope, Discharge, or W/S Elevation When the S/Q/y/n tab has been selected, to calculate a slope that satisfies the uniform flow equations for the current cross section, simply enter values into the Discharge and a W/S Elev fields and press the Compute button. A value for the slope is then automatically entered into the Slope field. Likewise, for solving for discharge or water surface elevation, enter values for the other two parameters. The roughness values are automatically taken from the geometry file, but these can be changed to better represent the bed characteristics of the cross section. In addition to changing the value of the roughness factor (in the default case, Manning’s n), the function for defining roughness can be changed. To do this, click on any cell in the equation column of the table and select a function from the dropdown list. The available functions to choose from are Manning’s, Keulegan, Strickler, Limerinos, Brownlie, and five grasslined channel methods. Each of these functions is discussed in detail in Chapter 15 of the Hydraulic Reference Manual. For the Limerinos and Brownlie functions, gradation distribution is necessary and can be entered by pressing the Gradation button. Only one gradation 15-3
Chapter 15 Stable Channel Design Functions distribution can be used for a given cross section and should be applied only to the main channel, as these functions were developed for bed material. The Gradation window is shown in Figure 15-2. The following gradation variables are defined as the following: d84: The sediment particle size for which 84% of the sediment mixture is finer (mm). d50: The sediment particle size for which 50% of the sediment mixture is finer (mm). d16: The sediment particle size for which 16% of the sediment mixture is finer (mm).
Figure 15-2. Gradation Window
The Brownlie function requires a sediment specific gravity to be entered and the Keulegan function requires a temperature to be entered. The Compute button only becomes active once all required input is entered. To solve for a roughness value, click on and delete only one of the roughness values in the table. Only one roughness section can be solved for at a time. Make sure Slope, Discharge, and W/S Elev are specified and all other required input are entered. RAS then computes a Manning’s n value to satisfy the uniform flow equation for the portion of the cross section that is desired. Then, the roughness value is back-calculated to match the selected roughness function. Only Manning, Keulegan, and Strickler functions can be used to solve for roughness, since the other functions do not have a representative roughness value to solve for. Once one computation has been made, the value that was solved for will be shown in bold font. For subsequent computations, any of the four uniform flow parameters that is emboldened will be what is solved for to avoid having to delete out the value every time. Once a new parameter is deleted out, it will then be solved for and emboldened.
Solving for Bottom Width Bottom width can be solved for the uniform flow equation only with a compound channel that is defined by the user. The compound channel may contain up to three trapezoidal templates, a low flow channel, the main channel, and the overbank channel. The bottom width of either the main channel or the overbank may be solved for. The addition or subtraction of 15-4
Chapter 15 Stable Channel Design Functions width may be applied to right of centerline, left of centerline or equally to both sides. When the bottom width tab is selected, the window shown in Figure 15-3 is displayed. To define the compound channel, enter the appropriate values into the compound channel table, which is located below the station elevation table. Data for the Overbank, Main, and Low Flow channels can be entered, however data for the low flow channel can only be applied if a main channel is also defined. The following variables are defined as follows: SSL: The side slope of the left side of the channel. Entering a value of “0” provides a vertical slope (1Vertical : __Horizontal). SSR: The side slope of the right side of the channel. Entering a value of “0” provides a vertical slope (1Vertical : __Horizontal). WL: The bottom width of the left side of the channel from the centerline of the channel to the toe of the side slope (ft or m). WR: The bottom width of the left side of the channel from the centerline of the channel to the toe of the side slope (ft or m). Height: The height of the respective channel from its invert to the top of its side slope (ft or m). Invert: The invert of the respective channel (ft or m). Once the channel template data is entered, the user may plot the data by selecting Apply Geometry. When this button is selected, the channel design is shown in the plot window and entered in the station elevation table with the default roughness information. A Manning’s n value of 0.03 will be applied to each of the channel templates defined. The user may then adjust the roughness values, change the roughness functions, or add more roughness change locations within the cross section on the station elevation table. Any changes made can be reapplied to the plot by pressing Apply Geometry. See Figure 15-4. If either the Brownlie or Limerinos functions are chosen, gradation data will have to be entered. A value for the energy slope, discharge, and water surface elevation must be entered in the appropriate fields. The user can then select how to solve for the bottom width by using the dropdown boxes in the “Compute Widths” section. Either the main channel or the overbank channel can be solved for and the width can be applied to the left side of the channel (Left of CL only), the right side of the channel (Right of CL only), or equally to both (Total).
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Chapter 15 Stable Channel Design Functions
Figure 15-3. Bottom Width Calculation
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Chapter 15 Stable Channel Design Functions
Figure 15-4. Example Bottom Width Data Entry
When all required data is entered, the Compute button will become active. The computations are constrained from creating unrealistic geometries. One example is the overbank bottom width cannot become less than the top width of the main channel. Likewise, the main channel bottom width cannot become less than the low flow channel top width. If this situation occurs within the computations, the user is notified and a course of action is suggested. However, if the top width of a lower channel becomes greater than the bottom width of the channel above it within the calculations, the program automatically increases the upper channel’s bottom width to compensate. When a solution is obtained, the new widths are updated in the compound channel table, the station elevation table and the plot.
Applying Uniform Flow Data to the Geometry File The resulting cross section, displayed in the plot window can be added to the existing geometry data by clicking on the “Copy XS to Geometric Data” command button. The following window will appear: 15-7
Chapter 15 Stable Channel Design Functions
Figure 15-5. Copy Cross Section Window
Enter in the river station you want this cross section to be applied to. If the selected river station already contains a cross section, RAS will ask if you want to copy over it. If there is no cross section at the entered river station, RAS will automatically adjust the distances between the new cross section and its adjacent ones. Make sure that once the new cross section has been copied to the geometry, appropriate values for the bed elevations are reentered. This can easily be done by selecting “Adjust Elevations…” in the Option menu of the Cross Section Data window.
Saving Uniform Flow Data To save the uniform flow data, click on File…save. This will add all pertinent data from all the HD Functions to an ascii file with the extension *.h##. The content of this file can easily be read within any word processing program.
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Chapter 15 Stable Channel Design Functions
Stable Channel Design Stable channels can be computed using three different methods:
Copeland
Regime
Tractive Force
To access the stable channel design window, click on Type…Stable Channel Design in the Hydraulic Design Window. The following window will become active:
Figure 15-6. Stable Channel Design Window
Copeland Method To use the Copeland Method, select the tab named “Copeland.” There are a number of required and optional fields to enter data into for both the design
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Chapter 15 Stable Channel Design Functions section and the upstream section. To enter in data for the design section, simply add data to the fields shown. Discharge: The design discharge. Can be the 2-year, 10-year, bankfull, etc. Must represent the channel forming discharge (cfs or m3/s). Specific Gravity: Self-explanatory. Default is 2.65. Temperature: A representative temperature of the water. Default is 55 degrees F or 10 degrees C. Valley Slope: (Optional) The maximum possible slope for the channel invert (i.e. no channel sinuosity). If the slope returned is greater than the valley slope, HEC-RAS will indicate that this is a “sediment trap.” Med. Channel Width: (Optional) Median channel width. The median width of the array of 20 bottom widths that are solved for. There will be 9 widths less than and 10 widths greater than the median channel width all at an increment of 0.1 X Med. Channel Width (ft or m). If this is left blank, the median width assigned will be equal to the regime width by the following equation: B = 2Q0.5 Side Slope: Slope of the left and right side slopes. (1Vertical : __Horizontal). Equation: Can choose from Mannings or Strickler to solve for the side slope roughness. n or k: If Mannings is selected, enter a Mannings “n” value. If Strickler is selected, enter a “k” value (ft or m for k values). Gradation of the sediment is required for Copeland method and can be entered by clicking on the Gradation command button. Values for d84, d50, and d16 must be entered. The user has the ability to designate the default regime for the computations. The HEC-RAS default is lower regime, but this can be changed by clicking on the “Default Regime…” button and selecting “Upper Regime”. Any time the computations result in a solution that is in the transitional regime, the default regime will be used and the user will be notified in the output table that this occurred. See chapter 12 of the Hydraulic Design Manual for more information. Once all required data for the design section has been entered, click on the “Inflow Sediment…” command button to input information about the upstream section for sediment concentration computations. The window shown in Figure 15-7 becomes active. The user can either enter in a value for the inflowing sediment concentration or let HEC-RAS calculate it. If HEC-RAS is to calculate the inflow sediment concentration, then the following information about the upstream section must be entered: Supply Reach Bottom Width: Width of the bed of the supply reach (ft or m).
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Chapter 15 Stable Channel Design Functions Supply Reach Bank Height: A representative value of the bank elevation minus the channel invert elevation of the supply section. This is only used in the computations to target a depth and does not limit the solution to this height (ft or m). Supply Energy Slope: A representative energy slope at the supply section. Water surface slope is typically used. Side Slope: Slope of the left and right side slopes of the supply section. (1Vertical : __Horizontal). Equation: Can choose from Mannings or Strickler to solve for the side slope roughness of the supply section. n or k: If Mannings is selected, enter a Mannings “n” value. If Strickler is selected, enter a “k” value for the supply section (ft or m for k values).
Figure 15-7. Inflow Sediment Concentration Window
Click OK to apply the input and return to the main HD Functions window. Once all of the required input has been entered, the Compute button will be activated. Click the Compute button to run the computations. When the computations are complete, the output table will be shown. The output table lists all of the channel widths solved for along with the corresponding depth, slope, composite n value, hydraulic radius, velocity, Froude number, shear stress and bed transport regime. An example is shown in Figure 15-8. There will be twenty different stable channel geometries plus one for the minimum stream power. The user can select one of these geometries for display on the plot window. Once the desired section is selected, click OK and the HD Functions window will become active with the selected section plotted in the plot window. When the computations have been run, the Table button, the two Stability Curve buttons and the Copy to Geometry button become active. The Table 15-11
Chapter 15 Stable Channel Design Functions button simply allows the user to pull up the output table again, and select a different stable section, if desired. Clicking on the Stability Curve 1 button will bring up a plot of the stability curve showing slope versus width, indicating for what slope/width combination aggradation or degradation can be expected. Figure 15-9 shows an example.
Figure 15-8. Copeland Method Output Table
Figure 15-9. Stability Curve
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Chapter 15 Stable Channel Design Functions Stability Curve 2 brings up a similar plot, only with slope compared to depth. In addition to viewing the plots, the table tab can be clicked to view the stability curves in tabular form. As with the uniform flow computations, the section that has been plotted from the Copeland Method can be applied to the current geometry file by clicking on the Copy to Geometry button.
Regime Method To use the Regime Method, select the tab named “Regime.” The window shown in Figure 15-10 becomes active.
Figure 15-10. Regime Method
Enter in all required input which are: Discharge: Channel forming discharge (cfs or m3/s). d50: Median particle size (mm). Sediment Conc, C ppm: The bed material sediment concentration, in ppm. Temperature: A representative temperature of the water. Default is 55 degrees F or 10 degrees C. Side Factor, Fs: The side factor as defined by Blench. Blench suggests 0.1 for friable banks, 0.2 for silty, clayey, or loamy banks, or 0.3 for tough clay banks. Default value is 0.2.
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Chapter 15 Stable Channel Design Functions Once these values are entered, the compute button becomes activated and the stable channel regime values for depth, width, and slope will be solved for and entered into the appropriate fields. In addition, the plot window will display the resulting cross section. The displayed cross section can be added to the existing geometry file by clicking on “Copy XS to Geometric Data.”
Tractive Force Method To use the Tractive Force Method, select the tab named “Tractive Force.” The window shown in Figure 15-11 becomes active.
Figure 15-11. Tractive Force Method
Enter in all required input which are: Discharge: Design discharge (cfs or m3/s). Temperature: Temperature of the water. Default is 55 degrees F or 10 degrees C. Specific Gravity: Specific gravity of the sediments for the left side slope, bed, and right side slope. Angle of Repose: The angle of repose of the sediment for the left side slope, bed, and right side slope. See figure 12-9 in the HEC-RAS Hydraulic Reference Manual for suggested values. Side Slope: Left side slope and right side slope (1Vertical : __Horizontal). 15-14
Chapter 15 Stable Channel Design Functions Equation: the roughness equation for the left side slope, bed, and right side slope. Mannings and Strickler are available for use. n or k: If Mannings is selected, enter a Mannings “n” value. If Strickler is selected, enter a “k” value for the left side slope, bed, and right side slope (ft or m for k values). Method: Solve for critical shear using either Lane, Shields, or by entering in your own critical mobility parameter. The remaining values are the dependant variables. Only two can be solved for at a time. The other two must be entered by the user. The three fields for particle diameter (left side slope, bed, right side slope) are considered one variable such that any one of the remaining variables plus any or all of the particle diameters can be solved for. d50/d75: The particle diameter in which 50%/75% of the sediment is smaller, by weight. d50 is used for Shields and user-entered. d75 is used for Lane (mm). D: The depth of the stable cross section (ft or m). B: The bottom width of the stable cross section (ft or m). S: The slope of the energy grade line at the stable cross section. Once the required values plus two of the dependent variables are entered, the compute button becomes activated and the stable channel values for the remaining dependent variables will be solved for and entered into the appropriate fields. In addition, the plot window will display the resulting cross section. The displayed cross section can be added to the existing geometry file by clicking on “Copy XS to Geometric Data.”
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Chapter 15 Stable Channel Design Functions
Sediment Transport Capacity The sediment transport capacity computations can only be run once steady or unsteady flow computations have been run. Sediment Transport Capacity for any cross section can be computed using any of the following sediment transport functions:
Ackers-White
Engelund-Hansen
Laursen
Meyer-Peter Müller
Toffaleti
Yang
To access the sediment transport capacity window, click on Type…Sediment Transport Capacity in the Hydraulic Design Window. The following window will become active:
Figure 15-12. Sediment Transport Capacity Window
15-16
Chapter 15 Stable Channel Design Functions To perform sediment transport capacity computations, the user must define one or more sediment reaches. A sediment reach indicates for which cross sections transport rates will be computed and contains information necessary to fulfill the computations. Sediment reaches can vary spatially within the geometry, can have different input parameters such as temperature, specific gravity, and gradation, or can simply use different sediment transport functions. A sediment reach cannot span more then one river reach, however there can be multiple sediment reaches within one river reach. Sediment reaches cannot have overlapping cross sections. When the sediment transport capacity window is opened, if there are not any previously defined sediment reaches defined for the current hd file, the user will be automatically prompted to name a new sediment reach. To create a new reach otherwise, click on File…New Sediment Reach. The user also has the option of copying, deleting, and renaming existing sediment reaches under the File menu option. The name selected for the new sediment reach will appear in the Sed. Reach dropdown box along with all other existing sediment reaches for the particular hydraulic design file. Once a new sediment reach has been named, the user must define its spatial constraints by selecting the river, reach, and the bounding upstream and downstream river stations. Next, one of the existing profiles must be selected. Sed.Reach: Indicates which sediment reach is active. This dropdown box lists all existing sediment reaches for the current hydraulic design file. River: The river where the current sediment reach is located. Reach: The reach where the current sediment reach is located. US RS: The upstream bounding river station of the current sediment reach. DS RS: The downstream bounding river station of the current sediment reach. Profiles: The profile to be used in the sediment transport computations for the current sediment reach. River Sta: The river station currently displayed on the plot. Temperature: Temperature of the water. Default is 55 degrees F or 10 degrees C. Specific Gravity: Specific gravity of the moveable sediments. Default is 2.65. Bed Sta Left/Right: The cross section stations that separate the left overbank from the main channel from the right overbank for sediment transport capacity computations. Defaults are the main bank stations. These values can be changed for every cross section within the sediment reach. The selected stations appear on the cross section plot as yellow nodes, and are bracketed by “MB” (mobile bed) location arrows on the top of the plot.
15-17
Chapter 15 Stable Channel Design Functions Conc. of Fines (opt): The concentration of fine sediments (wash load) in the current sediment reach. This is an optional value and is used to adjust the transport rate based on Colby’s (Colby, 1964) findings regarding the effects of fine sediment and temperature on kinematic viscosity, and consequently particle fall velocity. Values are given in parts sediment per one million parts water, by weight. Functions: The user can select one or more sediment transport functions from this list box. By clicking the checkbox, a check will appear and RAS will compute for that function. When clicking on the name of the function, a brief description of the function and its applicability will appear in the text box below. Gradation: This is entered for the left overbank (LOB), main channel (Main) and right overbank (ROB) as defined by the left and right bed stations. The user can enter nothing or up to 50 particle size/percent finer relationships. By right-clicking on one of the tabs, the grid can be expanded for easier viewing. Right-click again to return the grid to its compact display. Typically 5 to 10 gradation points are enough to represent a typical gradation curve. The particle diameter is entered in mm under the column header Diam, mm, and the percent of the representative sediment that is finer than that particle diameter is entered under the column header %Finer. RAS then takes this gradation input to determine the fraction of the sediment that is in each standard grade size class. If a zero percent value and/or a 100% value are not entered by the user, the program will assign zero percent to the next lowest grade class and 100% to the next highest grade class. See the hydraulic reference manual for more detail. Plot Gradation: This button gives the user a graphical representation of the sediment gradation. The user has the option to compute sediment transport capacity rates for the currently selected sediment reach (Compute for this Sediment Reach) or for all existing sediment reaches (Compute for all Sediment Reaches) within the currently opened hydraulic design file. A text box is provided for brief descriptions of selected transport functions. In addition to a summary of the selected function, the range of input parameters, from both field and laboratory measurements, used in the development of the respective function is also provided. Where available, these ranges are taken from those found in the SAM package user’s manual (Waterways Experiment Station, 1998) and are based on the developer’s stated ranges when presented in their original papers. The ranges provided for Engelund and Hansen are taken from the database (Guy, et al, 1966) primarily used in that function’s development. The following variables are used in the summaries:
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d, overall particle diameter
dm, median particle diameter
s, sediment specific gravity
V, average channel velocity
D, channel depth
Chapter 15 Stable Channel Design Functions
S, energy gradient
W, channel width
T, water temperature
Defaults: The Defaults button will restore all input boxes for the currently selected sediment reach to the default values. Apply: The Apply button will be enabled any time new input has been added which has not been stored into memory. By clicking on the Apply button, all input for the current sediment reach will be stored to memory. Compute: The compute button will be enabled once all required input is entered. Pressing the compute button initiates the computations for sediment transport capacity. Options Menu: The Options Menu drop down list is on the top of the Sediment Transport Capacity form and includes: Fall Velocity: This option allows the user to select the method of fall velocity computation. If “Default” is selected, the method used in the research and development of the respective function is chosen. Otherwise, any functions used in the computations will use the selected fall velocity method. The three fall velocity methods available are: Toffaleti, Van Rijn, and Rubey. Depth/Width: This allows the user to select which depth and width parameters to use in the solution of the transport functions. If “Default” is selected, the program will use the depth/width combination used in the research of the selected functions(s). If any of the other depth/width combinations is used, all selected functions will be solved using those specific parameters. Eff. Depth/Eff. Width: Used in HEC 6, this is the effective depth and effective width. Effective Depth is a weighted average depth and the effective width is calculated from the effective depth to preserve aD2/3 for the cross section: 2
n
EFD
Davg ai Davg3
n
i 1
n
2 3 avg
a D i 1
i
EFW
2
ai Davg3 i 1
EFD
5 3
Hyd. Depth/Top Width: The hydraulic depth is the area of the cross section divided by the top width. Hyd. Radius/Top Width: The hydraulic radius is the Area divided by the wetted perimeter. Is equivalent to hydraulic depth for relatively wide, shallow streams. Hiding Factor for Ackers-White: An optional “hiding factor” adjustment is available for the Ackers-White function only. The user can choose whether or not to use this feature. The default is “No.” 15-19
Chapter 15 Stable Channel Design Functions Compute for Small Grains Outside Applicable Range: By default, RAS will perform calculations for grain sizes which are smaller than the applicable range of a given transport function. By selecting “No”, the user can override this and have RAS compute for only the grain sizes within the applicability range of each sediment transport function, as defined in Table 12.7 in the Reference Manual. Sediment Rating Curve Plot/Table: This button displays a plot of the sediment transport capacity rates for a selected cross section within a sediment reach. It is only enabled once computations for that reach have been performed. Display options can be selected from the dropdown buttons. Figure 15-13 shows a sediment rating curve plot. In addition to viewing the plots, the table tab can be clicked to view in tabular form.
Figure 15-13. Sediment Transport Capacity Rating Curve
Sediment Transport Profile Plot/Table: This button displays a plot of the sediment transport capacity rates along a selected sediment reach. It is only enabled once computations for that reach have been performed. Display options can be selected from the dropdown buttons. Figure 15-14 shows the sediment transport profile plot. In addition to viewing the plots, the table tab can be clicked to view in tabular form.
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Chapter 15 Stable Channel Design Functions
Figure 15-14. Sediment Transport Capacity Plot
Both plot windows have a list box at the bottom with warning messages. These warnings are meant to make the user aware of how sediment transport rates are being computed. If the user selects the option to compute sediment transport rates for all grade sizes within the user-specified range, a warning stating this will be shown. If the user selects the option to compute sediment transport rates for only those grade sizes within the respective function’s applicability range, then a warning a different warning message will appear. The “Compute for Small Grains Outside Applicability Range” option is located in the menu item “Options” on the Hydraulic Design window for sediment transport capacity. Report: The Report button is located in the plot window and generates a report summarizing the input and output data. The output data is displayed as per the selections made in the dropdown buttons. Because the amount of output has the potential for being quite large, the report that is generated can likewise be very large. Figure 15-15 shows an example of the sediment transport capacity report. As with other report windows found in HEC-RAS, the user has the ability to send this report to the clipboard, print it, or save it as a text file.
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Chapter 15 Stable Channel Design Functions
Figure 15-15. Sediment Transport Capacity Report
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Chapter 16 Advanced Features for Unsteady Flow Routing
CHAPTER
16
Advanced Features for Unsteady Flow Routing HEC-RAS has several advanced features that can be used when modeling complex unsteady flow situations. These features include mixed flow regime capabilities (subcritical, supercritical, hydraulic jumps, and draw downs); the ability to perform a dam break analysis; levee overtopping and breaching; hinge pool calculations for navigation dams; how to model pressurized pipe flow in HEC-RAS; and using generic rules to control gate operations at hydraulic structures.
Content:
Mixed Flow Regime
Dam Break Analysis
Levee Breaching and Overtopping
Pump Stations
Navigation Dams
Modeling Pressurized Pipe Flow
User Defined Rules for Controlling Gate Operations
16-1
Chapter 16 Advanced Features for Unsteady Flow Routing
Mixed Flow Regime Modeling mixed flow regime (subcritical, supercritical, hydraulic jumps, and draw downs) is quite complex with an unsteady flow model. In general, most unsteady flow solution algorithms become unstable when the flow passes through critical depth. The solution of the unsteady flow equations is accomplished by calculating derivatives (changes in depth and velocity with respect to time and space) in order to solve the equations numerically. When the flow passes through critical depth, the derivatives become very large and begin to cause oscillations in the solution. These oscillations tend to grow larger until the solution goes completely unstable. In order to solve the stability problem for a mixed flow regime system, Dr. Danny Fread (Fread, 1986) developed a methodology called the “Local Partial Inertia Technique.” The LPI method has been adapted to HEC-RAS as an option for solving mixed flow regime problems when using the unsteady flow analysis portion of HEC-RAS. This methodology applies a reduction factor to the two inertia terms in the momentum equation as the Froude number goes towards 1.0. The modified momentum equation is show below:
Q2 A Q h gA S f 0 t x x
(16-1)
and
FT Frm 0 where: σ
16-2
( Fr FT ; m 1 ) ( Fr FT )
=
LPI factor to multiply by inertial terms.
FT
=
Froude number threshold at which factor is set to zero. This value should range from 1.0 to 2.0 (default is 1.0)
Fr
=
Froude number.
m
=
Exponent of equation, which changes the shape of the curve. This exponent can range between 1 and 128 (default value is 10).
h
=
Water surface elevation
Sf
=
Friction slope
Q
=
Flow rate (discharge)
A
=
Active cross sectional area
Chapter 16 Advanced Features for Unsteady Flow Routing g
=
Gravitational force
The default values for the equation are FT = 1.0 and m = 10. When the Froude number is greater than the threshold value, the factor is set to zero. The user can change both the Froude number threshold and the exponent. As you increase the value of both the threshold and the exponent, you decrease stability but increase accuracy. As you decrease the value of the threshold and/or the exponent, you increase stability but decrease accuracy. To change either the threshold or the exponent, select Mixed Flow Options from the Options menu of the Unsteady Flow Analysis window. When this option is selected, the unsteady mixed flow options window will appear as shown in Figure 16-1. As shown in Figure 16-1, the graphic displays what the magnitude of the LPI factor will be for a given Froude number and a given exponent m. Each curve on the graph represents an equation with a threshold of 1.0 (FT) and a different exponent (m).
Figure 16-1. Unsteady Mixed Flow Options Window
By default, the mixed flow regime option is not turned on. To turn this option on, check the Mixed Flow Regime box, which is contained within the computational settings area of the Unsteady Flow Analysis window. This window and option is shown in the Figure 16-2.
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-2. Unsteady Flow Analysis Window with Mixed Flow Regime Option Turned On
In general, when modeling a river system that is completely subcritical flow, you should not turn on the mixed flow regime option. If the system is mostly subcritical flow, with only a few areas that pass through critical depth, then this option can be very useful for solving stability problems. However, there may be other options for modeling the areas that pass through critical depth. For example, if the system has a location with a drops in the bed where flow passes through critical depth over the drop, but is subcritical just downstream of the drop, this would be a good location to model the drop as an inline weir within HEC-RAS. By modeling the drop as an inline weir, the program is not modeling the passing through critical depth with the momentum equation, it is getting an upstream head water elevation for a given flow from the weir equation. If the river system has several areas that pass through critical depth, go supercritical, and go through hydraulic jumps, then the mixed flow methodology may be the only way to get the model to solve the unsteady flow problem. A profile plot of a mixed flow regime problem is shown in Figure 16-3. This example was run with the unsteady flow simulation capability within HEC-RAS using the mixed flow regime option. The example shows a steep reach flowing supercritical, which then transitions into a mild reach. A hydraulic jump occurs on the mild reach. The mild reach then transitions back to a steep reach, such that the flow goes from subcritical to supercritical. Because of a high downstream boundary condition (for example backwater from a
16-4
Chapter 16 Advanced Features for Unsteady Flow Routing lake), the flow then goes from supercritical to subcritical though another hydraulic jump. Mixed Flow
Plan: UNSTEADY FLOW
2:59:20 PM
Mixed Reach Mixed Reach
74
Legend WS 01JAN2000 1700
72
Crit 01JAN2000 1700 Ground
70
OWS 01JAN2000 1700
Elevation (ft)
68
66
64
62
60
58
56
500
1000
1500
2000
2500
3000
Main Channel Distance (ft)
Figure 16-3. Example Mixed Flow Regime Run with Unsteady Flow Routing
16-5
Chapter 16 Advanced Features for Unsteady Flow Routing
Dam Break Analysis The failure of several dams in this country (Buffalo Creek, West Virginia 1972; Teton dam, Idaho 1976; Laural Run Dam and Sandy Run Dam, Pennsylvania 1977; Kelly Barnes Dam, Georgia 1977; and others), has led our nation to take a strong look at dam safety. One aspect of dam safety is to answer the question, “What will happen if the dam were to fail?” The ability to evaluate the results of a dam failure has been added into the HEC-RAS software. HEC-RAS can be used to model both overtopping as well as piping failure breaches for earthen dams. Additionally, the more instantaneous type of failures of concrete dams (generally occurring from earthquakes) can also be modeled. The resulting flood wave is routed downstream using the unsteady flow equations. Inundation mapping of the resulting flood can be done with the HEC-GeoRAS program (companion product to HEC-RAS) when GIS data (terrain data) are available. Dams are modeled within HEC-RAS by using the Inline Structure editor. The Inline Structure editor allows the user to put in an embankment, define overflow spillways and weirs, and gated openings (radial and sluice gates). Gated openings can be controlled with a time series of gate openings or using the elevation control gate operation feature in HEC-RAS. For more information on modeling inline hydraulic structures within HEC-RAS, please review Chapter 6 of this manual (Entering and Editing Geometric Data). The lake area upstream of the dam can either be modeled with cross sections or by using a storage area (Figure 16-4.). If cross sections are used, then HEC-RAS will perform full unsteady flow routing through the reservoir pool and downstream of the dam. If a storage area is used, HEC-RAS uses level pool routing through the lake, then unsteady flow routing downstream of the dam. When using a storage area to represent the reservoir pool, HEC-RAS requires two cross sections inside of the reservoir pool, then the inline structure representing the dam, and then the downstream cross sections (see Figure 16-4). The routing reach is hydraulically connected to the reservoir (storage area) with the first (most upstream) cross section. This cross sections water surface is forced to the elevation of the water surface in the storage area during the unsteady flow routing. The second cross section in the pool area is required as a bounding cross section for the inline structure (the dam). One additional caution for using a storage area to represent the pool area: When the initial conditions are computed by backwater analysis, it is up to the user to ensure that the water surface computed just upstream of the dam (for the two cross sections) is consistent with the starting water surface entered for the storage area. If this is not the case, the model will most likely have stability problems at the very start of the unsteady flow routing. There are two ways to ensure these water surfaces are consistent. The first is to adjust the low flow gate openings and initial base flow in the reach to produce a water surface that is consistent with the desired starting pool elevation. The second way is to use the option that allows the user to force the water
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Chapter 16 Advanced Features for Unsteady Flow Routing surface at a cross section during the initial conditions calculations. This option is called Internal RS Initial Stages, and is available from the Options menu of the Unsteady Flow Data editor. This option can be used to set the water surface just upstream of the dam to the same elevation as the storage area. An example of using the Inline Structure feature to model a dam is shown in Figure 16-5. As shown in the Figure, the user enters the embankment and overflow spillway as one piece using the Weir/Embankment editor. The embankment is shown as the gray filled in area above the ground. The overflow spillway is the rectangular notch on the upper left hand side of the embankment. The main outlet works consist of two rectangular gates, which are entered through the gate editor. The gates are shown towards the bottom of the embankment in this example.
Reservoir (Storage Area) Dam (Inline Structure)
Figure 16-4. Alternate Methods for Modeling a Dam and Reservoir Pool in HEC-RAS.
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-5. Inline Structure Editor with Example Dam Shown
Entering Dam Break Data Entering dam breach information is accomplished by pressing the button labeled Breach (plan Data). The breach information is stored as part of the current Plan. This was done to facilitate evaluating dam and levee breaching in a real time river forecasting mode. By putting the breach information in the Plan file, the geometric pre-processor does not have to be run again, thus saving computation time during forecasting. The user can also access dam breach information by selecting Dam Breach (Inline Structure) from the Options menu of the Unsteady Flow Analysis window. Once the Breach button is pressed, the Dam Breach window will appear as shown in Figure 16-6.
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-6. Dam Breach Data Editor with Example Dam
The data required to perform a dam breach analysis are as follows: Inline Structure. This field is used to select the particular inline structure that you want to perform a breach analysis on. The user can enter breach data and perform a breach for more than one dam within the same model. Delete This Breach. This button is used to clear all of the dam breach information for the currently opened inline structure. Delete All Breaches. This button is used to delete the dam breach information for all of the inline structures in the model. Breach This Structure. This check box is used to turn the breaching option on and off without getting rid of the breach data. This box must be checked in order for the software to perform the dam breach. When this box is not checked, no breaching will be performed on this structure. Center Station. This field is used to enter the cross section stationing of the centerline of the breach. The stationing is based on the inline structure that is shown in the graphic. Final Bottom Width. This field is used to enter the bottom width of the breach when it has reached its maximum size. Final Bottom Elevation. This field is used to enter the bottom elevation of the breach when it has reached its maximum size.
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Chapter 16 Advanced Features for Unsteady Flow Routing Left Side Slope. This field is used to enter the left side slope for the trapezoid that will represent the final breach shape. If a zero is entered for both side slopes, the breach will be rectangular. Side slopes are entered in values representing the horizontal to vertical ratio. For example, a value of 2 represents 2 feet horizontally for every 1 foot vertically. Right Side Slope. This field is used to enter the right side slope for the trapezoid that will represent the final breach shape. If a zero is entered for both side slopes, the breach will be rectangular. Side slopes are entered in values representing the horizontal to vertical ratio. For example, a value of 2 represents 2 feet horizontally for every 1 foot vertically. Full Formation Time (hrs). This field is used to enter the time required for the breach to form. It represents the time from the initiation of the breach, until the breach has reached its full size. The modeler should be very careful in selecting this time. If a linear breach progression rate is selected, then the breach time should be limited to when the breach begins to significantly erode and up to when the major portion of the breach is formed. More information on the breach full formation time is provided later in this chapter. Failure Mode. This selection box contains two options for the failure mode of the breach, a Piping failure or an Overtopping failure. The overtopping failure mode should be selected when the water surface overtops the entire dam and erodes its way back through the embankment, or when flow going over the emergency spillway causes erosion that also works its way back through the embankment. The Piping failure mode should be selected when the dam fails due to seepage through the dam, which causes erosion, which in turn causes more flow to go through the dam, which causes even more erosion. A piping failure will grow slowly at first, but tends to pick up speed as the area of the opening begins to enlarge. At some point during the breach, the embankment above the breach will begin to sluff, at which time a large mass wasting of the embankment will occur. Piping Coefficient. This field is only used if the Piping failure mode has been selected. The user enters an orifice coefficient into this field. The orifice equation is used to calculate the flow through the breach opening while it is acting in a piping flow manner. Once the embankment above the opening sloughs, and the water is open to the atmosphere, the program transitions to a weir equation for computing the breach flow. Initial Piping Elev. This field is used to enter the elevation of the center of the piping failure when it first begins to occur. Trigger Failure At. This field is used to enter the mode in which the breach initiation will be triggered. There are three options available within HEC-RAS for initiating the start of the breach: a water surface elevation (WS Elev), a specific instance in time (Set Time), and a combination of exceeding a water surface elevation for a user specified duration (WS Elev + Duration). With the third option (WS Elev + Duration) the user enters a threshold water surface elevation to start monitoring the location. A duration is also entered. If the water surface remains above the threshold value for the user entered duration, then the breach is initiated. Additionally the user can enter a water surface elevation labeled “Immediate Initiation WS.” If the water surface
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Chapter 16 Advanced Features for Unsteady Flow Routing elevation gets up to or beyond this elevation, the breach is immediately initiated. Starting WS. This field is only used if the user has selected a trigger failure mode of water surface elevation (WS Elev). The user enters a water surface elevation into this field. The water surface represents the elevation at which the breach will begin to occur. Start Date. This field is only used if the user has selected the Set Time option for the failure trigger mode. The user enters the date at which the breach will begin to occur. The time of the breach initiation is entered into the next field. The date should be entered in a month/day/year format (ex. 05/23/2002). Start Time. This field is used to enter a starting time to initiate the breach. The time is entered as a military time (ex. 1800 for 6:00 p.m.). Breach Plot. When this tab is selected, a plot of the inline structure will show up in the graphic window. The plot will show the proposed breach maximum size and location in a red color. Breach Progression. When this tab is selected a table will appear in the graphic display window. The table is used to enter a user defined progression curve for the formation of the breach. This is an optional feature. If no curve is entered, the program automatically uses a linear breach progression rate. This means that the dimensions of the breach will grow in a linear manner during the time entered as the full breach formation time. Optionally, the user can enter a curve to represent the breach formation as it will occur during the breach development time. The curve is entered as Time Fraction vs. Breach Fraction. The Time Fraction is the decimal percentage of the full breach formation time. The breach fraction is the decimal percentage of the breach size. Both factors are entered as numbers between zero and one. An example of a user entered nonlinear breach progression rate is shown in Figure 16-7.
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-7. Dam Breach Editor with Nonlinear Breach Progression
Once all of the Dam Breach data are entered, press the OK button to have the data accepted. However, the data is not saved to the hard disk at this point. You must save the currently opened plan in order for the breach information to be save to the hard disk. Breach Repair (Optional). This option allows the user to have the breach fill back in during a simulation. This would most often be used for levee breaches, but could also be used for a dam breach if the user were running a long term simulation or if it was assumed that some effort would be put in place to fill a breach back in during a failure. When the Breach Repair tab is selected, the editor will appear as shown in Figure 16-8.
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-8. Dam Break Editor with Breach Repair Tab Active.
The Breach Repair Option requires the user to enter three pieces of information: Number of hours after full breach to start repair: This field is used to enter the amount of time (in hours) it takes to start the repair process after the breach has occurred. Total repair time (hours): This field is used to enter the total amount of time that it will take to perform the breach repair, in hours. Final filled in elevation: This field is used to enter the top elevation of the final repaired breach.
Estimating Dam Break Parameters The key parameters that must be estimated in any dam breaching analysis are the breach formation time and the maximum size of the breach opening. Several researchers have developed regression equations to estimate breach sizes and times from historical dam breach information. Additionally, a few researchers have tried to develop computer models to simulate the physical breach process. The bulk of the research in this area has been summarized in a 1998 publication entitled “Prediction of Embankment Dam Breach Parameters”, by Tony L. Wahl of the U.S. Bureau of Reclamation. Wahl documents the data from most of the historical dam breaches that have
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Chapter 16 Advanced Features for Unsteady Flow Routing occurred in the world, as well as describing the equations and modeling approaches developed for predicting the dam breach parameters. For the HEC-RAS software, the modeler must estimate the maximum breach dimensions and breach formation time outside of the program. Because the breaching process is complex, it is suggested that the modeler try to come up with several estimates of the breach parameters, and then put together a matrix of potential breach sizes and times. One example would be to use two different sets of regression equations and one of the breach simulation models to estimate the breach parameters. In several studies performed at HEC we have used both the Froelich (1995), MacDonald\Langridge-Monopolis (1984), and the Van Thun and Gillete (1990) regression equations, as well as the BREACH model by Dr. Danny Fread (Fread, 1988). All four methods give different answers for the breach dimensions, as well as the time for the breach to form. In general, a range of breach parameter estimates should be run as separate trials within HEC-RAS in order to test the sensitivity of the results to the breach dimensions and times. It is always good to test the sensitivity of the breaching parameters, since they are the most unknown factor in this process. Each of the breach parameter estimates will yield a different outflow hydrograph from HEC-RAS. However, once these hydrographs are routed downstream, they will tend to converge towards a common result. How close they get to each other will depend on the distance they are routed, the steepness of the stream, the roughness of the river and floodplain, and the amount of floodplain storage available for attenuating the hydrograph. If the populated areas below the dam are quite a distance away (say 20 miles or more), then the resulting hydrographs from the various dam breaches may be very similar in magnitude by the time they reach the area of interest. However, if the areas of interest are closer to the dam, then the resulting breach hydrographs could produce a significant range in results. In this situation, the selection of the breach parameters is even more crucial.
HEC-RAS Output for Dam Break Analyses Several plots and tables are available for evaluating the results of a dam break analysis within HEC-RAS. Graphics include cross section, profile, and 3 dimensional plots, all of which can be animated on a time step by time step basis in order to visualize the propagation of the flood wave. An example cross-section plot of a dam while it is breaching is shown in Figure 16-9. Additionally, the corresponding water surface profile for the same instance in time is shown in Figure 16-10. Hydrographs can be viewed at any location for which the user requested hydrograph output. Shown in Figure 16-11 is a series of hydrographs from the breach shown in the previous figures. These hydrographs represent the flow leaving the dam and then subsequent locations downstream as the flood wave moved through the river system.
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Figure 16-9. Example Plot of Dam While Breaching
Figure 16-10. Example Profile Plot of Dam Breaching
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Chapter 16 Advanced Features for Unsteady Flow Routing
BALD EAGLE LOC HAV FLOW PIPINGFAIL
800000
FLOW (CFS)
600000
400000
200000
2300 19Feb99
2400
0100
0200
0300 Time
0400 0500 20Feb1999
0600
0700
0800
Figure 16-11. Flow Hydrographs from Dam to Downstream Locations
Levee Overtopping and Breaching Levee overtopping and breaching can be analyzed within HEC-RAS by modeling the levee as a lateral structure. When modeling a levee with a lateral structure, the area behind the levee should not be included in the cross section data of the main river. The cross sections should stop at the bottom of the levee. The lateral structure (levee) can be connected to a storage area or another river reach. The strategy for modeling the area behind the levee will depend upon what will happen to the water if the levee overtops or breaches. If the water going over or through the levee will pond, then a storage area would be more appropriate for modeling the area behind the levee. If the water will continue to flow in the downstream direction, and possibly join back into the main river, then it would be more appropriate to model that area as a separate river reach. Shown in Figure 16-12 is an example schematic with a levee modeled as a lateral structure connected to a storage area to represent the area behind the levee. An example cross section with a lateral structure (levee) on the right hand side is shown in Figure 16-13.
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Figure 16-12. Schematic with Example Levee and Storage Area
The user defines the levee by entering a series of station and elevation points that represent the top of levee profile. This station and elevation data is then used as a weir profile for calculating the amount of water going over top of the levee. An example levee entered as a lateral structure is shown in Figure 16-14.
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Levee Breach Example RS = 5.39 .15
220
.2
.04
Plan: Unsteady with Lat
1:31:50 PM
Downstream of bridge embankment
.2
.15
.2 Legend WS 11FEB1999 0100
218
Crit 11FEB1999 0100 216
Lateral Structure
214
Bank Sta
Elevation (ft)
Ground
212 210 208 206 204 202
500
1000
1500
2000
Station (ft)
Figure 16-13. Example Cross-Section with Lateral Structure
Figure 16-14. Lateral Structure Editor with Levee Modeled as a Weir
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Chapter 16 Advanced Features for Unsteady Flow Routing In the example shown in Figure 16-14, the levee is connected to a storage area that will be used to represent the area behind the levee. As the levee overtops and/or breaches, the storage area will fill up until it reaches the same elevation as the water in the river. After the flood passes, the water in the storage area can pass back out any breach that may have occurred. The levee information is entered as station and elevation data in the Lateral Weir/Embankment editor shown on the Lateral Structure editor. The station elevation data represents the top of the levee. An example of this editor with levee data is shown in Figure 16-15.
Figure 16-15. Lateral Weir/Embankment Editor with Example Levee Data
As shown in Figure 16-15, the user enters the width of the levee (which is only used for drawing purposes); the head reference for weir flow calculations; the lateral weir coefficient; the distance that the upstream end of the levee is from the nearest upstream cross section; and the station and elevation data representing the top of levee. For more information on this editor, see Lateral Structures in Chapter 6 of this manual. Once the physical levee information is entered, the user can press the Breach button in order to bring up the levee breach editor. An example of the levee breach editor is shown in Figure 16-16.
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Figure 16-16. Levee Breach Editor with Example Levee and Breach
As shown in Figure 16-16, the information required to perform a levee breach is the same as performing a dam break. To get the details of each data field, please review the information found under the Dam Break section of this chapter. After all of the data are entered and the computations are performed, the user can view output for the lateral structure (levee). Plots such as the profile plot, lateral structure hydrographs, and storage area hydrographs, can be very helpful in understanding the output for a levee overtopping and/or breach. Shown in Figure 16- is an example profile plot with a levee breach. Shown in Figure 16-18 is a stage and flow hydrograph plot for the lateral structure. In this plot there are three stage lines and three flow lines. The stage lines represent; the stage in the river at the upstream end of the levee (Stage HW US); the stage in the river at the downstream end of the levee (Stage HW DS); and the stage in the storage area (Stage TW). The river is always considered to be the headwater, and the storage area is the tailwater. The flow lines on the plot represent: the flow in the river at the upstream end of the levee (Flow HW US); the flow in the river at the downstream end of the levee (Flow HW DS); and the flow leaving the river over the lateral weir to the storage area (Flow Leaving).
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Figure 16-17. Profile Plot with Levee Breach
Figure 16-18. Lateral Structure Stage and Flow Hydrographs
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Chapter 16 Advanced Features for Unsteady Flow Routing In addition to the profile plot and the lateral structure hydrographs, it is a good idea to plot the stage and flow hydrographs for the storage area. This allows the user to easily see the amount of flow coming into and out of the storage area, and the change in the water surface elevation. Shown in Figure 16-19 is the stage and flow hydrograph for the storage area in this example.
Figure 16-19. Stage and Flow Hydrograph Plot for Storage Area
Referring to Figure 16-18 and Figure 16-19, as the levee breaches, the flow going into the storage area and the stage increase quickly, while the stage and flow in the main river drop. In addition to the graphics in HEC-RAS, tabular results are also available. Shown in Figure 16-20 is a detailed output table for the lateral structure. The user can select a specific time line for viewing the output by selecting a specific profile. The profiles are labeled by the date and time they occurred in the model simulation.
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Figure 16-20. Detailed Tabular Output for Lateral Structure
Modeling Pump Stations Pump stations can be connected between storage areas; a storage area and a river; and between river reaches. HEC-RAS allows up to ten different pump groups at a pump station, and each pump group can have up to twenty identical pumps. Each pump can have its own on and off trigger elevation. To learn how to connect a pump, enter pump data, and use pump override rules, please review the section on pumps in Chapter 6 of this user’s manual. Pump stations can be used for many purposes, such as pumping water stored behind a levee (interior sump) into the main river. An example schematic of an interior ponding area behind a levee is shown in Figure 16-21. Note that the pump is connected from the storage area to a river station at the downstream end of the levee.
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-21. Example Pumping Station for Interior Ponding Area
In the example shown in Figure 16-21, a lateral structure was entered to represent the levee. This structure has a gravity draining culvert with a flap gate. The flap gate only allows water to drain from the storage area to the river. Additionally, a pump station is included to pump flows over the levee during a rainfall event. The pump station was drawn by selecting the Pump Station tool, then drawing a connection from the storage area to the cross section at river station 5.39. In this example, there is a hydrograph attached to the upstream end of the river reach, which represents the incoming flood wave to this reach. There is also a lateral inflow hydrograph attached to the storage area, which represents the local runoff collecting behind the levee. The pumps are used to pump water from the storage area, over the levee, to the river. The top of the levee is at elevation 220 feet. Therefore, the pump station is constantly pumping to a head of 220 feet. The Pump station data editor is shown in Figure 16-22.
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-22. Pump Station Data Editor with Example Data
The second tab on the editor brings up the Pump Group Data (Figure 1623). As shown is Figure 16-23, there is one pump group with three identical pumps (pumps are the same size and flow capacity). However, each of the pumps has a different on and off trigger elevation. The pump efficiency curve is used for all three of the pumps.
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-23. Pump Group Data on Pump Editor.
The third tab, Advanced Control Rules, allows the user to enter rules to override the normal pump station operations. When this tab is selected a window will appear as shown in Figure 16-24.
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-24. Advanced Control Rules editor for Pump Station.
A shown in Figure 16-24, two rules have been applied to this pump station. The first rule sets an absolute maximum pump flow of 800 cfs and a minimum of 1cfs for the entire pump station. This rule will always be applied. The second rule sets a maximum flow of 700 cfs to be applied only between 12 April 0000 and 24 April 0000, but only if the water surface at storage area Bayou is greater than 209. Also, the second part of this rule sets a minimum flow for the same time period, but only if the stage at storage area Bayou is less than 206.5. Details of how to use the rules can be found in chapter 6, under the section on Pumps. After the computations are performed, the user can view output for the pump station by selecting the stage and flow plotter, then selecting Pump Stations from the Type menu at the top of the window. An example stage and flow plot for the pump station is shown in Figure 16-25. As shown in the figure, the stage for the tailwater location (Stage TW) is a constant 220 ft. This is due to the fact that the pump is constantly pumping over the levee at elevation 220. The stage at the headwater location (stage HW) is the water surface elevations in the storage area. The storage area elevation starts out at an elevation of 205 ft., goes up to around 206.6, and then back down to around 205.1. The flow through the pumps was zero until an elevation of 206
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Chapter 16 Advanced Features for Unsteady Flow Routing was reached within the storage area, which triggered the first pump. The second pump turned on when the storage area got to elevation 206.2, and the third at elevation 206.5. On the falling side of the hydrograph the pumps began to turn off as the stage went down in the storage area. Shown in Figure 16-26 are the stage and net inflow to the storage area. The net inflow represents all the inflows minus the outflows at each time step.
Figure 16-25. Stage and Flow Hydrographs for Pump Station
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-26. Stage and Flow Hydrographs for the Storage Area
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Chapter 16 Advanced Features for Unsteady Flow Routing
Navigation Dams This section discusses the navigation dam option in HEC-RAS. For a navigation dam, the program will try to maintain both a minimum and maximum water surface at one or more locations along a navigation channel. The program does this by controlling the gate settings on an inline structure. The user enters a target water surface (and various other calibration data) and the program will adjust the gate settings at user specified time intervals in order to meet the target water surface as closely as possible. This section describes the data requirements for a navigation dam and includes a general discussion of how the gate operations are performed. The first step in modeling a navigation dam is to add the physical data for the navigation dam by selecting the Inline Structure option on the Geometry Data editor and entering the appropriate information. The next step is to add the inline weir as a boundary condition on the Unsteady Flow editor and then click the Navigation Dams button. The editor, as shown in Figure 16-27, will appear (note: the fields will be blank when the editor first appears).
Figure 16-27. Navigation Dam Editor with Flow Monitor
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Chapter 16 Advanced Features for Unsteady Flow Routing Normal gate change time increment – This field states how often the program will adjust the gate settings. In the example shown in Figure 16-27, the program will only make adjustments to the gates every six hours under normal operations. Rapidly varying flow change increment – This field represents the minimum length of time between gate setting adjustments. For example, during rapidly changing conditions, the program can adjust the gates up to once an hour in order to maintain the appropriate water surfaces. Initial gate change time – This field is the time (military style) for when the first gate change will take place. In this example, it is 10:00am. If the simulation starts after 10:00am then the gates will be first adjusted at 4:00pm, 10:00pm, or 4:00am as appropriate. Gate minimum opening – This field is the minimum opening for the first gate group (the first gate group as defined on the Geometry editor). The program will keep the gates on this gate group open to at least 0.1 feet. The other gate groups may be closed completely (see discussion of gate opening and closing below). Gate maximum opening – This field is the maximum opening for the first gate group (the first gate group as defined on the Geometry editor). The program will not allow the gates on this gate group to open more than the specified value. If this field is left blank, then the default is the physical gate maximum opening from the geometric data. The final two fields [Gate opening and Gate closing rate] are the maximum speed that the gates in any gate group can be opened or closed. Generally this rate is determined by the physical speed with which the gates can be adjusted. Sometimes, however, opening or closing the gates too quickly can cause instability in the unsteady solver. In this case, it may be necessary to reduce the opening or closing rate. A shorter time step may also help.
Pool Only Control There are several types of navigation dam operations. The simplest is pool only control (as shown in Figure 16-27). In this case, the program tries to maintain the water surface immediately upstream of the dam within user specified targets. In the other operations (see below), the target water surface is located some distance upstream of the dam and there may or may not be limits on the water surface right at the dam. In order to keep the water surface at the dam within the user specified limits, while only infrequently changing the gate settings (i.e., every six hours), the program needs to know what the approximate inflow at the dam will be some time into the future. This is done by monitoring the flow at an upstream cross section. The user must enter this location. In this example (Figure 1627), the Flow Monitor tab has been activated and the flow monitor location has been entered as river station 315.5. The flow monitor location should be chosen so that the river travel time between the monitor location and the navigation dam is on the order of (or somewhat less than) the normal gate
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Chapter 16 Advanced Features for Unsteady Flow Routing increment. In this example (Figure 16-27) the gate time increment is every six hours, so a location a few hours upstream would be appropriate. The calibration of the navigation dam control data involves some empirical decisions and trial and error experimentation. This is true of the flow monitor location as well as most of the remaining data explained below. The flow monitor location must be a normal cross section in the model. This means that cross sections must be extended far enough upstream of the dam to account for this location. Note also that the monitor point can be located upstream of other hydraulic structures, including other navigation dams. As long as another upstream navigation dam does not have a significant storage capacity, it should not affect the results of the flow monitor. After the flow monitor location has been chosen, the Pool Control tab can be pressed bringing up the editor shown in Figure 16-28.
Figure 16-28. Navigation Dam Editor with Pool Control
The user enters a range of water surfaces and corresponding Flow Factors. In this example, the ideal target water surface has been entered as 459.35. The primary target range is from 459.2 (Target Low) to 459.5 (Target High). In general, if the water surface is between Target Low and Target High and it is
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Chapter 16 Advanced Features for Unsteady Flow Routing time to change the gate settings, then the program will adjust the gates to get an average of the current flow at the dam and the monitor flow. For instance, assume that at time 10:00 the current discharge from the navigation dam is 10,000 cfs, 11,000 cfs of flow is observed at the monitor location, and the water surface at the dam is 459.4 feet. Since 459.4 is in the primary target range, the program will compute the average of the flows, 10,500 cfs. By trial and error, the program will change the gates (and compute the corresponding flow) until there is 10,500 cfs (plus or minus the tolerance) of discharge at the dam. The tolerance is 1% of the flow, in this case 105 cfs. So the program will actually stop iterating whenever it first determines a gate setting that results in a flow that is between 10395 cfs and 10605 cfs. After the gates have been changed, they won’t (normally) be adjusted for the next six hours. The flow from the dam willl vary as the water surface at the dam fluctuates. As the water surface at the dam gets out of the primary target range, then the flow (that is, the discharge from the dam) is adjusted by the Flow Factors. In general, when the stage is between Target High and Maximum, then the flow is multiplied by Flow Factor Target High (in this case 1.03). Between Maximum and Maximum High, it is multiplied by at least 1.07. Between Maximum High and water surface Open River, the flow is rapidly increased up to at least Flow Open River (listed as 50,000). Flow Open River does not represent a cap. If the flow at the monitor location gets high enough, the discharge at the dam can go above Flow Open River based on the Flow Factors. Above water surface Open River, all the gates are opened all of the way. The operations below the target zone work the same way. Flow Factor Target Low and Flow Factor Minimum are applied in the same way. Between Minimum Low and water surface Close Gates, the flow will be rapidly decreased to Flow Minimum, but again, this is not an absolute minimum. If the water surface remains low enough, the program will continue to close the gates and reduce flow. The only absolute minimum is that the program will not close the first gate group below the gate minimum opening. The water surface targets are basically calibration knobs and no particular water surface targets have to exactly match the operationally prescribed limits on the pool surface. However, the best response will probably be obtained if the Maximum and Minimum are close to the prescribed limits.
Hinge Point Only Control The next type of navigation dam operation is hinge point control. This is similar to pool control. The main difference is that instead of the water surface targets being located right at the face of the dam, the water surface targets are located some distance upstream. Figure 16-29 shows the Hinge Point Only editor. (Hinge point control is selected by clicking on the drop down box near the top right of the editor.) In this example, the navigation dam is located at river station 714.35, and the hinge point is located at river station 728.28. The program will adjust the
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Chapter 16 Advanced Features for Unsteady Flow Routing gates at the dam in order to maintain an approximate water surface of 645.55 feet (the target water surface) at river station 728.28. The target water surfaces and Flow Factors behave the same as for pool control. A flow monitor location is still needed. It should be located an appropriate distance upstream of the hinge point. For this dam, it is located a few hours upstream at river station 750.1. The Steady Profile Limits Table is an optional feature (see Figure 16-29). It can make the navigation dam operations more robust for rapidly changing flow. It addresses the situation where the water surface for a given flow at the dam diverges significantly from the water surface that would be expected at the dam for a steady state, uniform flow condition. A typical example is the trailing end of a high flow hydrograph. For instance, the flows at the hinge point and monitor location may have fallen considerably below the Open River condition, but the water surface at the dam is still a little high (compared to the flow). When the program computes a desired flow at the dam, e.g. 10,000 cfs, it adjusts the gates to get this flow. Over the next six hours, however, as the water surface at the dam continues to fall toward a lower equilibrium, the discharge can drop significantly below 10,000. This means that the navigation dam response is either sluggish in returning to the target water surface at the hinge point and/or the gates have to be changed more frequently. This is where the table becomes useful. The data in this table give the water surfaces at the dam that will produce the target water surfaces at the hinge point for steady state conditions. For this navigation dam, it is desired to keep the water surface at the hinge point between 645.35 and 645.65 feet (Minimum and Maximum values from the water surface elevations table). If, for instance, there is a long term (steady state) flow of 10,000 cfs between the hinge point and the dam, then maintaining a water surface at the dam of 645.19 feet will result in a water surface of 645.35 feet at the hinge point. Similarly, a water surface of 645.59 at the dam will result in a water surface of 645.65 feet at the hinge point, for the same 10,000 cfs flow. The user can generate these profile limits by putting together a steady flow run from the dam up to the hinge point location. An iterative process of forcing elevations at the dam and computing them at the hinge point is required. The user must find the elevations at the dam that will get to the high and low hinge point elevations from a steady flow backwater computation. Then the values used at the dam to produce the max and min at the hinge point should be entered into this table.
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-29. Navigation Dam Editor with Hinge Control
Continuing on with the 10,000 cfs flow example, before the program starts to iterate, it checks the current water surface at the dam against the table. If the current water surface is between the limits (in this case 645.19 and 645.59), the program continues normally as it would if the table was not being used (that is, the user had left it blank). However, let’s assume that the water surface at the dam is 646.0 feet. This would mean that the water surface at the dam is above the limits. In this case the program will temporarily assume a headwater of 645.59 feet at the dam and determine the gate settings that will result in a discharge of 10,000 cfs for this lower, assumed, headwater. After this has been done, the program will use the new gate setting and continue on normally. This gate change will result in the flow at the dam initially being above the 10,000 cfs target. However, as the water surface at the dam drops, the flow should also drop down towards the 10,000 cfs range. This will, hopefully, produce a faster response without over shooting the target water surface at the dam. If the water surface is on the low side, it works the same way except the lower limit is used. If the water surface at the dam were 645.0 then the gate setting would be based on an assumed water surface of 645.19. The profile table is optional and can be left blank. However, it can produce a better response, at least for some data sets. That being said, it should also be
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Chapter 16 Advanced Features for Unsteady Flow Routing noted that the table will not perform as well when the flow at the dam is being heavily influenced by tailwater conditions.
Hinge Point and Minimum Pool Operations The hinge point navigation dam operation can also be combined with limits on the water surface at the dam. Hinge point and minimum pool operation will try to maintain the water surface within targets at the hinge point, but only when the water surface at the dam is above certain limits. When the water surface at the dam drops too low, the program will adjust the gates based on the water surface at the dam, essentially reverting to pool only control. The hinge point and the minimum pool operation are each treated as separate control points. In addition to the water surfaces and Flow Factors for the Hinge control, the pool minimum has its own full set of water surfaces and Flow Factors as shown in Figure 16-30 (these are accessed by clicking on the Min Pool Control button). Even though the minimum pool control is only trying to maintain a minimum water surface at the dam, a full range of water surfaces and Flow Factors are needed. These include the “too high” numbers such as Maximum High and Flow Open River. This allows the program to smoothly transition between hinge control and pool control. It also allows the pool control response to be fully calibrated between sluggish and overly sensitive transitions. For hinge and minimum pool navigation dams, the program independently determines a desired flow for each control point (that is, the hinge and the pool minimum). It will then take the lower of the two flows and use that for determining the gate settings. For example, assume the flows at the monitor location and the hinge point are 40,000cfs and that the water surfaces at the hinge point and the dam are 645.6 and 644.9 respectively. Based on the hinge point conditions (water surface at hinge point, Targets and Flow Factors for the hinge point), the program might compute a desired flow of 41,000 cfs. Next, the program will look at the conditions, targets, and Flow Factors at the dam and compute a desired flow of, perhaps, 42,000 cfs. Since the desired flow for the hinge point targets is lower than the desired flow for the navigation dam targets, the pool minimum is not a limiting factor. The program will adjust the gate settings to get 41,000 cfs and the navigation dam is operating under hinge control.
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-30. Navigation Editor with Hinge Point and Minimum Pool Operations and Control
The next time the gates are adjusted, assume the flow at the monitor and hinge point are still basically 40,000 cfs, but that the water surfaces have dropped to 645.5 feet at the hinge and 644.4 feet at the dam. The new computed flows might be 40,000 cfs at the hinge and 39,000 at the dam. In this case the program would use the 39,000 cfs figure and the dam would be under pool minimum control. In other words, the water surface at the dam has dropped to the point that the program has to operate the gates to maintain a minimum water surface at the dam regardless of what is happening at the hinge point. The hinge and pool minimum operation is usually under hinge control for low and normal flows. At high flows the water surface at the dam must be lowered in order to keep the hinge point within the target range. At even higher flows, the water surface at the dam cannot be lowered far enough to keep the hinge point in range, thus the dam reverts to pool minimum control. Ideally, the pool would be kept at the specified absolute minimum (perhaps 644.1 feet in the above example) until the hinge point dropped back down into the target range. This is not possible without continuous adjustments of the gates, which is not practicable. Instead, the water surface at the dam will fluctuate slightly even when it is operating under pool minimum control (just like it would fluctuate for pool
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Chapter 16 Advanced Features for Unsteady Flow Routing only control). This is reflected in the range of target water surfaces for pool minimum control. The spacing of the target water surfaces has to be determined by trial and error. For example, if the water surface Target, Target High, and so on, are set to relatively high elevations (compared to the desired value), then the water surface at the dam might stay significantly above the minimum of 644.1. This is not desired when the water surface at the hinge point is above the targeted range. Moving the dam target water surfaces closer together (closer to 644.1) will cause the program to increase the flows more quickly in order to drive the water surface back down. However, this can also cause the program to overshoot the desired target leading to frequent gate changes and/or bouncing water surfaces. If the pool minimum is a hard minimum (a hard minimum might be, the pool should not be allowed to drop below 644.1 feet), then this minimum should be coded as one of the lower target water surfaces. For example, if 644.1 is the operationally prescribed absolute minimum and the user coded the primary water surface Target as 644.1, then the pool would fluctuate around the value of 644.1 during pool control. It would be better, in this case, to code it to the Minimum Low. On the other hand, if the minimum is a “soft” minimum (a soft minimum might be 644.45 +/- .25 feet) then setting Target Low or even perhaps the primary Target to 644.45 might give better results. As already mentioned, the user should be prepared to take a trial and error approach in order to get the best results. For hinge point and minimum pool operation, the Steady Profile Limits table can still be optionally used. This table is only used when the dam is operating under hinge control. The water surface values in the table can be lower elevations than the actual limits on the pool. These values are still used, but the pool control minimum will still apply. For example, the values in the table go below the 644.1 desired minimum at the pool. During rapidly changing conditions, when the water surface for a given flow diverges from the steady state water surface (for that flow), these lower values can still be used and will (in some cases) give a faster response. However, if the water surface actually drops down to around the 644 to 645 level, the flow based on pool control will eventually be lower than that based on the Hinge/Steady Profile table and the dam will revert to pool control (which, again, does not use the tables).
Hinge Point and Minimum and Maximum Pool Control The final type of navigation dam operations is combining hinge point control with both a minimum and maximum limit on the water surface at the dam. This editor has a third button as shown in Figure 16-31 and Figure 16-32. The minimum and maximum pool controls are treated as separate control points even though they are both located immediately upstream of the dam. They each have a full set of target water surfaces and Flow Factors. The program will compute a desired flow for each control point. Therefore, there will be a flow based on the hinge point targets, a flow based on the minimum pool elevation, and a flow based on the maximum pool elevation. During normal operations, the flow will be based on the hinge point target. However,
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Chapter 16 Advanced Features for Unsteady Flow Routing the desired flow will not be allowed to go below the minimum pool control flow and it will not be allowed to go above the maximum pool control flow. Having separate control points for the minimum and maximum control allows a smooth transition between pool control (either high or low) and hinge control for a full range of flows. It also provides the greatest control and sensitivity for allowing the water surface at the pool to be maintained within the tightest tolerances. The optional steady profile limits table may still be used. As before, it only applies to hinge control.
Figure 16-31. Navigation Editor with Hinge and Maximum and Minimum Control (Min Pool Control Shown)
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-32. Navigation Editor with Hinge and Minimum and Maximum Control (Max Pool Control Shown)
Modeling Pressurized Pipe Flow
HEC-RAS can be used to model pressurized pipe flow during unsteady flow calculations. This is accomplished by using the Priessmann slot theory applied to the open channel flow equations. To model pressure flow with HEC-RAS, the user must use cross sections with a Lid option. The cross section is entered as the bottom half of the pipe and the Lid is entered as the top half of the pipe. Any shape pipe can be modeled, however, the details of the pipe shape will depend on how many points the user puts in for the bottom (cross section) and the top (Lid). An example of adding a lid to a cross section is shown in Figure 16-33.
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-33. Cross Section with the Lid Option entered.
To enter a Lid at a cross section, select Add a Lid to XS from the Options menu on the Cross Section editor. When this option is selected, a window will appear as shown in Figure 16-34.
Figure 16-34. Cross Section Lid Editor
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Chapter 16 Advanced Features for Unsteady Flow Routing Additionally, the user must instruct the program to use the Priessmann Slot option for that particular cross section. The Priessmann Slot option can be turned on for an individual cross section from the Cross Section Lid Editor by checking the box at the top of the editor. The user can also bring up a table that will show all of the locations where cross sections with lids exist. This table can be viewed by selecting Priessmann’s Slot on Lidded XS’s from the Tables menu on the Geometric Data Editor. When this option is selected, a window will appear as shown in Figure 16-35.
Figure 16-35. Priessmann Slot Table for Cross Sections with Lid's.
The Priessmann Slot table will show all cross section locations that contain lids. The user can turn on or off the Priessmann slot option by simply checking the box next to the desired cross section location. All of the check boxes can be turned on or off simultaneously by clicking on the Add Priessmann Slot column heading at the top of the table. In general, lids can be added to any cross section in the HEC-RAS model. Several cross sections in succession with lids can be used to represent a pipe. Multiple interconnected pipes can be modeled. Lidded cross sections can be used around stream junctions to represent pressurized junctions. However, HEC-RAS does not compute minor losses at junctions, bends, or where pipes
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Chapter 16 Advanced Features for Unsteady Flow Routing change size. This is currently a limitation in modeling pressurized pipe flow with HEC-RAS. Lateral flows can be modeled by either using lateral structures with culverts, or by directly inputting hydrographs as lateral flow boundary conditions. The lateral structure option can be used to mimic drop inlets connecting the surface flow to the pipe. An example of a pressurized pipe with lateral structures connected to the surface is shown in Figure 16-36.
Figure 16-36. Example Pressurized Pipe modeled with Cross Sections and Lids.
For the computational details of how the Priessmann slot option works, please see the section on modeling pressurized pipes in the Hydraulic Reference Manual.
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Chapter 16 Advanced Features for Unsteady Flow Routing
User Defined Rules for Hydraulic Structures The operating procedures for determining and controlling the releases from reservoirs and other types of hydraulic structures can be quite complex. HEC-RAS allows flexibility in modeling and controlling the operations of hydraulic structures through the use of rules (Figure 16-37).
Figure 16-37. Selecting Rules from the Unsteady Flow Data Editor
The rules can be used to operate the height of the gate openings. Alternately, the rules can directly control (or constrain) the flow despite the gate openings (or even without gates at all). Examples of variables that could be used to control releases from a hydraulic structure are: current flows and water surfaces at the structure, current flows and stages at downstream or upstream cross section locations, time considerations (winter,
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Chapter 16 Advanced Features for Unsteady Flow Routing morning, etc), and/or previously computed values (accumulated outflows, running averages, etc). These variables can be combined with math operations and conditional operations to produce sophisticated controls. Rule operations in HEC-RAS are available for inline hydraulic structures, lateral hydraulic structures, and storage area connections.
Entering Rule Operations Rules for controlling hydraulic structures can be entered after an inline structure, lateral structure, or storage area connection has been added to the project. From the Unsteady Flow Data editor, add or select the given structure and then click on the Rules button (figure 16-37). This will bring up the Rule Operations editor as shown in figure 16-38. In the Gate Parameters table near the top of the editor, some initial information can be entered for any gate groups that are in the hydraulic structure.
Figure 16-38. Gate Rule Operations Editor
The Open and Close Rate controls how fast the gates can move. So if, for example, a rule operation required the gate to open one additional foot and the gate opening rate was 0.1 ft/min and the user had selected a one minute time step, it would take ten time steps for the gate to reach the new opening height. The Open and/or Close Rate can be left blank, which means the gate can move to any new setting in a single time step. The Max and Min Opening will constrain the maximum and minimum gate opening settings. Building on the above example of opening the gate one additional foot, if the gate was at 3.5 feet and the maximum was set to 4 feet (even though the gate was 6 feet tall), over a five minute period, the gate would open to 4 feet and then stop. If the Max is left blank, then the gate maximum opening is limited only by the height of the gate. If the Min is left blank, then the minimum opening is fully closed (i.e. 0.0).
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Chapter 16 Advanced Features for Unsteady Flow Routing The Initial Opening provides the first setting for the gate. This opening height will be used during the initial backwater computation. The gate will be left at this setting until it is changed by a rule operation. The Initial Opening is required for all gate groups, if any, in the hydraulic structure and may not be left blank. At the top of the editor, the user has the option of entering a description of the rule set. This can be a useful tool for documentation especially if the user has multiple plans with different rule operations. The Summary of Variable Initializations is described below.
Rule Sets A group of rules for one hydraulic structure is referred to as a rule set. At the start of each time step, each rule set is evaluated to check for changes to the operation of the given hydraulic structure. Rule operations are performed from the first (top) rule to the last (bottom) rule. By default, each rule operation is evaluated once. However, branching operations (If/Then/Else, etc) can cause some rule operations to be skipped. No looping or jumping to prior rule operations is allowed. That is (during a given time step), a rule operation may not be performed more than once. Note: A rule set is only called once during a time step, even if the program iterates during that given time step. (Whatever rules are “in force” at the start of the time step will apply during all of the iterations). At some point, a user option may be added to RAS to let the rule set be called for every iteration. This would allow the rule set to use a more “current” water surface and/or respond to stability problems. When the Rule Operations editor is opened, the rules for that hydraulic structure are displayed beneath the Rule Operations heading. The rule set shown in figure 16-38 has 7 types of operations. In this example, operation #1 gets the current gate opening. Operation #2 gets the current flow going through the gate. Operation #3 checks if the flow is less than 500 cfs. If it is, then operation #4 sets the gate opening to the current opening + 0.1 feet. After operation #4, control would jump to after the End If (operation #7). However, since there are no more operations after the End If, the rule set would be done for this time step. If the flow is greater than or equal to 500 cfs, then operation #3 is false. In this case, control would jump to operation #5. Operation #5 checks if the flow is greater than 750 cfs. If it is, then operation #6 will close the gate 0.1 feet. In either case, the rule set would again be finished for this time step.
Operation Rules To add, delete, or edit rule operations, click on the Enter/Edit Rule Operations… button at the bottom of the Rule Operations editor. This will bring up the Operation Rules editor as shown in figure 16-39.
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Chapter 16 Advanced Features for Unsteady Flow Routing
Figure 16-39. Operation Rules Editor
Seven different types of operations can be added by clicking one of the buttons under the Insert New Operation field. A brief overview is given immediately below and this is followed by a detailed description of each rule operation type.
Operation Types:
Comment. Provides a user entered line of text (for documentation only).
New Variable. Allows the user to create a variable and give it a custom name.
Get Simulation Value. A variable is set equal to a given value in the simulation, such as the flow at a cross section or the time of day.
Set Operation Parameter. Changes the operation of the hydraulic structure, for example, adjusting the gate height or setting a maximum discharge.
Branch (If/Else). Controls which operations are executed on the basis of an If-Then test (e.g., do different gate operation checks based on seasonal considerations).
Math. Performs math operations such as summing flows or averaging water surfaces.
Table. This operation allows the user to enter a table and perform table lookups to get a value.
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Comment: Clicking the Comment button allows the user to enter a line of text. This “operation” is not used during the computations. Rather, it is intended to make the rule set operating procedure easier to understand by allowing the user to document the rules inside of the rule set (see figure 16-40). Note: because RAS uses a comma as an internal delimiter, it will not allow a “,” to be part of a comment line.
Figure 16-40. Operation Rules Editor with comment line shown
New Variable: The New Variable button brings up the editor as shown in figure 16-41. The name of the variable must be entered in the User Variable Name field. The name must be unique. That is, it can’t be the same as any other variable name in the given rule set. A duplicate name will cause a run time check error, as discussed below. By default, the variable type is real (which includes fractional numbers such as 11.35). The alternative type is integer (counting numbers such as -2, 0, 1, 5, 10, etc). If the user selects integer, the value of the variable will always be an integer. So if the current value of a user integer is 4 and a math operation (see below) adds 1.7 to it, the final value will be rounded to the nearest integer (in this case 6). The user may enter an initial value for the variable (by default the value is zero). The variable is only initialized to this value at the start of the simulation. It will equal this value until (or unless) it is changed by another
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Chapter 16 Advanced Features for Unsteady Flow Routing rule. For example, if the user variable, “Test Case” has an initial value of 3 and at the start of the fourth time step it is changed (by another rule) to a value of 6. At the fifth time step, it will equal 6 (it is not “reinitialized” to 3) and will continue to equal 6 until/unless it is changed again.
Figure 16-41. Operation Rules Editor with New Variable operation shown
If the user checks the optional “Include Variable in Summary,” then the variable will be listed on the main Rule Operation editor as shown in figure 16-42. The initial value can then be entered or changed directly on the Rule Operations editor.
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Figure 16-42. Rule Operations Editor with Summary of Variable Initializations
Get Simulation Value: The Get Simulation Value operation provides information about the current state of the model. In the example shown in figure 16-43, the operation is getting the day of the month at the beginning of the time step and putting it into a new variable called “Day Beg time step.” For example, if the simulation time window went from 01Jan2000 to 03Jan2000 and the run was about halfway through, the “Day Beg time step” variable would be set to 2. This is another way to create a “new variable” (a variable does not have to be created with the New Variable button). However, variables created in this manner cannot be integers (they may only be real types), they cannot be assigned an initial value (or rather, the initial value is always zero), and they cannot be included in the Variable Summary.
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Figure 16-43. Operation Rules Editor with Get Simulation operation shown
If the user selected to change the Assign Result to Existing Variable, then a drop down menu would appear as shown in figure 16-44. Selecting one of the previously defined variables would put the result (day of the month in this example) into that variable instead of creating a new one.
Figure 16-44. Get Simulation Value assigning to an existing variable
Note: on renaming New/Existing variables. A new variable (whether it is from a user variable, get simulation, math, or table operation) can be renamed by typing in a new name. However, any references to that variable will not be automatically renamed! A reference to a nonexistent variable will result in a run time check error. The user will have to manually change all references to that variable (whether on the Assign Result
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Chapter 16 Advanced Features for Unsteady Flow Routing to an Existing Variable or using an Existing Variable in an Expression, see below). This is also covered in the discussion of Check Rule Set, below. There are currently seven categories of simulation variables (more may be added later). These are Time, Solution, Cross Section, Inline Structure, Lateral Structure, Storage Areas, and Storage Area Connectors. Clicking on the “+” will expand the list for that category. A complete list and definition of each variable is given at the end of this section of the manual. For all the variables under Time, the user can select to use the time at the beginning of the time step (default), the end of the time step, or the previous time step. For example, assume the time step was 30 minutes long and the program had just finished the time step that ended at 12:15 (the program had just gone from 11:45 to 12:15 and was getting ready to go from 12:15 to 12:45). The minute of the hour at the beginning of time step would be 15. The hour of the day (fractional) for the beginning of the time step, end of time step, and previous time step would be 12.25, 12.75, and 11.75, respectively. The hour of the day at both the beginning and end of the time step would be 12. The previous hour of the day would be 11. To get a water surface or flow at a normal cross section, expand the Cross Section list and highlight either the Flow or WS Elevation field as shown in figure 16-45. This will also display the standard node selector to allow the user to select the river, reach and river station for the desired cross section (also shown in figure 16-45).
Figure 16-45. Selecting water surface at a cross section
At the far right is another drop down menu that allows additional choices for when and how the simulation value (water surface in this example) is computed, see figure 16-46. The default is the Value at the current time step. In the time example above, this would be the water surface at 12:15
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Chapter 16 Advanced Features for Unsteady Flow Routing for river station 73.75. Alternately, the user could select the Value at previous time step, which would be the water surface at 11:45 in the above example.
Figure 16-46. Changing how simulation values are computed
The remaining choices provide various options for the value further back in time. Selecting Value at specified lag, will display a window where the lag time in hours is specified. So continuing to build on the same example, a lag time of 1.5 hours would get the water surface at the given cross section at 10:45 (12:15 minus 1.5 hours equals 10:45). For the next four options, the user must specify a starting and ending lookback time. Selecting Sum over previous time window, the user could enter 1.5 for the starting time and 0 for the ending time (for the specified time window to end at the current time step, the ending time should be 0). So if the user selected flow, this would sum the flow for the previous 1.5 hours. In other words, it would return the volume of water passing the given node for the previous 1.5 hours (since the value takes into account the length of each time step, this is more technically an integral instead of a sum). Or the user could select water surface, select Average over the previous time window, enter 2 hours for starting time and 1 hour for ending time, see figure 16-47. This would return the average water surface at the cross section between 10:15 and 11:15. The next two options will return either the maximum or minimum value over the given user specified time window (e.g. the highest water surface).
Figure 16-47. One hour average water surface starting two hours ago
Under the inline structure simulation variables as shown in figure 16-48, some of the choices are shown in bold font. The variables shown in bold are user settable operational parameters (Figure 16-49) for the current hydraulic structure (which happens to be an inline structure). These variables are also provided under the Get Simulation Value. This provides a way to check what an operational parameter has been set to (if it has been set at all). So, for instance, if a maximum flow had been set for the structure, then the Structure – Flow Maximum variable would return the value that this had been set to. The variables listed in bold are only available for the current
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Chapter 16 Advanced Features for Unsteady Flow Routing inline structure—this is the hydraulic structure that this particular rule set is attached to.)
Figure 16-48. Getting an Operational Parameter
Set Operational Parameter: Clicking the Set Operational Param button brings up the editor that allows a change to be made to the hydraulic structure operations (i.e. adjusting a gate opening). Changes can only be made for the hydraulic structure that the rule set is attached to. This is the hydraulic structure (inline, lateral, or storage area connection) that was selected on the Unsteady Flow Data editor (figure 16-37).
Figure 16-49. Setting an Operational Parameter
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Chapter 16 Advanced Features for Unsteady Flow Routing Figure 16-49 shows a change being made to the gate opening height (the gate will be moved to the new gate setting based on the Opening/Closing rate and maximum and minimum values, if any). If a gate variable is used, the appropriate gate group must be selected from the drop menu at the bottom. The new value is set equal to the value of the expression. (The expression might be a constant, such as the number “5”, a user variable as shown in figure 16-49, or a simple math operation. See Math operations, below, for a detailed description of using expressions). The Opening Rate or Closing Rate changes the rate at which the gate can open or close. This overrides any value that the user may have entered on the Rule Operations editor (figure 16-37). Gate - Flow (Fixed) sets the gate flow to the given value. However, setting or changing the Flow (Fixed) value does not affect the gate opening height (the given amount of flow will be released through this gate group from the reservoir regardless of the gate opening height). The Fixed flow value will be used each time step until the Fixed flow is changed or removed. To remove the Gate - Flow (Fixed) parameter, set the Flow (Fixed) expression to “Not Set” as shown in figure 16-50 (see Math below for editing expressions).
Figure 16-50. Turning off the Gate Fixed Flow
Gate - Flow Maximum and Minimum puts limits on the gate flow. For example, if the user sets the flow maximum to 1000 cfs, the program would first compute the flow through the given gate based on the gate opening and water surface(s), if the flow is below 1000, it would use that value. If the computed flow was larger, the program would restrict the flow to the user entered amount (1000 cfs). Note, however, that the Gate Maximum and Minimum flow will not override a Gate Fixed flow value. Gate Maximum and Minimum flow can be removed by using the “Not Set” value. Gate - Flow (Desired) will cause the program to adjust the gate opening in order to give the given, desired, flow (based on the water surfaces and the gate characteristics) through the gate group. Once the gate opening is determined, the program will use this opening height to compute the actual flow. Since determining the gate opening is an inexact, iterative process, the actual computed flow may not perfectly match the desired flow. Note also that the program will not open/close the gates faster than the current Opening/Closing Rate, if any, allows. The program will adjust the gate opening each time step as long as the Gate Flow Desired has a value. This feature can be turned back off by using the “Not Set” value. Having the Desired value on will not prevent the final gate flow from being overwritten by
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Chapter 16 Advanced Features for Unsteady Flow Routing a Maximum, Minimum, or Fixed flow. If the user wishes to force a given gate flow, but also wishes to know the [approximate] gate settings that would result in that flow, then this can be done by setting both the Fixed flow and the Desired flow to the same value (e.g. 3000 cfs). The rule set shown in figure 16-51 illustrates how these gate features can be used and combined. The resulting output is shown in figure 16-52.
Figure 16-51. Desired and Fixed Flow Gate Operations
The initial gate opening is 6.0 feet. The If/Then test on row #2 is false until the time reaches 1:15 (this is 1.25 in fractional hours). At that point, row #4 is executed and the Desired Flow is set to 600 cfs. The gates open (at the user set opening rate) until the flow is approximately 600 cfs. After 1:30, the Desired Flow is turned off (“not set”). The gates then remain at that, current, gate height (6.1295 feet). At 1:45 row #8 is executed and the flow is fixed at 700 cfs (but the gate opening height is not changed). At 2:30, the Desired Flow is turned back on by setting it to 700 cfs (same as the fixed flow). This causes the gates to adjust. However, the actual release remains exactly 700 cfs because the Fixed Flow is still set to 700.
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Figure 16-52. Output from Desired and Fixed Flow Gate Operations
In addition to the gate control operations for individual gate groups, the user can set limits on all of the gates groups combined: Structure - Total Gate Flow sets the flow for all of the gate groups. Instead of summing the flow from each gate group (and regardless of whether the gate group flow is “natural” or “fixed”), this flow is used instead. Structure - Total Gate Flow Maximum sets a maximum flow for all of the gate groups. It will not override Structure - Total Gate Flow. Structure - Total Gate Flow Minimum sets a minimum flow for all of the gate groups. It will not override Structure - Total Gate Flow. The next category of structure operation parameters are for weirs: Weir – Flow fixes the amount of flow over the weir. Weir - Flow Maximum sets the maximum flow over the weir. It will not override Weir – Flow.
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Chapter 16 Advanced Features for Unsteady Flow Routing Weir - Flow Minimum sets the minimum flow over the weir. It will not override Weir – Flow. Weir - Weir Coefficient sets the weir coefficient for the weir. (Tip: this allows a straight forward way to adjust the weir coefficient based on the depth and/or velocity of flow over the weir). Weir - Minimum Elev for Weir Flow changes the minimum weir elevation that is required before the program will compute flow for the weir. Weir - C Simple (Positive) sets the linear routing coefficient for positive flow (linear routing weirs only). Weir - C Simple (Negative) sets the linear routing coefficient for negative flow (linear routing weirs only). The final category of structure operation parameters are for the overall structure: Structure - Total Flow (Fixed) forces the given flow for the inline structure. This flow is used regardless of the flow from the gates and/or weir. Structure - Flow Maximum sets a maximum flow for the inline structure. It will not override the structure Fixed flow. Structure - Flow Minimum sets a minimum flow for the inline structure. It will not override the structure Fixed flow. Structure - Flow Additional will add in the additional given flow to the inline structure. It will not override the structure Maximum, Minimum, or Fixed flow. Structure - Total Flow (Desired) computes gate settings to provide the total given flow for the inline structure. It works in a similar manner to Gate Flow (Desired). However, it will open or close any/all of the gate groups to get the correct flow. (To increase flow, gate groups are opened in a left to right manner. To decrease flow, gate groups are closed from right to left.) Weir flow (and Flow Additional) is included in the desired flow (if the desired flow is 2000 cfs and the weir flow is 500 cfs, the gates will be adjusted to get 1500 cfs of flow). Structure - Total Flow (Desired) will not override Structure - Total Flow (Fixed). However, it will still adjust the gate group settings. Branch(If/Else). The branching operation allows for decision making based on the value of two (or four) expressions. Figure 16-53 shows a simple example. If the gate flow is less than 500 cfs, then the program will go from row #4 to the next operation at row #5. Otherwise, it will skip down to the first row after the End If (row #7). Note that the editor automatically indents the operations between the If and the End If.
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Figure 16-53. Branching Operations
Clicking the Branch(If/Else) button brings up a blank (i.e. not set) If/Then as shown in figure 16-54. The user must define a value for both expressions. Going back to figure 16-53, the first expression is the flow through the gate and the second expressions is the constant 500. The user must also choose a comparison test from the drop down menu between the two expressions. In figure 16-53 (row #4), the comparison is less than. So the If/Then test is true when the first expression is a smaller number than the second (i.e., the flow is under 500 cfs). The comparisons that the user can choose from are: less than, less than or equal to, greater than, greater than or equal to, equal to, or greater than or less than (i.e. not equal to).
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Figure 16-54. Creating a (blank) If/Then Operation
The user must add the End if that is associated with each If/Then. This is done by clicking on the Branch(If/Else) which brings up another blank If/Then as shown in figure 16-55. For the Branching Line Type, select End If as shown in figure 16-56.
Figure 16-55. Adding another blank If/Then Operation (first step in adding an End if)
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Figure 16-56. Changing the Branching Line Type to an End if
Important! There must be one, and only one, End If for each If/Then. The rule set as shown in figure 16-57 is not valid because it has three If/Then operations but only two End Ifs. Notice how the last operation, row #10, is indented. If the last row is indented, then the rule set is missing at least one (if not more than) End if. Although not required, it is highly recommended that the user create the End If operation immediately after adding the If/Then operation. When the If/Then is added (figure 16-55), all the remaining operations are indented, which can look confusing. Figures 16-55 and 16-56 show the steps in adding the End if. To add the End If, the Branch(If/Else) is clicked again, which adds another If/Then that causes the remaining operations to be indented even further (figure 16-55). However, once the rule operation is changed from an If/Then to an End If (figure 16-56) the remaining operations return to their appropriate location. With the desired If/Then and End If in place, the If/Then operation can be defined and additional rule operations can be inserted between the If/Then and the End If as desired. The program will
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Chapter 16 Advanced Features for Unsteady Flow Routing allow the operations to be added in any order. So the user could, of course, create the If/Then and then add the additional operations, before finally creating the End If. However, up until the End If is added, the indentation on the display is liable to cause confusion. Warning! When an If/Then rule operation is deleted, the user must also delete the appropriate End If. Just as it is possible to have more If/Then operations than End If operations, it is also possible to have too many End If operations. There is an erroneous End If in row #7 of figure 16-57 (if there is an End If that does not have an If/Then, it will be displayed in red). If a rule set has a large number of operations with complex, nested If/Then operations, it may be worthwhile to note which End If corresponds to which If/Then before beginning to delete either rule operation.
Figure 16-57. Erroneous “End If” is displayed in red
Instead of simple If/Then, a two part if test can be done by selecting the If And/Or Then option under the Branching Line Type. This requires the user to define four expressions and select a logical operator, figure 16-58. In the first part, the first two expressions are compared (using a greater than for the example in figure 16-58). In the second part, the third and fourth expressions are compared (using a less than or equal to, in the example). The final step is testing the two parts with the logical operator. For figure 1658, And has been selected from the drop down menu between the two sets of expressions. If both the first part and the second part are true, then the overall If test is true. If either, or both, the first or second part are false, then the overall If test will be false. The two parts can also be tested with an Or logical operator by changing the selection on the drop down menu. If this is done, the test will be true if either the first or the second part is true. Only if both parts are false will the If test then be false.
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Figure 16-58. Two part If/Then test
If/Then operations can be nested. Figure 16-59 shows an example where the check for the gate adjustment is made at the top of the hour and half past the hour. If the first If/Then is false (not an appropriate time), then control will jump to after the corresponding End If (as shown by the level of indentation) at row 12. Continuing the example, If the first If/Then is true (time to make flow check) then the second If/Then will be evaluated (row #5). Control will go to row #6 or row #8 depending on whether the second If/Then is true or false.
Figure 16-59. Nested If/Then test
After an If/Then and corresponding End If have been added, an Else can be added as shown in figure 16-60. When the original If is false, control will go to the first line after the Else (row #12 in the example). When the original If
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Chapter 16 Advanced Features for Unsteady Flow Routing is true, the operations between the If and the Else will be performed. Once control reaches the Else, it will jump to the End If (after row #10 control will jump to row #14).
Figure 16-60. Else Operation
Instead of a simple Else, another option is an ElseIf. In this case, there is a second conditional. The operations after the ElseIf will only be performed if the initial If is false and the second If (that is, the ElseIf) is true. Additional ElseIfs can be added as shown in figure 16-61. An Else can also be combined with the ElseIf(s). However, there can only be one Else and it must come after the ElseIf(s). Therefore, after a simple Else operation, there may not be any more ElseIf or Else operations. (The limitations on ElseIfs/Else only apply to branching types at the same level of indentation, that is, in the context of the given If/Then End If. There may still be other “nested” conditionals with their own ElseIfs and Else operations).
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Figure 16-61. Elseif Operations
The display of the rule operations between an If/Then and the corresponding End If may be “collapsed.” Note the “-” at the beginning of each If/Then rule. Clicking the “-” will change it to a “+” and the display of the rules between the If/Then to End If will collapse as shown in figure 16-62. These rules are still in effect (collapsing rules does not change their operation). This option merely changes the display, and it is intended to make large rule sets easier to understand and manage. Clicking the “+” will expand the rules back to their original form. Note: all of the operations under the Current Selection Changes (cut, copy, paste, etc), see below, function normally even on collapsed regions. In the above example where the collapsed region is highlighted, clicking the Delete button would delete rules 47 through 66.
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Figure 16-62. Collapsed If-Then
Math: Clicking the Math operation button creates a [blank] math operation as shown in figure 16-63. The result of the math operation can be assigned to either a new variable or an existing variable (in the same way that a get simulation variable can be assigned, as above). The math operation itself is composed of up to four different “expressions.” Each expression that is defined will return a real number. Expressions should be defined from left to right. So if a math operation is composed of two expressions, the left two expressions should be defined and the right two expressions should be left as “[not set]” (i.e. they should be left blank). If more than one expression is defined, then the user must choose an algebraic connector from the drop down menu between them. The choices are: addition, subtraction, multiplication, and division. The value of each individual expression is determined and then the remaining algebraic operations are performed from left to right. So if the math operation has the three expressions as shown in figure 16-64, the first two expressions are added together and that sum is then divided by the third expression.
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Figure 16-63. Blank Math Operation
Figure 16-64. Three Expression Math Operation
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Chapter 16 Advanced Features for Unsteady Flow Routing Expression. To define an expression, click on the Edit button to bring up the Edit Rule Expression editor as shown in figure 16-65. If no values have been entered (or if the Clear Expression button has been clicked), then the current expression will be shown as “[not set].”
Figure 16-65. Blank Rule Expression
Up to five fields in the expression editor can be defined. Any that are not defined are ignored. The simplest expression is to enter a single number in the Constant field as shown in figure 16-66. In this example, this expression will always have a value of 5.
Figure 16-66. Rule Expression set to a constant
Another simple example is shown in figure 16-67. Here a preexisting variable has been selected from the drop down menu. This expression will return the current value of this variable. An optional coefficient can be added in front of the selected variable and a value may also still be added under the constant field.
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Figure 16-67. Rule Expression set to an existing variable
The Variable can also be raised to an exponent by entering a value in either or both of the fields inside of the parenthetical. If only the Exponent Coefficient or the Exponent Variable is defined, then the variable is raised to the given value of the Exponent Coefficient or Exponent Variable, see figure 16-68. If both are defined, then the Exponent Variable is multiplied by the Exponent Coefficient and the given Variable is raised to the resulting product.
Figure 16-68. Variable raised to an exponent multiplied by a coefficient
Note: each expression is always determined before operations between expressions are performed.
Table: The final operation type is a table lookup. Clicking the Table operation button creates a table operation as shown in figure 16-69. The result of the table lookup can be assigned to a new variable or an existing variable. The table can be either one or two dimensional.
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Figure 16-69. Table lookup operations
Figure 16-69 shows a one dimensional table operation. The table data can be entered (and/or viewed) by clicking on the Enter/Edit Table Data… button. This brings up the Rule Table editor as shown in figure 16-70.
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Figure 16-70. Rule Table Editor
When the table operation is performed, the program will determine the value of the Expression, which starts out ‘Canal Dam’ in the example in figures 1669 and 16-70. The location of the Expression value is determined in the left hand column of the table (figure 16-70) and the corresponding lookup value is determined from the right hand column. In the above figure, if the value of
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Chapter 16 Advanced Features for Unsteady Flow Routing the expression happens to equal 13.08, the result of the table lookup would be to assign the value 1.2 to the variable “Head Opening #1.” The Argument Name (“Flow MGD” in the above example) is used as the heading for the left column. This is only used as a label. (Alternately, the program could have used the numeric formula in the given expression as the heading label, but this could be rather long and awkward.) This label is only used as a heading in the Rule Table editor (it is not a user selectable variable). The right hand column is labeled with the assignment result. In this example, the result of the table lookup is being assigned to a new variable called “Head Opening #1.” By default, the lookup will interpolate between values. So in the above example, if the expression equaled 14.78, the lookup would return 1.3. This can be changed by the drop down menu that is just above the Enter/Edit Table Data. There are three other choices. “Nearest index value” will move up or down to the nearest value (14.7 would return 1.2 and 14.8 would return 1.4, in the above table). “Index <=value” and “Index >=” will go down or up to the next value in the table. These other options can be useful for forcing exact gate settings. For instance, if it was desired that the gates only be opened to the nearest tenth of a foot, values in tenths (e.g., 3.0’, 3.1’, 3.2’, etc) could be entered in a table and “Nearest index value” selected. The result of the table lookup could then be used to set the gate. Tip: Another possibility for forcing exact gate settings is to use an integer user variable. Assume that the gate can be opened in hundredths of a foot (e.g., 3.00’, 3.01’, 3.02’, etc.). These could be [tediously] entered into a table. Alternately, the approximate gate opening could be determined, say for example, 3.028 feet. This value could be multiplied by 100 to get 302.8. This value, 302.8 could be assigned to an integer user variable which would result in 303. Finally, this could be divided back by a 100 (assigning the result back to a real variable) to get 3.03 that could then be used to set a gate opening. Instead of a one dimensional table, the other option is a two dimensional table as shown in figure 16-71. The editor now has two expressions and two argument Names (the top argument name corresponds with the left expression and the bottom argument name corresponds with the right expression). Clicking Enter/Edit Table Data brings up an expanded Rule Table as also shown in figure 16-71. As before, the left most column corresponds to the value in the first expression. The top row now corresponds to the second expression. The value in the table is determined by two way interpolation (or nearest value depending on the interpolation option). So in the table shown, if the first expression (“Inline Flow”) is equal to 5000 and if the second expression (“Hour”) is equal to 9, then the value from the table lookup would be 400.
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Figure 16-71. Two dimensional Table
Current Selection Changes. On the right hand side of the Operation Rule editor (figure 16-72), are six buttons for manipulating the current, highlighted rule (or selection of rules). The Cut, Copy, Paste, and Delete buttons operate in a normal, Windows manner. One or more rules may be selected using the keyboard (e.g. Shift + down arrow) or the mouse pointer (e.g. Ctrl + click) as shown in figure xxx. A copy of the rule(s) can be put on the Clipboard with the Copy button and can then be pasted (using the Paste button) to another location, as shown in figure 16-73.
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Figure 16-72. Copying Highlighted Rules
The copy function makes an exact duplicate of the selected rules. This can generate potential “errors” that the user will have to correct. For instance, in the above example of using the copy function, rule 6 is a Get operation that assigns the result to the “New Variable” named Gate Elv New. The copy of this rule, rule 13 in figure 16-73, is also assigning the result to the same “New Variable” named Gate Elv New. After copying this rule, the user must change one of the “Gate Elv New” names to something else. Or, if it is intended that the copy use the same variable, the user should change the assign result for the copied rule to “Existing Variable” and then select Gate Elv New from the drop down menu. Since the copy function uses the standard Windows Clipboard, rules can be pasted into a completely different rule set, or the user can even open up a different plan (or different RAS project) and paste the results. The user will have to correct any erroneous variable names or references (different cross section river stations, different gate group names, etc.).
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Figure 16-73. Pasting Rules
The Cut button will move the highlighted rule(s) to the clipboard. After the rules have been removed by cutting, the Paste button can then be used as a “move” operation. The Delete button permanently removes the highlighted rules. There is no “undo” operation, so care should be exercised when using the Delete button. However, if a mistake is made, the Cancel button will cancel all the changes that have been made since the Operation Rules editor was opened. Tip: frequently saving the changes made in the Operation Rules editor allows the Cancel button to be used as an “undo” operation without canceling too much work. Note: If a collapsed If/Then-End If block is highlighted, then it will still be subject to copy, paste, and delete/cut, just as it would be in its fully expanded state. Tip: The standard Windows shortcut keys: Ctrl + “x”, “c” or “v” may be used instead of clicking on the Cut, Copy, or Paste buttons. Checking the Disable button is a quick way to temporarily remove the highlighted operations (it will cause the highlighted operations to be displayed as green comment lines with a strikethrough), see figures 16-74 and 16-75. These operations will no longer be performed by the program (be careful disabling Branching Line Types). Clicking the Enable button will restore the operations.
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Figure 16-74. Disabling Highlighted Rules
Figure 16-75. Disabled Rules
The Copy Rules Text to Clipboard will copy the display text of the entire rule set to the clipboard (figure 16-76). This can then be pasted, for instance, as simple text into Notepad or a Word document report (figure 1677). This copy if for “display” only and may not be pasted back into a rule set.
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Figure 16-76. Copying Rule Text to the Clipboard
Figure 16-77. Text Pasted into Notepad
Clicking the right mouse button (on a given row) will display a popup editor as shown in figure 16-78. In addition to the functions described above, the Insert New Operation functions are also available in this manner.
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Figure 16-78. Right mouse click on a line
Clicking the Check Rule Set… button will cause RAS to check the rule set for common user errors. All of the rule sets in the model will also be checked when an unsteady flow run is launched. The Check Rule Set… button is just a convenient way to find and fix rule errors for the given rule set while the Operation Rules editor is opened. If no errors are found, RAS will display a message stating that no inconsistencies were found. Otherwise, RAS will display a list of the mistakes and the line numbers they occur. An example is shown in figure 16-79. Common problems are: a variable name that has been defined more than once, a reference to a non-existent variable (the variable was renamed or deleted), “unbalanced” If/Then End If operations, or a reference to a nonexistent node (e.g. a river station that has been removed from the project).
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Figure 16-79. Checking the Rule Set
Font. In the upper right hand corner of the Operation Rule editor, there is a drop down menu where the user can change the rule operation font size. The font can also be toggled between normal and bold by checking the Bold Font box. Detailed Log Output. If the detailed log output is turned on, then results from each rule set will be sent to the log file during runtime, see figure 16-80. On the left side is the row number of the operation followed by the result of the operation. For instance, the operation at row #19 results in the variable ‘Tampa Dam Flow’ being set equal to 324.9499. Row #32 is an If/Then test that came back false. For this time step, for this operation, the first and second expressions are both equal to 4. This results in the less than or greater than (i.e. ‘not equal to’) test being false. Since the test is false (and there is not a corresponding ElseIf or Else), control jumps to after the End If, which happens to be row #121. Row #121 is a two part If/Then test that is also false. It is expected that additional, (tabular and graphical) output from rule set operations will be added to future versions of HEC-RAS. For the current version of RAS, the log output may be the best way to track down user programming mistakes. Rule operations that are valid (as far as RAS is
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Chapter 16 Advanced Features for Unsteady Flow Routing concerned), but do not produce the result desired by the user. For instance, a Get operation that references the wrong cross section node.
Figure 16-80. Detailed log output
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Simulation and Operational Variables. The following is a list of the currently available simulation output variables and operational variables that can be set.
Time variables: Julian Day: Days since December 31, 1899 (e.g. 01Jan2000 = 36525). Year: Year (e.g. 2006). Month: Month of the year (e.g. August = 8). Day of Year: e.g. Jan 1 = 1. Feb. 1 = 32. Dec 31 is 365 (non-leap year). Day of Water Year: e.g. Oct 1 = 1. Sept 30 = 365 (non-leap year). Day of Month: e.g. 22Jan2000 = 22. Day of Week: Integer day starting on Sunday. e.g. Sunday = 1, Monday = 2, Saturday = 7. Hour of Day: Integer hours since midnight (e.g. 01Jan2000 1245 = 12). Minute of Hour: Integer minutes after hour (e.g. 01Jan2000 1245 = 45). Second of Minute: Integer seconds after minute (e.g. 01Jan2000 1245:15 = 15). Hour of Day (fractional): (fractional) Hours since midnight (e.g. 01Jan2000 1245 = 12.75). Hour of Simulation: (fractional) Hours since simulation started.
Solution variables: Time Step: Length of current time step in hours. Iteration Number: Number of iterations, for given time step (“current time step” will not have relevance until rules are allowed for every iteration, see above, but “previous time step” will return the number of iterations from the last time step). WS Error Max: Maximum error, for given time step, in computed water surface at any cross section (“current time step” will not have relevance until iterations are allowed, see above, but “previous time step” will return the maximum error from the last time step).
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Chapter 16 Advanced Features for Unsteady Flow Routing Flow Error Max: Maximum error, for given time step, in computed flow at any cross section (previous time step only). WS SA Error Max: Maximum error, for given time step, in computed water surface at any storage area (previous time step only).
Cross Sections variables: WS Elevation: Water surface. Flow: Flow. WS Change: Change in water surface, for given time step (previous time step only). Flow Change: Change in flow, for given time step (previous time step only). WS Error: Error in water surface, for given time step (previous time step only). Flow Error: Error in flow, for given time step (previous time step only).
Inline Structures, Lateral Structures, and Storage Area Connections variables: Structure - Total Flow: Total flow for the inline structure. Structure - Total Flow (Fixed): Force the given flow for the inline structure. Structure - Total Flow (Desired): Compute gate settings to provide the total given flow for the inline structure. Structure - Flow Additional: Add in the additional given flow to the inline structure. Structure - Flow Maximum: Set a maximum flow for the inline structure. Structure - Flow Minimum: Set a minimum flow for the inline structure. Structure - Total Gate Flow: Flow for all of the gate groups. Structure - Total Gate Flow Maximum: Set a maximum flow for all of the gate groups. Structure - Total Gate Flow Minimum: Set a minimum flow for all of the gate groups. Weir - Flow: Flow over the weir. Weir - Flow Maximum: Set a maximum flow over the weir.
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Chapter 16 Advanced Features for Unsteady Flow Routing Weir - Flow Minimum: Set a minimum flow over the weir. Weir - Weir Coefficient: Weir coefficient for the weir. Weir - Minimum Elev for Weir Flow: Minimum weir elevation for flow for the weir (water surfaces below this elevation will not produce weir flow). Weir - C Simple (Positive): Linear routing coefficient for positive flow (linear routing weirs only). Weir - C Simple (Negative): Linear routing coefficient for negative flow (linear routing weirs only). Weir - Submergence: Fractional submergence for the given weir (e.g. 0.97). Gate - Flow: Flow through the gate group. Gate - Flow (Fixed): Force the given flow for the gate group. Gate - Flow (Desired): Compute gate setting to provide the given flow for the gate group. Gate - Flow Maximum: Set a maximum flow through the gate group. Gate - Flow Minimum: Set a minimum flow through the gate group. Gate - Opening: Gate opening height for the gate group. Gate - Submergence: (fractional) Gate submergence for the gate group (e.g. 0.88). Gate - Opening Rate: Gate opening rate for the gate group. Gate - Closing Rate: Gate closing rate for the gate group. Lake Superior (Plan 1977A): This get simulation value will determine the nominal monthly outflow for Lake Superior as specified by the Plan 1977A regulations. This computation is based on the value of user defined variables that must be in a specific order. The first twelve rule operations (excluding comment lines) must be defined in the order show in figure 16-41.
Storage Areas variables: WS Elevation: Water surface elevation for the given storage area. Net Inflow: Net inflow for the given storage area (e.g. Total Inflow - Total Outflow). Total Inflow: Total inflow for the given storage area (gross inflow, ignores outflow).
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Chapter 16 Advanced Features for Unsteady Flow Routing Total Outflow: Total outflow for the given storage area (gross outflow, ignores inflow). Area: Current surface area of storage area. Volume: Current volume of storage area.
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Chapter 17 Performing a Sediment Transport Analysis
CHAPTER
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Performing a Sediment Transport Analysis This chapter shows how to perform a mobile bed sediment transport analysis with HEC-RAS. A sediment model requires a geometry file, a quasi-unsteady flow file, a sediment file and a sediment analysis plan file. Instructions on creating a geometry file can be found in Chapter 6 of this User’s Manual. The other three files are described in this chapter.
Entering and Editing Sediment Data Once the geometric data are entered, the modeler can enter the sediment data required to develop a mobile bed sediment transport model. However, it is suggested that the modeler first run a series of profiles using the Steady Flow Analysis option. This will allow the modeler to work out any problems with the river hydraulics calculations, and to develop a robust hydraulic model before attempting the mobile bed calculations. To access the sediment data editor, select Sediment Data from the Edit menu or press the sediment data icon. The sediment data editor will appear as depicted in Figure 17-1.
Figure 17-1. The Sediment Data Editor.
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Chapter 17 Performing a Sediment Transport Analysis
Initial Conditions and Transport Parameters The sediment data editor has two tabs. The default tab when the window appears is Initial Conditions and Transport Parameters. From this editor the user can specify: transport function, sorting method, fall velocity method, sediment control volume and the bed gradation associated with each cross section.
Transport Function A transport function can be selected from the drop down box near the top of the form. There are currently seven transport functions to select from: Ackers and White England and Hansen Copeland’s form of Laursen Meyer, Peter and Muller Toffaleti Yang (sand and gravel eqns.) Wilcock Warning: Sediment transport results are strongly dependent on which transport function is selected. Carefully review the range of assumptions, hydraulic conditions and grain sizes for which each method was developed, and select the method developed under conditions that most closely represent the system of interest.
Sorting Method Select a sorting method to compute active layer thickness and vertical bed layer tracking assumptions. Two methods are currently available:
Exner 5 – A three layer active bed model that includes the capability of forming a coarse surface layer that will limit erosion of deeper material thereby simulating bed armoring (default method).
Active Layer – This is a simplified two layer active bed approach. The active layer thickness is set equal to the d90 of the layer. This assumption is only appropriate for gravel beds and is intended for use with the Wilcock transport method in particular.
These methods can be selected from the drop down box titled Sorting Method.
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Chapter 17 Performing a Sediment Transport Analysis
Fall Velocity Methods Several methods are available for computing fall velocity and the user should select the most appropriate algorithm. The options include: Ruby Toffaleti Van Rijn Report 12 (Default method in HEC-6)
Maximum Depth or Minimum Elevation Within the Initial Conditions and Transport Parameters tab is a grid with a record corresponding to each cross section in the model, including columns identifying associated River, Reach, River Station and invert. These cross sections can be filtered with the River: and Reach: drop down lists to focus the display on a particular study river or reach. In the HEC-RAS sediment framework, a sediment control volume is associated with each cross section as depicted in Figure 17-2. The control volume starts midway from the next cross section upstream and ends midway to the next cross section downstream. The width and vertical thickness of the control volume, however, must be specified by the user. The vertical extent of the control volume is shown by the dotted line in the profile and cross section plots in the sediment data editor (Figure 17-1). The vertical thickness of the sediment control volume can be specified in either of the columns labeled Max Depth or Min Elev. The Max Depth column allows the user to set the control volume depth as a distance below the original invert of the channel. When this option is used, the software will compute the Minimum Erodible Elevation as the original channel invert elevation minus the Max Depth. The second option, Min Elev, allows the user to enter a set elevation below which the model cannot erode. This option is often used to specify a bedrock control, grade control structure, a flume bottom, or a concrete channel lining. The model will allow erosion as long as the thalweg exceeds this elevation. However, if the cross section thalweg drops to the minimum elevation, no further material entrainment or channel degradation will occur.
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Chapter 17 Performing a Sediment Transport Analysis
Upstream Cross Section
Erodible Limits
Downstream Cross Section
Midpoint Between Cross Sections Sediment Control Volume
Max Depth/ Minimum Elevation
Figure 17-2. Schematic of sediment control volume associated with each cross section.
Mobile Cross Section Limits Width is the final dimension required for each sediment control volume. Lateral limits for erosion and deposition are specified in the Sta Left and Sta Right columns. HEC-RAS will allow deposition to occur along the entire wetted perimeter of a cross section but will only erode the channel between defined mobile bed limits. Since the mobile bed stations are fixed, there must be a station-elevation point at this location on the cross section. If there is not already a station-elevation point, the program will add one automatically. HEC-RAS will only raise or lower wetted cross section points between these lateral limits. Lateral limits should be selected carefully to ensure that deposition does not cause elevations in the channel to rise above the bank stations, unless physically justified. The Use Banks for Extents button allows the user to set the erodible bed limits to the main channel bank stations as an initial estimate.
Bed Gradation Each cross section must have an associated bed gradation. HEC-RAS first requires the creation of bed material gradation templates. Then the bed gradation templates can be associated with the appropriate range of cross sections using pick and drag functionalities. Bed Gradation Templates: To assign bed gradations to the cross section, first create bed gradation templates. In many applications, these templates will correspond to individual bed samples taken in the project reach. Templates are created and edited by pressing the Define/Edit Bed Gradation Button, which will launch the dialog depicted in Figure 17-3. The gradation of the bed
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Chapter 17 Performing a Sediment Transport Analysis sample can be input in either of two forms by toggling between the radio buttons at the bottom of the form:
% Finer: as a cumulative bed gradation curve with percent finer associated with the geometric mean of each grain class. The diameter listed for each grain class is the upper bound of that grain class and values should be entered as percents. (e.g. since this is specified in Percent Finer, 50% should be input as 50 and NOT as 0.5)
Grain Class Fraction/Weight: the sample fraction of each grain class is specified. (e.g. if 20% of the sample is fine sand, input the value 20). These values will be normalized so values do not have to add up to one or 100% and can be input as simple masses if preferred. The upper bound grain diameter is associated with each grain class to delineate the range of the class.
Figure 17-3. Gradation template editor.
Selecting a Template: After the sediment templates are defined, they are available in a drop down pick list under the Bed Gradation column of the Sediment Data grid. Clicking on a cell of the Bed Gradation column generates a drop down list of the defined bed sample templates (Figure 17-4a). A single bed sample is frequently associated with multiple cross sections. Therefore, once selected; a sample can be easily copied into multiple cells by placing the mouse pointer over the bottom right corner of the selected cell and dragging vertically (Figure 17-4b).
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Chapter 17 Performing a Sediment Transport Analysis
(a)
(c)
(b)
(a)
(b)
(c)
Figure 17-4. Illustration of process of associating sample templates with cross sections.
Interpolation: In cases where channel geology justifies assumptions of gradual bed gradation transitions between samples the option to interpolate between specified gradational templates is available. To interpolate, select the appropriate bed gradation templates for the known cross sections, leaving the other rows of the Bed Gradation field blank (Figure 17-5). Then press the Interpolate Gradations button on the Sediment Data editor (Figure 17-1).
(a)
(b)
Figure 17-5. Gradation interpolation process.
HEC-RAS will interpolate a bed gradation at any station that occurs between two defined gradations within a reach and write “Interpolated” in the Bed Gradation field for those nodes. If a cross section occurs between one defined gradation and either the upstream or downstream end of the reach, the closest gradation template will be copied to the node as depicted in the first two fields of Figure 17-5.
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Chapter 17 Performing a Sediment Transport Analysis
Sediment Boundary Conditions On the boundary conditions tab, sediment loads can be specified in a variety of locations and formats as shown in Figure 17-6. The form will automatically list external boundaries of the model. Sediment boundary conditions must be specified for all external boundary conditions. Lateral boundary conditions can be added as appropriate.
Figure 17-6. Boundary conditions tab of the sediment data editor.
Add Sediment Boundary Location Although HEC-RAS will automatically list external boundaries, the user must specify internal locations where sediment boundary conditions are required. To add an internal boundary, press the Add Sediment Boundary Location(s) button, which will launch the river station selector depicted in Figure 17-7. One or more of these river stations can be selected by double clicking on the list or selecting locations while holding down the control or shift button. Stations can be removed from the Selected Locations list by double clicking on them or pressing the Clear Selected List button. When the locations are selected, they will appear in the Sediment Data Editor and the load type designation will initially be blank. Sediment boundary conditions are chosen by selecting the grid cell associated with the location of interest. Based on the properties of the given location, different sediment boundary conditions buttons will be available. Choose
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Chapter 17 Performing a Sediment Transport Analysis from the available sediment boundary condition type buttons in order to set the boundary condition and begin entering data.
Figure 17-7. Editor for selecting a lateral flow load boundary location.
Equilibrium Load Equilibrium load, available only for upstream external cross sections, is determined by transport capacity. HEC-RAS will compute sediment transport capacity, for each time step, at the specified cross section and this will be used as the sediment inflow. Since load is set equal to capacity for each grain size, there will be no aggradation or degradation at this cross section.
Rating Curve A rating curve determines a sediment inflow based on water inflow. The water inflow can be the upstream boundary flow series, a lateral flow series or a uniform lateral series. One of these flow series must be associated with a river station in order for the rating curve option to be available at that cross section. If a rating curve is selected for a station with a uniform lateral series, the load will be distributed along the cross sections in the same manner as the flow. Selecting this option will open the Load Specification editor depicted in Figure 17-8. Since sediment rating curves correlate inflowing sediment load with water discharge, a series of Flow-Load pairs must be defined. The number of columns, one for each Flow-Load pair, is set using the Number of flow-load points drop down box at the top of the dialog. Blank columns are not allowed. A range of flows should be entered that completely encompasses the flows expected during the simulation. If flows occur that exceed the upper bound of the rating curve, HEC-RAS will not extrapolate, but will use the largest sediment load specified in the table. Flows below the smallest entered flow, will be interpolated assuming a zero sediment load at zero flow. The Plot… button plots flow versus total load in log space.
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Chapter 17 Performing a Sediment Transport Analysis Each column has a flow and an associated total load entered as mass per time (e.g. tons/day). The gradational character of the sediment loads must then be specified in each column. (Note: Do not use % finer here. These are incremental percentages or fractions not cumulative curves). Percentages (or decimal fractions) can be entered for each grain class for each load. If the total of the percentages (decimal fractions) does not equal 100 (or 1.0), HECRAS will normalize the total during computations (so that a given flow will produce the entered total load based on the ratios of the grain sizes).
Figure 17-8. Load specification editor.
Point Loads and Distributed Loads If a load is required that is not tied to a flow boundary, it can be entered as a sediment load series. Since this boundary condition is not dependent on a flow boundary, it can be introduced to any cross section except for the downstream node. Sediment loads are input (as a mass rather than a rate) in a similar fashion to the flow or temperature series data. Sediment time series loads, however still require grain size information. Therefore, a rating curve that defines grain size distributions for a range of loads must also be entered.
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Chapter 17 Performing a Sediment Transport Analysis
Figure 17-9. Point load series editor.
Downstream Pass Through Boundary In order to fix the downstream model boundary (prevent it from aggrading or degrading), a downstream pass through boundary can be used. This boundary is often used for flume studies or downstream boundaries defined with normal depth. When the pass through boundary is used, material transported out of the downstream control volume is precisely equal to the inflowing material, resulting in no degradation or aggradation. To use this boundary type, check the box labeled Set downstream pass-through boundary at the bottom of the Boundary Conditions tab of the Sediment Data Editor.
Sediment Properties Options There are a number of default values used by HEC-RAS that can be changed by the user. To change the defaults, select the various options available from the Options menu on the Sediment Data editor. Only make changes to the default settings if it is appropriate for the specific application. A list of the options available follows.
Set Sediment Properties Selecting the Set Sediment Properties menu option launches a simple dialog that allows the specific gravity and the Unit Weight of sediment to be changed (Figure 17-10).
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Chapter 17 Performing a Sediment Transport Analysis Specific Gravity: The default value of specific gravity for all sediment particles is 2.65. While generally appropriate, this assumption is not universally valid. Only one specific gravity can be specified for a given sediment data file. Shape Factor: The shape factor is the ratio of the shortest axis of a grain to the largest axis. A spherical particle will have a shape factor of 1 while an oblong particle that is twice as long as thick will have a shape factor of 0.5. The only place HEC-RAS uses the shape factor is in the computation of fall velocity with the Report 12 method.
Figure 17-10. Sediment property dialog.
Unit Weight/Density: Sediment unit weights or densities are used to convert deposited or eroded masses into volumes that translate into bed elevation changes. This is one of the only parameters that is different for SI and US customary units. US customary is defined in terms of unit weights (lb/ft3) while SI is defined in terms of density (kg/m3). Conversions between density and unit weight are performed internally in the program. Three unit weights or densities are used to define sand and gravel, silt and clay. These three parameters can be changed independently in the Sediment Properties editor (Figure 17-10).
Cohesive Options Cohesive methods and parameters can be specified by selecting Set Cohesive Options under the Options Menu. The method selected will be applied to silts and clays. Fine particle transport can either be computed in a standard transport capacity approach using the selected method to compute transport potential for the silt and clay grain classes, or alternately, the Krone and Parthenadies equations can be used for fine grain classes. If the Krone and Parthenadies option is selected, the erosion threshold, erosion rate, mass erosion threshold and mass erosion rate must be specified.
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Chapter 17 Performing a Sediment Transport Analysis
Figure 17-11. Cohesive options editor.
Bed Change Option Cohesive methods and parameters can be specified by selecting Set Cohesive
Figure 17-12. Bed change options editor.
Transport Function and Calibration The sediment transport functions are, to varying degrees, the results of theoretical and empirical science. Even the most theoretically detailed was fit to data using empirical coefficients. These coefficients represent the central tendencies of the data considered but will not likely reflect the transport of a specific site precisely, even if an appropriate transport function is selected. Therefore, some of these transport parameters have been ‘exposed’ to the user for four of the transport functions. To use calibration paramaters first click the box labled Overwrite Transport Equation with Variables Specified on This Form. If the box is not checked, variables can be edited and changed but they will not be used for transport calculations. Each of the four transport functions has a variable that quantifies the force or energy required to mobilize the particle. In Laursen-Copeland and MPM it is * the critical shear stress, c (also known as the Shields number), in Ackers-White it is the Threshold Mobility (A) and in Wilcock it is the reference Shear Stress rm*. When
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Chapter 17 Performing a Sediment Transport Analysis calibrating a sediment transport function using this feature, these mobility factors should be the main parameters adjusted, since they can be related to physical phenomena. For example, imbricated or vegetated particles will be harder to move than the critical Sheilds’ parameter would suggest, so a physical case could be made for a higher c*, which would decrease transport. Conversely, the presence of substantial fine particles could make it easier for the flow field to entrain coarser particles, resulting in a lower c*. Warning: These variables should only be adjusted within reasonable ranges in response to a hypothesis based on observed physical processes.
Figure 17-13: Transport Function Calibration and Modification Editor.
For Ackers-White and MPM, the coefficient and power of the equations have been exposed as well. The central equation of the Ackers-White equation is:
Fgr A Ggr C A
m
where Ggr is sediment transport, F is a dimensionless sediment mobility parameter, A is the critical mobility and C and m are empirical coefficients. MPM can be simplified into the form:
qb 8( c )3 / 2
,
c 0.047
where qb* is a dimensionless measure of transport (the Einstein number), * is the dimensionless shear stress (the Shields number) and t*c is the critical
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Chapter 17 Performing a Sediment Transport Analysis dimensionless shear. 8 and 3/2 are coefficients fit to the simple excess shear relationship in the original formulation. Exposing the critical shear stress, the coefficient and the power of the MPM relationship turns it into a generic excess shear formula that can be used to customize a site specific excess shear, power function. In fact, Wong and Parker (2006) recently reanalyzed the data set initially used to develop the MPM equation and found that the relationship
qb 4.93 ( c )1.6
,
c 0.047
actually fit the original MPM data better than the MPM equation. Pressing the Use Wong and Parker Correction to MPM button, will automatically set the coefficient and power to the corrected values.
User Defined Grain Classes HEC-RAS defaults to twenty grain classes that follow the scale (Parkers and Andrews, 1985) for which the grain class boundaries are defined by D=2, where is the set of integers between -8 and 11. The default grain classes are detailed in Table 17-1. Table 17-1. Default grain classes in HEC-RAS (mm).
Grain Classes Clay Very Fine Silt Fine Silt Medium Silt Coarse Silt Very Fine Sand Fine Sand Medium Sand Course Sand Very Course Sand Very Fine Gravel Fine Gravel Medium Gravel Coarse Gravel Very Coarse Gravel Small Cobbles Large Cobbles Small Boulders Medium Boulders Large Boulders
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Clay VFM FM MM CM VFS FS MS CS VCS VFG FG MG CG VCG SC LC SB MB LB
Lower Upper Mean Geometric Bound Bound Diameter Mean 0.002 0.004 0.003 0.00283 0.004 0.008 0.006 0.00566 0.008 0.016 0.011 0.0113 0.016 0.032 0.023 0.0226 0.032 0.0625 0.045 0.0447 0.0625 0.125 0.088 0.0884 0.125 0.25 0.177 0.177 0.25 0.5 0.354 0.354 0.5 1 0.707 0.707 1 2 1.41 1.41 2 4 2.83 2.83 4 8 5.66 5.66 8 16 11.3 11.3 16 32 22.6 22.6 32 64 45.3 45.3 64 128 90.5 90.5 128 256 181 181 256 512 362 362 512 1024 724 724 1024 2048 1448 1450
Chapter 17 Performing a Sediment Transport Analysis
The user can define a customized set of grain classes in order to focus more detail in a particular size range or model specific grain sizes. Selecting the User Defined Grain Classes item on the Options menu allows the user to override default HEC-RAS grain classes. This menu option will open the dialog depicted in Figure 17-14. The User Defined Grain Classes dialog defaults to the grain classes outlined in Table 17-1, and will write a text line at the bottom of the dialog with a “Currently Default” message if this is the case (Figure 17-14). HEC-RAS must have twenty grain classes that are adjacent and increasing even if fewer are used in the calculations. Because the sizes must be adjacent, all of the grain class lower bounds (except the first one) are set by the program as the upper bound of the previous class. The user can, therefore, edit the grain classes by changing the upper bounds (labeled max in Figure 17-14). Geometric means are computed by the program and can not be directly edited. When changes have been made to the grain classes, the dialog will show the “Currently Customized” message in the panel below the buttons. Grain classes can be reset to the default parameters by pressing the Defaults button.
Figure 17-14. User defined grain classes dialog.
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Chapter 17 Performing a Sediment Transport Analysis
Set Pass Through Nodes Pass through nodes remain fixed throughout the sediment simulation. All of the sediment that enters the control volume associated with that cross section exits that control volume. Capacity for the node is set equal to supply. So the cross section will not deposit or erode. This feature was designed for channel bends where multi-dimensional channel dynamics keep sediment from depositing but the one dimensional transport approach computes deposition. To use this option the user simply selects the river station locations in which they would like all sediment to pass through (i.e. no deposition or erosion)
Entering Observed Data Observed elevations can be entered in HEC-RAS which allows for comparison with simulated bed profiles or other known parameters. Selecting Observed Data under the Options menu of the Sediment Data Editor will open the Observed Data dialog. Observed data can be entered for one or more cross sections (Figure 17-15). These elevations will then be available for viewing along with other profile output.
Figure 17-15. Observed bed elevation dialog.
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Chapter 17 Performing a Sediment Transport Analysis
Entering and Editing Quasi-Unsteady Flow Data Current sediment capabilities in HEC-RAS are based on quasi-unsteady hydraulics. The quasi-unsteady approach approximates a flow hydrograph by a series of steady flow profiles associated with corresponding flow durations. Because this type of analysis requires different information than steady or unsteady flow, a separate quasi-unsteady flow dialog (Figure 17-16) is available by selecting Quasi-Unsteady Flow under the Edit menu of the main HEC-RAS dialog or by pressing the Quasi-Unsteady Flow shortcut button.
Figure 17-16. Quasi-Unsteady flow dialog.
Boundary Conditions Several different boundary conditions are available in HEC-RAS. Each upstream boundary (the most upstream cross section of an open ended upstream reach) must have a Flow Series boundary condition specified. Optional internal boundaries include Lateral Flow Series and Uniform Lateral Flow Series. Each downstream boundary (the downstream most cross section of an open ended downstream reach) can be either: Stage Time Series, Rating Curve, or Normal Depth.
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Chapter 17 Performing a Sediment Transport Analysis
Flow Series The quasi-unsteady flow editor will automatically list the cross sections that correspond to each external boundary condition. External boundary specifications are required to run a sediment analysis in HEC-RAS. For an upstream external boundary a Flow Series must be selected. Click the blank Boundary Condition Type field associated with the upstream node and then press the Flow Series button to open the Flow Series Editor.
Flow Series The flow series editor is depicted in Figure 17-17. Since Quasi-unsteady flow can have irregular (varying) time steps, each specified flow must also be accompanied by a time duration (over which the flow is constant). Additionally, a computational time step must be entered for each record (see discussion below).
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Chapter 17 Performing a Sediment Transport Analysis
Figure 17-17. Flow series editor.
Flow Duration: To approximate a flow hydrograph as a series of steady flows, each steady flow profile must have a flow duration. The duration is then broken up into a series of computational increments over which the sediment routing occurs. Due to the non-linear nature of alluvial sediment movement, transport is usually concentrated during large, peak flow events. These events are usually of relatively short duration and are characterized by rapidly changing flow. Because of this non-linearity, an irregular time step is desirable. Low flows, corresponding to small or moderate transport (or bed change), are often approximated with large time steps. More detail (shorter time step) is beneficial, however, during the large flow and high transport regions of the hydrograph.
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Chapter 17 Performing a Sediment Transport Analysis Flow Durations are specified in the corresponding column of the data grid. Each time (in hours) represents the duration of the flow for that part of the hydrograph. All fields in the quasi-unsteady flow data grid allow values to be dragged to other cells. Therefore, data for portions of the flow record having the same time step can be entered once and dragged over the pertinent region. The flow durations for different boundary conditions do not need to match. If boundary conditions have different time steps HEC-RAS will compute and use the smallest time step common to all boundaries. Computation Increment: Quasi-unsteady modeling is based on the assumption that changes in bed geometry between computations of hydraulic parameters are not enough to alter the hydrodynamics (i.e., the overall water surface profile) appreciably. This means that the hydrodynamics (“backwater computations”) do not need to be computed as often as the sediment transport computations, thus shortening the program runtime. However, sediment model stability and accuracy are dependent on how much bed change occurs between geometry updates and the effect geometry updates have on velocities and depths for a given control volume. If channel geometry is updated too infrequently, too much material could be eroded or deposited in a given time step, causing the model to over correct in the next time step, generating oscillations and instabilities in the model. Therefore, each flow duration is often subdivided into computational increments (Figure 17-17). Model bathymetry will be updated at the end of each of the computation increments. Additionally, the flow depths and velocities are recomputed. These hydraulics are based on the new cross section shape, but it still uses the water surface elevation that was determined from the prior hydrodynamics. However, if any cross section experiences more than a 0.1 foot (an editable variable) vertical change since the last time the hydrodynamics were computed, then the hydrodynamics will be recomputed. For example, in the first flow record in Figure 17-17 the flow is constant for 100.8 hours but the model geometry and hydraulic parameters are recomputed every 24 hours, while in record 18 the flow is 1160 cfs for 6.72 hours with geometry and hydrodynamics computed every 36 seconds (0.01 hours) of model time. When flows are large and transport is significant, more bathymetry updates will be required and smaller computation increments should be selected. For very large values, these increments may need to be very small in order for the model to be stable. While smaller computation increments will increase run time, re-computing geometry and hydraulics too infrequently (e.g. computation increments that are too large) is the most common source of model instability. Even if the model does not fail, severe inaccuracies can be introduced by selecting computation intervals that are too large.
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Chapter 17 Performing a Sediment Transport Analysis
Lateral Flow Series A lateral flow series can also be associated with any internal cross section node in a project. This feature is often used to account for inflows from unmodeled tributaries. Before a series can be input, however, the associated node must be added to the Quasi-Unsteady Flow dialog. A flow change location is added by pressing the Add Flow Change Location(s) button on the Quasi-Unsteady Flow dialog. This will open the Select River Station Locations dialog (used elsewhere in the program e.g. Figure 17-7). The user can select one or more river stations by double clicking on them or selecting them and pressing the arrow key. Pressing OK will cause rows for each of these stations to be added to the QuasiUnsteady Flow Editor.
Uniform Lateral Flow The option is also available to define a single flow series, which HEC-RAS will distribute over several cross section nodes. This feature is often used to distribute overland watershed runoff, computed from a hydrologic model. Specifying a uniform lateral flow is similar to the lateral flow series. A flow change location must be added before the boundary condition can be selected. The flow change location must be specified as the upstream node of the cross sections over which the uniform lateral load will be distributed. When the node is added to the Quasi-Unsteady Flow editor the user can then select the Uniform Lateral Flow button. When the Uniform Lateral Flow button is selected, an editor will appear as shown in Figure 17-18. This dialog is similar to the other flow series dialogs (Figure 17-17) except for a drop down selector that occurs at the top of the editor. HEC-RAS will distribute the flow by reach weighted averages (incremental main channel reach lengths between the individual cross section divided by the total main channel reach length between all cross sections) between the upstream node listed and the downstream node selected in the dropdown box. Uniform lateral flows cannot be specified across stream junctions.
Gate Time Series Gates, added to inline weirs can be controlled from the Quasi-Unsteady flow editor using the T.S. Gate Openings Boundary condition (Figure 17-19). Gates are internal boundary conditions that are not required. Therefore, to control the openings, the structure must be added as a boundary condition. Press the Add Flow Change Location(s) button and select the Inline Structure’s station. For a large model with many cross sections, just the inline structures can be displayed by pressing the Node Types button (Figure 17-7) and selecting “Inline Structures.”
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Chapter 17 Performing a Sediment Transport Analysis
Figure 17-18. Uniform lateral flow series dialog.
Figure 17-19. Time series gate editor.
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Chapter 17 Performing a Sediment Transport Analysis The gate time series editor is very similar to the other quasi-unsteady flow editors except that time series openings for all gate groups in the structure are defined on the same editor. A gate group is a set of up to 25 gates that are identical in every aspect except lateral position and are operated together. Up to 10 gate groups that open and close independently can be associated with an inline structures. If the structure has more than one gate group, a time series must be entered for each group. A user can move between gate groups by selecting them in the gate group drop down menu at the top of the editor.
Stage Time Series The Stage Time Series boundary condition allows the user to input a time series of stages at the downstream boundary. If this option is selected the Quasi-Unsteady flow editor opens a dialog that will allow the user to specify a time series of stages (Figure 17-20). This time series editor follows the standard irregular time series format of a duration associated with a stage. No computation increment is required for a Stage Time Series.
Figure 17-20. Specifying stage with a stage time series.
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Chapter 17 Performing a Sediment Transport Analysis
Rating Curve A rating curve can also be specified as the downstream boundary condition. By pressing the Rating Curve button a Flow-Stage curve for the downstream cross section can be entered (Figure 17-18). HEC-RAS will then interpolate a boundary stage from the rating curve, for each time step based on the flow at that cross section.
Figure 17-181. Rating curve editor for downstream boundary condition.
Normal Depth Normal depth is another option for a downstream boundary condition. Pressing the Normal Depth button will open a simple window allowing the user to enter a Friction Slope (slope of the energy grade line). Using the specified energy slope HEC-RAS will determine a downstream depth for each flow in the series using the slope-area method (solving Manning’s equation for depth). Caution: by setting the downstream boundary condition as a depth rather than a stage it is independent of the computed channel elevation at that point. Therefore, once a depth is computed based solely on unchanging cross section parameters, it will aggrade or degrade at a constant rate throughout the time step. Therefore, while convenient, normal depth is often a poorly posed boundary condition for sediment transport models, unless the downstream boundary is at a location where the stream is in equilibrium for the period simulated or a pass through downstream sediment-boundary condition is also used in conjunction with this boundary condition.
Temperature Because several aspects of sediment transport mechanics, particularly fall velocity, are sensitive to water temperature, HEC-RAS requires temperature
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Chapter 17 Performing a Sediment Transport Analysis information. Currently, only one temperature per time step can be specified for the entire model. To specify a temperature time series, press the Set Temperature button on the bottom of the Quasi-Unsteady Flow editor (Figure 17-16). This will open the temperature series editor depicted in Figure 17-22.
Figure 17-22. Specifying a temperature time series.
Performing a Sediment Transport Computation
Defining a Plan For sediment transport computations, the user is required to create a Plan file. A sediment plan includes a geometry file (.gxx), a sediment file (.zxx), a flow file (quasi-unsteady: .qxx), and some plan level options data. The plan dialog, which also serves as the compute window, can be accessed by selecting Sediment Analysis under the Run menu on the main HEC-RAS dialog or by pressing the Sediment Analysis button. When this option is selected, the Sediment Transport Analysis window will appear as shown in Figure 17-23. As with Unsteady Flow, a time window must also be specified for the sediment analysis. This requires start and end dates (in DDMMMYYYY format) as well as start and end times (2400 clock) in the Simulation Time Window (Figure 17-23).
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Chapter 17 Performing a Sediment Transport Analysis
Figure 17-23. Sediment analysis window.
Sediment Computation Options and Tolerances There are several settings and model coefficients under the Options menu of the Sediment Analysis dialog that the user can change. Many of these are the same as found in the Steady Flow Analysis dialog and documentation for these can be found in Chapter 7 of the User’s Manual. However, there are other options that are sediment specific. Sediment specific options include the Sediment Computation Options and Tolerances, Sediment Output Options and Transport Energy Slope menu items available from the Options menu of the Sediment Transport Analysis window. When this option is selected, a window will appear as shown in Figure 17-23.
Computational Options Bed Exchange Iterations: The Bed Exchange Iterations variable (SPI factor in HEC-6) is the number of iterations the sorting and armoring algorithms will perform per computation increment to account for changes in bed material availability. Sorting and armoring iterations are important to track supply limitation in order to keep the model from over predicting erosion. However, they also significantly affect run times. HEC-RAS will allow the user to specify between 1 and 50 iterations. While the default is set to 10 iterations, it is generally suggested to start with a high number and decrease the number of computations as far as possible without changing the results. (Figure 17-19)
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Chapter 17 Performing a Sediment Transport Analysis
Figure 17-19. Sediment computation options and tolerances dialog.
Minimum Bed Change Before Updating Cross Section: In order to decrease run times HEC-RAS does not update the cross sections every computational increment. Instead it tracks erosion or deposition at a cross section until a minimum bed change is achieved. When this threshold is exceeded at one of the cross sections the bathymetry is re-computed. The default is 0.02 feet which will generally be exceeded in streams with relatively active beds. This tolerance can be increased to lower run times or decreased to make the model more sensitive to bed change.
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Chapter 17 Performing a Sediment Transport Analysis Minimum Cross Section Change Before Recomputation of Hydraulics: Similarly, hydraulic parameters are not automatically computed after each computational increment unless one of the cross sections has undergone appreciable change. This conserves computational resources since the frequency with which hydraulic computations are performed drives run times. This parameter is, by default, set equal to the Minimum Bed Change Before Updating Cross Section parameter so that every time the cross sections change, the hydraulics are recomputed but can be edited separately. Volume Change Method: When HEC-RAS transfers depositional or erosional volumes into bed elevation it generates computational and remainder errors. Rather than significantly increasing run times by iterating to achieve a bed geometry that reflects the volume to a very small tolerance, HEC-RAS checks for errors and carries remainders over into the next time step. This option is selected by default and is recommended unless results are being compared with HEC-6 which does not use this option.
Cross Section Weighting Factors Sediment transport calculations can be highly sensitive to local changes in channel hydraulics. This sensitivity can generate instabilities. Therefore, it is often advantageous, from a stability perspective, to dampen the effects of abrupt changes between cross sections by averaging the hydraulic parameters (e.g., slope, depth, etc.) of several cross sections when performing sediment transport at a given cross section/control volume. Dampening the effects of these transitions by averaging parameters with one or more cross sections upstream and one or more cross sections downstream will increase the stability of the simulation but decrease the accuracy. There are five pieces of averaging information that a user may edit for an internal cross section (Figure 17-19). First, the number of cross sections to be averaged upstream, and the number of cross sections to be averaged downstream of the cross section of interest must be specified. The hydraulic parameters of these cross sections will be averaged with the hydraulics of the given cross section. The defaults are 1, meaning that only the cross sections immediately upstream and downstream of the computed node are used when averaging parameters. Next, the user must decide how much weight to assign to the upstream cross section(s), the given cross section of interest, and the downstream cross section(s). The defaults of 0.25/0.5/0.25 (Figure 17-19) designate a weighted average of hydraulic parameters that will give 50% weight to the computational node (cross section of interest) and 25% weight to the upstream and downstream cross sections. If there are multiple upstream or downstream cross sections, then the combined average of all of these cross sections together will be weighted 25% (in this example) for the final average hydraulics. The cross section weightings for upstream and downstream nodes must be handled differently. These parameters are separately specified for the upstream and downstream boundaries, since these have no upstream and downstream cross sections respectively to assign weight to. The default
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Chapter 17 Performing a Sediment Transport Analysis condition is unaveraged parameters for the upstream boundary condition (the upstream cross section accounts for 100% of the hydraulic parameters) and averaged parameters (50%/50%) for the downstream boundary node and the cross section immediately upstream.
Sediment Output Options and Tolerances Set Number of Variables The user can control the number of output variables, as well as the frequency at which HEC-RAS will generate them. The number of variables is controlled by selecting the Output Level. The Output Level can be chosen from the Sediment Output Options window. The default output level is 4, which generates a file reporting 14 variables at each time step. However, sediment output files can get very large, on the order of GB’s, and may have difficulty loading output for long runs into the viewers at that size. Therefore, more or fewer output variables can be requested by selecting the appropriate Output Level (Figure 17-20). At levels 5 and 6 several output variables are represented by grain class. Since there are up to twenty active grain classes, this can increase the output file size significantly. The variables associated with each level are detailed in Table 17-2.
Figure 17-20. Sediment output options editor.
Output in Mass or Volume? Converting between mass and volume in HEC-RAS is a relatively simple matter of multiplying or dividing by the unit weight of the material. Some users prefer to view the magnitude of erosion, deposition, transport or other variables either in mass or volume units. Volume is the default reporting unit but it can be switched to mass using this option.
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Chapter 17 Performing a Sediment Transport Analysis
Table 17-2. Variables associated with each level of output. Level 1 Level 2 Bed Elevation Bed Elevation WSE Observed Invert Change
Level 3 Bed Elevation WSE Observed Invert Change Velocity Flow Shear XS Mass Out Tot
Level 4 Level 5 Level 6 Bed Elevation All From Level 4 and… All From Level 5 and… WSE d50 Cover Sediment Discharge (Tot) Observed Data d50 Surface Channel Manning's n Invert Change d50 Inactive Channel Froude # Flow Cover/Active Layer Thickness Shear Velocity (u*) Velocity Subsurface Layer Thickness d90 Cover Shear Mass Cover (All) d90 Surface Energy Grade Slope Mass Surface (All) d90 Inactive Mass Out Tot Mass Inactive (All) Effective Depth Mass Out Cumulative Sediment Concentration Effective Width Mass Bed Change Tot Dredged Volume Mass Bed Change Cum Tot Mass Capacity tot Mean Effective Channel Invert Mean Effective Channel Invert Longitudinal Cum Mass Change (All) = This variable is output as a total for all materials and separately for each of the 20 grain classes Tot = Only total for all grain sizes combined Cum/Cumulative =Cumulative mass from the beginning of the simulation to the current time WSE = Water Surface Elevation Delta Bed = Change of bed elevation
Set Output Increment Another way to reduce the size of the output files, without reducing the number of variables reported, is to increase the Number of Increments Between Outputs (Figure 17-20). First the basic ‘unit’ of the output increment must be selected. The actual frequency of output will be specified as multiples of this increment. Output Increment: HEC-RAS has four options for specifying the sediment transport computational increment. Two are numerical time steps used in the quasi-unsteady flow formulation and the other two are absolute time. The default output increment is the computation increment specified for each flow in the Quasi-Unsteady Flow Editor (Figure 17-16). By specifying the computational increment output could be viewed multiple times during each specified flow. If this option is selected, output will also be provided at the end of each flow duration whether it is a multiple of the computation increment or not. The second option is to use the flow duration, or the end of each separately specified flow. By specifying either the Computation Increment or the Flow Duration as the Output Increment the time series or animation could be skewed in time. Since these are irregular time steps and it is common to use a smaller concentration increment during large or rapidly changing flows these options will distort the temporal scale. However, since high and rapidly changing flows are often where most of the bed change occurs, it can be useful for visualizing changes. The other two options, hours and days, set the output increment to a constant absolute time. This avoids the temporal distortion in the time series plots and animations but can result in skipping over interesting changes.
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Chapter 17 Performing a Sediment Transport Analysis Once a base output increment is selected the actual elapse time between outputs is controlled by a multiple of the increment. Output for the spatial and time series plots are controlled separately from the cross section plots. The spatial (profile) output and the time series output are generated based on a multiple of the base increment. For example, in Figure 17-20 output is generated every 10 computation increments. It is worth noting that output like Mass Out (the mass leaving the control volume in a time step) or Mass In (the mass entering the control volume in a time step) or Mass Bed Change (the amount of deposition or erosion in the control volume in a time step) only reports results for the last computational increment. Therefore, if the output increment is longer than the computational increment, the data is not summed and reported at the output increment. Instead a series of ‘snapshots’ in time are reported. The cumulative variables are usually more helpful for tracking mass through a simulation.
Set Cross Section Bed Change Output HEC-RAS can plot and animate cross section changes computed as part of a sediment run. Cross section data is usually much more memory intensive since the vertical position of each node on each cross section is stored. Therefore, cross section output is turned off by default and must be selected by checking the Cross Section Bed Change Output check box. When the cross section output is turned on the multiple that controls cross section output is multiples of the profile output. For example, in Figure 17-20 output is generated every 10th profile output or every 100th computation increment. To set the cross section output to write for every profile the Number of Profiles Outputs Between XS Outputs would be 1. As a default HEC-RAS will output variables after every tenth computational increment and at the end of each flow duration. Choosing a less frequent output interval yields a minor improvement in run times. WARNING: Choosing a high Output Level (ex. 6) and a low time increment between outputs (ex. 1) can produce extremely large sediment output files.
Dredging Sometimes sediment is removed from the bed mechanically by processes that can not be predicted by physical computations. In particular, dredging can remove significant material from sediment transport systems that cannot be predicted by standard transport computations. Sometimes calibration periods span dredging events or the very purpose of a model is to predict system response to dredging configurations. Therefore, HEC-RAS includes the capability to define dredging events as part of a sediment simulation. The Dredging Events Editor (Figure17-26) can be found in the Options menu of the Sediment Analysis Editor.
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Chapter 17 Performing a Sediment Transport Analysis
Figure 17-26. Dredging events editor.
To simulate dredging you must first create a Dredging Event by pressing the New Dredging Event button . A dredging event consists of a date and time and a set of dredging ‘templates’ defined at one or more cross sections. Each dredging event happens at a specific time so that a given cross section could be dredged several times in a given simulation. Presently, only a rectangular dredging template is available. The dredged template at each cross section for a given dredging event is defined by right and left extents and an elevation. Any material between the left and right extents and above the specified elevation will be removed from the cross section at the defined time (Figure 17-27). The Dredging Event Editor also includes several tools to facilitate template construction and interpolation. These tools are analogous to those found on the Channel Modification Editor. A template can be projected upstream or downstream from a given cross section on a constant slope.
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Chapter 17 Performing a Sediment Transport Analysis
(a) Left Dredge Limit
Right Dredge Limit
Volume Removed Dredge Elevation
(b)
Figure 17-27. (a) Schematic of a dredging template and an example cross section. All of the material inside the volume is removed at the dredge event time. The resulting cross section is depicted in (b).
Sediment Transport Energy Slope
Most sediment transport equations are highly sensitive to the energy slope used. By default HEC-RAS computes this slope locally at the cross section by back calculating the friction slope from Manning’s equation. However, at times, HEC 6 took the actual slope of the Energy Grade Line as the slope used in the sediment transport equations. This option (Average Energy Slope) is therefore available under OptionsSediment Transport Energy Slope on the Sediment Analysis Editor but will be rarely used (Figure 17-28).
Figure 17-28. Sediment transport energy slope editor.
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Chapter 17 Performing a Sediment Transport Analysis
Running a Sediment Analysis Once a plan is created, and geometry, quasi-unsteady flow and sediment files are specified, a sediment analysis can be performed by pressing the Compute button. A progress screen will appear, showing the status of the run. Results can be viewed at any time during the simulation up to the time step that HEC-RAS is calculating.
Viewing Results When sediment computations are performed, detailed sediment and hydraulic output are written to a separate binary file. The detailed output from the sediment computations can be displayed as spatial plots and tables, as well as time series plots and tables. To access these plots and tables go to the View menu on the main HEC-RAS window and select Sediment Spatial Plot or Sediment Time Series Plot.
Profile Plot There are a wide array of variables that can be accessed either in plot or table form by selecting Sediment Spatial Plot from the View Menu of the main HEC-RAS dialog. These include: thalweg elevation, water surface elevation, velocity, bed change, and an array of weights and volumes tracked by layer and grain size.
Figure 17-21. Sediment spatial plot.
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Chapter 17 Performing a Sediment Transport Analysis One or more variables can be plotted on the same plot, and these variables can be plotted for one or more profiles, as shown in Figure 17-21. The profile can also be animated to view profile change over time. It should be noted, however, that HEC-RAS outputs a profile at the end of each computational increment, so animated profiles may be distorted in time if portions of the simulation have different computation increments. Tabular output can also be viewed under the Table tab of the Sediment Spatial Plot viewer. Additionally, a schematic plot is available (Figure 1730) where the variables can be viewed in plan view with a graded color scheme. The color scheme can be edited by selecting View Color Scheme on this window.
Figure 17-30. Schematic representation of depth of deposition.
Time Series Plot Similarly, by selecting Sediment Time Series and RC Plot from the View Menu of the main HEC-RAS dialog a user can plot the change in the same variable(s) over time at a single cross section (Figure 17-31).
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Chapter 17 Performing a Sediment Transport Analysis
Figure 17-31. Sediment time series plot.
XS Bed Change Plot Initial and final station-elevation plots are generated for each cross section after a sediment transport analysis. Addition or more frequent cross section output can be obtained by selecting XS Bed Change Output in the Sediment Output Options dialog. Cross section output is available under ViewSediment – XS Bed Change Plot… on the main HEC-RAS window. Cross section shape change can be animated or plotted simultaneously at various times (Figure 17-32). It is important to view at least the final cross section shapes to make sure that erosion and deposition were simulated in physically believable patterns. Additionally, it may be advantageous to generate a new geometry file based on an intermediate or final adjusted bathymetry generated by a sediment simulation. This can only be done from the Sediment XS Changes menu. Select the simulation time at which you would like to generate a new geometry file and press the Create Geometry File… button. You will then be prompted to save a new geometry file under a new name. The new geometry file will include the cross sections shapes displayed in the Sediment XS Changes editor for that profile including any erosion or deposition computed by the program.
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Chapter 17 Performing a Sediment Transport Analysis
Figure 17-32. Cross section shape for a single station after four different simulation times.
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Chapter 18 Sediment Impact Analysis Methods (SIAM)
CHAPTER
18
Sediment Impact Analysis Methods (SIAM) SIAM is a sediment budget tool that compares annualized sediment reach transport capacities to supplies and indicates reaches of overall sediment surplus or deficit. SIAM is a screening level tool to compute rough, relative responses to a range of alternatives, in order to identify the most promising alternatives (which should then be modeled in more detail). The algorithms in SIAM evaluate sediment impact caused by local changes on the system from a sediment continuity perspective. The results map potential imbalances and instabilities in a channel network and provide the first step in designing or refining remediation. Users can begin with existing geometry and flow data and develop a set of sediment reaches with unique sediment and hydraulic characteristics. The SIAM program will then perform sediment transport capacity computations to determine potential imbalances and instabilities in a channel network. SIAM does not predict intermediate or final morphological patterns and does not update cross sections, but rather indicates trends of locations in the system for potential sediment surpluses or deficits. The results can be used to design or refine remediation efforts in the system.
Getting Started SIAM is located in the Hydraulic Design Functions module and can be accessed by selecting Hydraulic Design Functions under the Run menu or by pressing the HD button. SIAM is not the default Hydraulic Design tool, so it must be selected from the Type menu from the Hydraulic Design editor. The SIAM window in the HD editor is depicted in Figure 18-1.
Defining a Sediment Reach The HEC-RAS hydraulic model must initially be subdivided into sediment reaches. A sediment reach is a grouping of cross sections with relatively consistent hydraulic and sediment properties. Hydraulic parameters are averaged over the cross sections comprising a sediment reach and a single set of sediment data is entered for it. When the user first opens SIAM, they will be prompted to provide a name for the first sediment reach. Additional sediment reaches can be created by selecting New Sediment Reach under the sediment menu. Sediment reaches must be defined such that all cross sections are included within one and only one sediment reach. The four drop down selectors: River, Reach, US RS, and DS RS are designed to designate
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Chapter 18 Sediment Impact Analysis Methods (SIAM) the upstream and downstream cross sections that form the limits of the sediment reach. (Figure 18-1) Sediment reaches cannot cross junctions and, therefore, must exist entirely within the same hydraulic reach. For example, if an HEC-RAS model contains three hydraulic reaches, it must have three or more sediment reaches. Hydraulic reaches should be subdivided into sediment reaches if they have significantly distinct hydraulic properties, hydrology or sediment data.
Figure 18-1. SIAM editor in the hydraulic design window with the bed material tab active.
Once the sediment reaches are defined, they must be populated with data. There are five data tabs: Bed Mat’l – Bed material gradation data Hydro – Annualized flow distribution Sed Prop – A variety of sediment properties required to run the model Sources – Accounting of local and annual sediment sources to the reach Hydraulics – Reach weighted averaged hydraulic parameters for the sediment reach (automatically populated by HEC-RAS) Each of these data tabs must be completed before the model will run.
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Chapter 18 Sediment Impact Analysis Methods (SIAM)
Entering Data
Bed Material Each sediment reach requires bed material information. However, any number of bed material sampling records can be defined in the Bed Mat’l records tab. A given bed material sample can be used exclusively for one sediment reach or can be shared by more than one sediment reach. The record shown in the sampling drop-down box when data is saved will be the record assigned to the sediment reach active at that time. When a new sediment reach is selected, the contents of the tabs records are automatically saved to the previously active sediment reach. The SIAM window with the bed material tab activated is shown in Figure 18-1. When a new SIAM project is started, after the user enters a name for the new sediment reach, a prompt will be given to name a new bed sampling record. Once this is done, the new bed sampling record will appear in the Sampling drop-down box, as will all other created bed material records. Once a bed sample template is created the gradation can be specified in the grid. Twenty grain classes are available. The name and geometric mean grain size for each class are displayed. Gradation is entered as the percent of the total sediment gradation, which is finer than the listed particle diameter, by weight (e.g a number between 0 and 100). Any grade classes that are not assigned a percent finer value will be treated as if they do not exist in the bed material. A text box is located at the bottom of the tab for the user to add notes that identify or otherwise describe the currently active bed material record. Buttons available on the bed material tab are: Create a new Bed Material Sampling record. Rename the current Bed Material Sampling record. Delete the current Bed Material Sampling record. Plot the current Bed Material Gradation curve.
Hydrology Before a SIAM model can be developed a standard, steady flow HEC-RAS model must be created and run. The SIAM Hydro tab is automatically populated with Hydrology records when a new sediment reach is defined. By default, the new hydrology record will be named “Hydro – (Sediment Reach Name)”. Although this record must remain with the sediment reach it was
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Chapter 18 Sediment Impact Analysis Methods (SIAM) created with, the user can change its name. The Hydro tab is shown in Figure 18-2.
Figure 18-2. Hydrology data tab.
The Profile column is automatically populated with the profile associated with the current plan file. The Ch Q column is also automatically populated with a sediment reach length-weighted channel discharge. These values update if the bounding cross sections of the sediment reach change. SIAM predicts annual trends and is based on an annualized flow duration curve. Therefore, the populated profiles must be distributed over 365 days. The user enters duration increments in the Duration column, for each profile, in units of days per year. These durations should sum to 365. SIAM will utilize all of the days input for its annualized flow and will not normalize to a year. Water temperature is also required for each profile. This allows the user to vary the temperature seasonally. Buttons available on the Hydro tab are:
Rename the current Hydrology record. Plot the current Duration curve.
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Chapter 18 Sediment Impact Analysis Methods (SIAM) An example plot of a full hydrologic record is shown in Figure 18-3.
Figure 18-3. Plot of annualized duration curve.
Sediment Properties Sediment Property records are similar to the Bed Gradation templates in that a given Sediment Property record can be used exclusively for one sediment reach or can be shared by more than one sediment reach. These properties are defined in the Sed Prop tab. The record shown in the Prop. Group drop-down box at the time data is saved, will be the record assigned to the currently active sediment reach. The SIAM window with the Sed Prop tab activated is shown in Figure 18-4.
Transport Function SIAM uses one of six transport functions to compute the annualized transport capacity. The appropriate equation is selected from the drop down box labeled Transport Function. Results are very sensitive to the transport function selection so care should be taken when selecting this option. For more description of these functions see Chapter 17 as well as the technical reference manual. Separate transport functions can also be applied for different grain classes by selecting Multiple Transport Functions by Grain Size from the Options menu item. When this is selected, the transport function dropdown box becomes a command button with the caption Multiple Transport Eqs. Clicking this button to accessed the multiple transport functions grid, as shown in Figure 18-5.
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Chapter 18 Sediment Impact Analysis Methods (SIAM)
Figure 18-4. Sediment property tab.
This grid lists the 20 grain classes and their respective geometric mean particle diameter size. By clicking on a cell in the Transport Eq. Column, the user can access a drop down box which allows selection of a grain class specific transport equation. Once selected, all cells below the currently active cell populate with the same transport equation. Note: The grain class specific transport equation feature should be used with caution. When two different transport functions are used to compute transport potential for adjacent grain classes a discontinuity is often introduced. This could result in such difficulties as a larger computed potential for the larger grain class or an unreasonable drop in transport potential from one grain class to another. If this option is selected, pay careful attention to the results for material around the size of the transport transition(s). A similar caution should also be observed when attributing different transport equations to different reaches. This will cause spatial rather than gradational discontinuities and should be approached with similar caution.
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Chapter 18 Sediment Impact Analysis Methods (SIAM)
Figure 18-5. Grain class specific grain class function feature.
Fall Velocity Method The Fall Velocity Method drop down box allows the user to select the method of fall velocity computation. If Default is selected, the method associated with the respective transport function in the literature is used. Otherwise, the selected fall velocity method will be used. The three fall velocity methods available are: Toffaleti, Van Rijn, and Rubey.
Wash Load Max Class, Diameter Wash load is the material in the system, but not present in appreciable quantities in the bed. SIAM does not apply the standard transport equations to compute a mass balance for wash load materials. Instead, it automatically passes them through the sediment reach. If the wash load threshold drops from one sediment reach to the next adjacent downstream reach, the material in the grain class(es) that is no longer wash load is added to the bedload and subjected to the standard mass balance approach.
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Chapter 18 Sediment Impact Analysis Methods (SIAM) A wash load threshold must be set for each sediment reach. The drop down box labeled Wash Load Max Class lists 10 grade classes (clays through sands for the standard grain classes) and their upper bound particle size in mm.
Specific Gravity The specific gravity of sediment is also required. It can be entered in the field labeled Specific Gravity. The default is 2.65.
Conc. of Fines (opt) The concentration of fine sediments is an optional value used to adjust the transport rate for high concentration scenarios. The adjustment is based on Colby’s (Colby, 1964) findings regarding the effects of fine sediment and temperature on kinematic viscosity, and consequently particle fall velocity. Values are given in parts sediment per one million parts water, by weight.
Sediment Sources In order to compare capacity to supply, sediment supply data must be entered. In SIAM sediment annual source information is entered for each sediment reach. This information is specified on the Sources tab. (Figure 18-6) Each Sediment Reach requires a Source Group, a collection of sediment source records. A given Sediment Source Group record can be used exclusively for one sediment reach or can be shared by more than one sediment reach. Before sediment supply information can be selected for a Source Group source templates must be created. Sediment source records can be created or edited by selecting the Define/Edit Sediment Sources button. An inset window will appear for source definition. Press the “new record” button and name the source template. In addition to naming the source, a source Type also must be selected. A source can be labeled: gully, bank, surface erosion or other. This is only a grouping descriptor and has no impact on the computations. Once the sediment load template is generated the annual load must be specified by grain size. The second column of the table displays the upper grain size diameter limit of each grain class. Annual loads in tons/year are entered in the third column (Figure 18-6). When the sediment sources are specified, close the source editor by pressing the OK button or the << Define/Edit Sediment Sources button, then select the appropriate source templates for each source group. A source record can be selected by clicking on the Name column of the Sources table. A drop down menu will appear populated with the source templates previously created. When a source record is selected the Type column will automatically populate. The Multiplier column defines the relative magnitude of the load.
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Chapter 18 Sediment Impact Analysis Methods (SIAM) If the load record represents the load coming into the reach precisely then enter a multiplier of 1 and the numbers entered will be used.
Figure 18-6. Define new sediment sources on the sources tab.
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Chapter 18 Sediment Impact Analysis Methods (SIAM)
Figure 18-7. Source selection.
However, if the load was entered per linear bank foot or per watershed acre (see note at the bottom of the load template in (Figure 18-6) then the material entering the reach will be the source record multiplied by the multiplier entered. For example, in Figure 18-7 the coarse bank material is entered in annual load generated by each linear foot of bank. In Figure 18-7 this load is then multiplied by the length of exposed banks in the sediment reach. Additionally, a negative multiplier can also be entered, which will cause material to be removed from the sediment reach.
Hydraulics The final tab is the Hydraulics tab. HEC-RAS computes this information and populates the table on this tab automatically. For each Hydro record, HECRAS computes a single set of hydraulic parameters for each sediment reach from the associated backwater profile, based on a reach weighted average of the included cross sections. The parameters in the grid are all sediment reach length-weighted values taken from the channel (not the full cross section) and are automatically updated if the bounding cross sections of the sediment reach are changed (Figure 18-8). Values cannot be changed directly on the grid by the user.
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Chapter 18 Sediment Impact Analysis Methods (SIAM)
Figure 18-8. Average hydraulic properties populated by HEC-RAS.
Options Several user capabilities are available in the Options menu. These options provide analysis flexibility in several aspects of the computations.
User Defined Particle Sizes The grain size bins used by default in HEC-RAS are based on a standard log base 2 scale based on the American Geophysical Union (AGU). This option allows the user to redefine the particle size class ranges to either simplify the analysis or provide more detail in a certain grain size range. The user can enter in the upper and lower bound of the first grade class and the upper bound of the rest of the grade classes (Figure 18-9). Lower bounds automatically adjust to eliminate gaps. The grain class labels can also be edited. Edited grain class names and sizes will appear in the corresponding dialogs. If the Defaults button is selected, all of the grid entries will return the AGU default values.
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Chapter 18 Sediment Impact Analysis Methods (SIAM)
Figure 18-9. Variable grain class boundary editor.
Multiple Transport Functions The multiple transport functions option allows the user to specify distinct transport functions for different grain classes. A more detailed description of this feature is included on page 5.
Remove Cross Section from Sediment Reach It may occasionally be desirable to omit one or more cross sections within a defined sediment reach from the hydraulic parameters averaging and sediment transport computation. If the hydrodynamics at a cross section are spurious and non-typical they may be omitted by de-selecting them in the editor depicted in Figure 18-10.
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Chapter 18 Sediment Impact Analysis Methods (SIAM)
Figure 18-10. Cross section de-selection editor.
Set Budget Tolerances When SIAM displays output, results are color coded in three categories: sediment deficit, surplus or equilibrium. Since the supply will never precisely equal the capacity, equilibrium is a range of acceptable deficit or surplus. This acceptable zone is strongly site and project specific and therefore must be entered by the user. The budget tolerance editor (Figure 18-11) allows the user to set a range of acceptable fluctuation (in tons/year) that will be displayed as equilibrium for each reach.
Command Buttons Four command buttons can be found in the upper right corner of the dialog. The Apply button will store the entries on the current window into memory. The Compute button launches a computational window depicted in Figure 18-12. To execute the SIAM computations press the Run SIAM Computations button. Computation times are generally short. Very complex models will run in several seconds. Finally, the Tables button provides access to SIAM output after an analysis is conducted.
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Chapter 18 Sediment Impact Analysis Methods (SIAM)
Figure 18-11. Sediment budget tolerance editor.
Figure 18-12. SIAM computation window.
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Chapter 18 Sediment Impact Analysis Methods (SIAM)
Model Output Once SIAM has completed computations it will update the inset schematic display to reflect the results. Sediment reaches for which a deficit is calculated are colored red while surplus reaches are colored blue and those that fall within the equilibrium tolerance will be green. (Figure 18-13) The quantitative local balance for each sediment reach can be queried by clicking on the colored region.
Figure 18-13. Plan view surplus and deficit display.
Plots and tables are available by pushing the Table button above the display. The standard output is Local Balance which reports the annualized sediment surplus or deficit for each sediment reach. Output can be viewed in tabular (Figure 18-14) or graphical format (Figure 18-15). All plots are bar graphs. In either tabular or graphical form multiple HD files and reaches can be selected or deselected too look at different scenarios or simplify the plot. Lists of the available reaches and HD files are available by pressing the HD File and Reaches buttons.
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Chapter 18 Sediment Impact Analysis Methods (SIAM)
Figure 18-14. Tabular local balance output.
Figure 18-15. Graphical local balance Output.
The following output options are available from the Type menu:
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Chapter 18 Sediment Impact Analysis Methods (SIAM) Local Balance: the annualized surplus or deficit for a given reach Sediment Transport Potential: the transport potential computed for each grain size as if it comprised 100% of the bed material. These numbers are prorated by their relative abundance in the bed to compute transport capacity. Supply and Balance: a summary plot that reports local supplies and the capacity which are compared to compute the local balance (also reported). It also breaks the supply into bed supply and wash supply components. Then there are several tables and plots where output is reported by grain size. Reaches can be activated or deactivated for these output options but because of the additional dimension multiple HD files cannot be viewed simultaneously. The grain size specific outputs are: Local Supply: sums the total annual sources applied to each sediment reach by grain size. Annual Capacity: reports the computed, cumulative, annual capacity for each reach and breaks it down into the capacity contribution of each grain class. Wash Material and Bed Material: summarize the total wash and bed material supplies for each reach and the relative contributions of each grain class. Local Balance: reports the same local balance output as depicted in Figure 18-14 except it also depicts the local balance for each grain class (Figure 18-16). It is of note in this figure that different grain classes can report deficits and surpluses in the same reach. Normalized Local Balance: Since longer reaches will generally have exaggerated local balances when compared to shorter reaches, the normalized local balance divides the result from each sediment reach by the reach’s channel distance. Therefore, local balance is reported per linear channel foot, making it easier to compare reaches of different lengths.
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Chapter 18 Sediment Impact Analysis Methods (SIAM)
Figure 18-16. Local balance by grain size.
Notes on Program Applicability and Limitations SIAM is not a sediment routing model. A mobile bed model will update hydraulics in response to sediment deficits and surpluses generally resulting in mitigated rates of erosion or deficit over time, as the channel adjusts its morphology. SIAM does not update the bed and, therefore, does not account for changing capacities in response to erosion or deposition. Therefore, SIAM should be used as a screening tool for sediment budget assessment. The numbers reported should be treated cautiously and interpreted as general trends of surplus and deficit not volumes of eroded or deposited material. One of the advantages of SIAM is the ease with which sensitivity, management or design alternatives can be evaluated. SIAM should be used to assess the impact of a wide range of alternatives in order to select the best few for more detailed modeling and analysis.
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Chapter 19 Water Quality Modeling
CHAPTER
19
Performing a Water Quality Analysis The water quality module uses the QUICKEST-ULTIMATE explicit numerical scheme (Leonard, 1979, Leonard, 1991) to solve the one-dimensional advection-dispersion equation. Individual sources and sinks as well as computed concentrations are available model output. The model simulates fate and transport of water temperature, arbitrary conservative and nonconservative constituents, dissolved nitrogen (NO3-N, NO2-N, NH4-N and Org-N), dissolved phosphorus (PO4-P, OrgP), algae, CBOD, and dissolved oxygen. In order to run the water quality model, a working, calibrated HECRAS unsteady or steady flow model must already be in place.
Getting Started There are three sets of water quality menus. The water quality data entry menu manages input data and calibration parameters; the water quality analysis menu manages simulation options and controls, and finally output tools manage model output files to facilitate viewing and exporting model results. Water Quality Data Entry Water quality boundary data, meteorological data and source and sink parameters are entered in the Water Quality Data Window. This window is accessed from the main water quality input either through the menu bar by selecting Edit… Water Quality Data or by selecting the Water Quality Data Icon. Water Quality Analysis All water quality data simulations are performed by first opening the Water Quality Analysis Window. This window is accessed from the main water quality input either through the menu bar by selecting Run… Water Quality Analysis or by selecting the Water Quality Analysis Icon . Water Quality Results
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Chapter 19 Water Quality Modeling Water Quality results are available in either spatial or time series format. Plots and tables are accessed from the main HEC-RAS window by selecting View… Water Quality Spatial Plot or View… Water Quality Time Series Plot.
Water Quality Data Entry The Water Quality Data Entry Window is opened by selecting Edit… Water Quality Data or by selecting the Water Quality Data Icon. This water quality data entry window is divided into three panes. The navigation bar is oriented as a vertical column at the far left. Its tree structure allows the user to access all input data and parameters. The two panes to the right of the navigation bar change in response to the selection on the left. For example, when a water quality data file (the top selection in the Navigation Bar) is selected, subwindows appear to the right allowing choice of WQ Constituents to be modeled, and to adjust water quality geometry (the lengths of Water Quality Cells), as shown in Figure 19-1.
Figure 19-1. Opening the main water quality data window.
To start a new water quality analysis, select the top row of the Navigation Bar (the line that says New Water Quality File), and enter a name for the data set. Next, in the Constituent Selection panel, select the desired water quality constituents to be modeled (Temperature Modeling Nutrient Modeling, and Arbitrary Constituents), any one or all of them can be turned on at the same time.
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Managing Water Quality Data Files To save, delete, or rename the current water quality file, highlight the file name at the top of the navigation bar. Once highlighted, select File… in the menu bar, as shown in Figure 19-2.
Figure 19-2. Managing water quality data files.
Water Quality Constituents The model organizes constituents and sources and sinks into three major groups. Temperature Modeling computes heat energy sources and sinks and water temperature. Nutrient Modeling simulates nutrients, dissolved oxygen, CBOD, and algae. Because most of the rate constants in the nutrient model are temperature dependent, nutrients may not be modeled unless water temperature is also simulated or set to a fixed value. Arbitrary Constituents are simple tracers, configured by the user. Arbitrary constituents are independent of water temperature and nutrients.
Water Quality Cells When the water quality model is opened for the first time, water quality cells are initially established between cross sections. Water quality computational points are located exactly between cross section pairs. An example of this default configuration is shown in Figure 19-3. In this schematic, water quality cells have been filled with alternating green (shaded) and yellow color to help distinguish them from their neighbors. In many cases, the water quality model can be run immediately using this default configuration and no further adjustment is necessary; however, in situations where hydraulic cross sections have been placed very close together (such as around bridges or other hydraulic structures), some of these (default) water quality cells may be very small. A very small water quality cell surrounded by larger cells is a challenging computational problem
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Chapter 19 Water Quality Modeling that may lead to instability. A single small water quality cell will force the model to choose a correspondingly small time step in order to satisfy the Courant and Peclet conditions (discussed later in this chapter in the Water Quality Analysis window under Water Quality Simulation Options). Smaller time steps lead to longer simulation times. If the small water quality cell is not necessary for purposes of water quality analysis, the user may wish to group small water quality cells together into larger ones.
water quality cells
hydraulic cross sections water quality computational points
Figure 19-3. Default water quality cell configuration: a single water quality cell has been placed between each pair of cross sections.
An example of the result of grouping water quality cells together into larger cells is shown in Figure 19-4. Note that regardless of grouping, water quality cells faces are always coincidental with hydraulic cross sections. When cells are combined, internal cross sections are sometimes incorporated into larger water quality cells. Once cells are combined, every water quality cell face will still be coincidental with a hydraulic cross section; however, not all cross sections must be associated with a water quality cell face. Computational points are always located at the center of a water quality cell. When cells are combined, the computational point is located at the center of the (new) combined cell. water quality cells hydraulic cross sections
water quality computational points
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Figure 19-4 Combined water quality cell configuration: five water quality cells combined into a single (larger) water quality cell.
Combining Water Quality Cells Grouping smaller cells into larger ones results in a reduction in the number of total cells in the system, increasing the length of the time step and reducing overall computation time. Water quality cells are combined by changing the minimum cell length. Minimum cell length is accessed in the Water Quality Data window as shown in Figure 19-5. Setting the minimum water quality cell length directs the software to combine water quality cells to ensure that the all cells are at least as long as this user specified minimum. The Set… button displays a subwindow allowing the user to input a minimum cell length as shown.
Figure 19-5. Combining water quality cells by choosing the maximum cell length.
The absolute minimum cell length that will be accepted is the smallest hydraulic reach length (this length is the default upon opening the program). To return to this default setting, decrease the Minimum Water Quality Cell Length to 1. The minimum cell length will then correspond with the minimum reach length in the system geometry. To assist in configuring water quality cell lengths, maximum and minimum cross section spacing and water quality cell lengths are printed to the window subpane. Cross section spacing is the maximum and minimum distance
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Chapter 19 Water Quality Modeling between cross sections. Water quality cell lengths indicate the longest and shortest water quality cells in the system. Changing the water quality cell length has no effect on hydraulic computations. It is not necessary to re-run the unsteady (or steady) flow simulation after adjusting water quality cell lengths. Fixed Faces… allows the user to establish particular cross sections as fixed faces. Such fixed faces will always be located at the boundary of water quality cells.
Viewing Water Quality Cell Configuration Once water quality cell lengths have been adjusted it is sometimes useful to review a table showing cross sections bounding each water quality cell and associated water quality cell lengths. Show Table of WQ Cells… displays the results of the current configuration. In this table, cells are listed along with River Stations that bound them and their lengths. An example is shown below in Figure 19-6. For example, WQ Cell 10 is bounded by the interpolated cross-sections 115581* and 115537* and it is 44.462 meters in length.
Figure 19-6. Table of water quality cell lengths.
Entering Boundary Condition Data A time series (boundary condition) must be specified for each modeled constituent at all locations where flow enters the system including: upstream boundaries of the main channel and its tributaries and lateral inflows. If the
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Chapter 19 Water Quality Modeling modeled reach is tidal, a boundary condition must also be included at the tidal boundary.
Viewing Required Boundary Condition Data Sets Locations of required boundary conditions are determined from hydraulic model output. Expanding the Boundary Conditions entry in the navigation bar lists locations of required boundary conditions with icons specific to type: Upstream boundary (positive flow across boundary) Lateral inflow (flow into water quality cell not at boundary) Downstream boundary (negative flow across boundary) In the figure below, data sets at four boundaries are required: the upstream boundary around RS 115959, and tributaries or lateral inflows around RS 98334, RS 85416 and RS 80521. Highlighting the water quality constituent (water temperature in this example) plots all data that will be applied at simulation time. For example, time series have already been entered for RS 115959 (blue) and RS 98334 (black). A constant value of 22˚C has been entered for RS 80521 (grey) and a constant value of 20˚C has been entered for RS 85416 (green).
Figure 19-7. Combined plot of all entered boundary conditions.
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Unattached Boundary Condition Data Sets There must be a boundary time series (or constant value) entered at every flow boundary. If the associated flow boundary of an existing water quality boundary time series is deleted, the water quality information becomes unattached. For this special case (when no flow information is available at a water quality boundary location) an icon with a question mark appears at the location.
The question mark indicates that the data is present in the water quality data file but it will not be used in the simulation.
Entering Boundary Condition Information To enter boundary condition information, select the desired location in the navigation bar. The Boundary Condition Data Entry Window will appear to the right and all data available for the location will be plotted in the lower window, as shown in Figure 19-8.
Figure 19-8. Entering boundary condition information.
The data entry window allows the user to enter data from up to four sources. In Figure 19-8, data has been entered from two sources. A constant value of 16 ˚C has been entered, and a time series has also been entered. Both are shown on the plot. Two other data sources are also available. Input data may also reside in an excel spreadsheet or in a DSS file and be referenced by the water quality model. The data source currently chosen in the Data Source pulldown is applied at simulation time. Data Units Although the water quality model uses SI units internally, data sets may be imported in a variety of units. For example, a temperature time series may
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Chapter 19 Water Quality Modeling be entered in degrees Fahrenheit or Celsius. If Fahrenheit is selected, the program converts the time series to Celsius when the data is read for use by the water quality model. Units are selected using the pulldown menu. Meteorological input data is always plotted in its original units.
Data Plotting Choosing Plot only the selected Data Source plots the currently selected data source. This is the data source that will be used the next time a water quality simulation is run. Choosing Plot all the defined Data Sources shows all data sources currently entered or referenced. Data Source Selection Data may be entered from one or all of the following sources: 1. Table Data – Data is entered directly into HEC-RAS and is stored with project model files. 2. DSS – Reference to DSS file is entered. Data is stored in DSS file. 3. Excel Worksheet – Reference to Excel Worksheet is entered. Data is stored in Excel file. 4. Constant value – Single value is entered into HEC-RAS. Value is stored with project model files. Note that the data entry window changes in response to the data source selection. 1.
Entering and Editing Table Data
Choosing Table Data from the Data Source selection field opens the Enter/Edit Table… subwindow. This time series editor includes tools to create a times series (and to manipulate the values of a time series once it has been created). The time series editor and its tools are summarized in Figures 19-9 through 19-15.
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Figure 19-9 Time series editor and tools.
Time Series Generation Tool The time series generation button sets the beginning and ending time of the time series as well as the data time interval. Make Date/Time Column Data creates an empty time series with a data and time column but with no data. Once this time series has been created, data can be typed into the table, or pasted into the table from the clipboard.
Figure 19-10. Using the time series generation tool.
Add Offset This tool adds the amount entered in the text box to the currently selected (highlighted) area range in the table or to the entire table. Note that if the the data column is selected, this number will also be added to the date. For dates the offset is in days (i.e. adding ten adds ten days).
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Figure 19-11. Using the add offset tool.
Multiply by Selected Amount This tool multiplies the amount entered in the text box with the currently selected (highlighted) area range in the table or to the entire table. Note that unlike the offset tool, dates are not affected by this command.
Figure 19-12. Using the multiply by selected amount tool.
Set to Specified Value This tool sets the selected cells or the entire table to the value entered in the text box. In this example, the selected cells will be set to the value 15.0.
Figure 19-13. Using the set selected cells to specified value tool.
Find and Replace Standard find and replace with the user entered value.
Figure 19-14. Using the find and replace tool.
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Chapter 19 Water Quality Modeling Time Series Interpolation Tool This time series tool uses linear interpolation to fill in missing values. Blank cells will be interpolated; the selection must include both the cell(s) with missing values, the cell before, and the cell after the blank cells. Cells must be highlighted before applying the interpolation tool. No submenu appears. Missing cells are simply filled in.
Add Rows to Time Series This tool adds the entered amount of blank rows to the current time series Facilitates cut and paste from the clipboard into a time series. Note that it is not necessary for the time series to be evenly spaced.
Figure 19-15. Using the add rows to time series tool.
2.
Referencing DSS Data
The Data Source selection DSS allows the user to select the DSS file and path (Figure 19-16). Once the path is selected and the DSS path sub window is closed, both the DSS file and path will be filled. DSS referenced data will continue to reside in the DSS file.
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Figure 19-16. Referencing data in a DSS file.
3.
Referencing Excel Worksheet Data
From the Data Source filed, select Excel Worksheet (Figure 19-17). The Worksheet name must be entered directly. The time series time column and value column do not have to be located side by side. However, the top and bottom row of the time and value columns must be the same. As is the case with the worksheet name, the Time Column, Value Column, Top Row and Bottom Row must all be entered directly. Data will continue to reside in the (referenced) excel worksheet.
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Figure 19-17. Referencing data in an excel worksheet.
3.
Setting a Constant Value
A constant value may also be entered. One value is entered into the subwindow as shown in Figure 19-18.
Figure 19-18. Entering a constant value.
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Entering Initial Conditions The Initial Conditions Editor is selected by choosing the Initial Conditions heading in the Navigation Bar. At least one initial condition value is required for each modeled constituent in each reach. Initial conditions may be read from a restart file or they may be entered by hand (as an initial temperature distribution). The initial conditions subwindow contains radio buttons to choose between a restart file and an initial condition distribution. The restart file (if one has been chosen) and the initial condition distribution entry table remain in the subwindow regardless of the active choice. The model schematic is color coded with the active choice and is supplied as a graphic in the lower pane.
Entering an Initial Distribution An initial distribution requires at minimum one value for each constituent for each reach. Once initial conditions have been entered, they are interpolated to generate a table of initial conditions at all water quality computational points. An example is shown in the Figure 19-19.
Figure 19-19. Entering initial conditions in the water quality data window.
Use Add RS location(s) to the table… to add cross section locations and icon to delete them if necessary. In order to view the result of use the the interpolation, select Show cell Interpolated values… . A second window will appear, displaying values at the center of each water quality cell.
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Chapter 19 Water Quality Modeling Each water quality cell is identified by the two cross sections that bound it as shown in the Figure 19-20.
Figure 19-20. Viewing table of initial conditions after interpolation across all water quality cells.
Note that because initial conditions are assigned to particular cross sections, it is not necessary to re-enter initial conditions if water quality cell lengths are changed.
Using Water Quality Restart Files Water Quality restart files allow the user to save results of a (previous) water quality simulation and to use those results as initial conditions in subsequent simulations. To specify a restart file, navigate to it using the file open icon. Water quality restart files are identified by the suffix “.wqrst”. Restart files are not automatically generated. Restart file options are accessed though the Water Quality Computation Options subwindow. See the Water Quality Analysis section later in this chapter for details on how to create a restart file.
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Entering Dispersion Coefficients Dispersion coefficients may be assigned to as few as one or as many as all cross sections. Selecting Dispersion Coefficients in the navigation bar brings up a list of locations where dispersion coefficients have already been entered, as shown in the Figure 19-21.
Figure 19-21. Entering and editing dispersion coefficients.
Use Add RS location(s) to the table… to add cross section locations and use the
icon to delete them if necessary.
If more than one dispersion coefficient is entered, values will be interpolated across all river stations. To view the result of the interpolation, select Show cell Interpolated values… Dispersion is a face property, so results of this interpolation are reported at individual cross sections (Figure 19-22).
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Figure 19-22. Viewing interpolated dispersion coefficient values.
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Using Computed Values for Dispersion Coefficients An alternative to user assigned dispersion coefficients is to have the water quality model compute dispersion coefficients based on hydraulic variables at each face. This method avoids potential model instability that may occur when a cross section with a large face area and large dispersion coefficient is immediately followed by a cross section with a small face area and small dispersion coefficient. Selecting Computed Values in the navigation bar, turns this option on, as shown in the Figure 19-23.
Figure 19-23. Choosing the Computed Values option for Dispersion Coefficients.
The equation for model computed dispersion coefficients is:
D m 0.011 m u w y u*
u 2 w2
(19-1)
y u* = = = = =
user assigned multiplier face velocity average channel width average channel depth shear velocity
and shear velocity is computed as:
(unitless) (m/s) (m) (m) (m/s) (19-2)
u * gdS g
=
gravitational constant
(9.81 m/s2)
d S
= =
average channel depth friction slope
(m) (unitless)
Note that the user assigned multiplier (m) is the Multiplier shown in the toolbar in Figure 19-23.
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Chapter 19 Water Quality Modeling Finally, Lower Limited for Computed and Upper Limit for Computed are user defined values that constrain the computed dispersion values. The equation for computed dispersion is based on the equation given by Fischer (1979). Fischer’s equation, which did not include the multiplier, is an estimate of shear flow dispersion based on hydraulic and geometric quantities (velocity, channel width, depth, and slope). Fischer’s comparisons of field observations and equation 19-2 suggest that this equation is a good estimation within a factor of four or so. Enter 1.0 to obtain Fischer’s original form. Enter 0.0 to obtain a diffusion coefficient of 0.0. No limits have been placed on the multiplier, aside from that it must be positive.
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Entering Meteorological Data In order to model water temperature, at least one full meteorological data set must be available. The model supports multiple meteorological data sets. Each water cell is individually assigned to a particular data set. Each meteorological data set must contain weather information including: atmospheric pressure; air temperature; humidity (vapor pressure, relative humidity, wet bulb or dew point); solar radiation; wind speed; and cloudiness. A time series of air temperature, humidity, and wind speed radiation with a sampling frequency of at least once per three hours is necessary for simulation of diurnal water temperature variation. A constant value for air temperature, humidity, wind speed, or solar radiation should not be used except for testing purposes. A time series of measured solar radiation is preferred. However, if solar radiation observations are not available, a time series of solar radiation may be computed based on the longitude and latitude of the site, the day of year, and the time of day. Atmospheric pressure is a required input to the water temperature model. If a time series cannot be obtained, it may be estimated from site elevation. In addition to meteorological time series, each data set includes a limited amount of physical information including latitude, longitude, and site elevation. Water temperature model calibration parameters are also stored with meteorological data sets. Calibration parameters include the dust coefficient (used only if a synthetic solar radiation time series is applied) and wind function parameters (used to control the magnitude of sensible and latent heat).
Organizing Meteorological Datasets If local meteorology varies significantly within the model geography, it is may be useful to apply multiple meteorological data sets. In Figure 19-24, time series information from four sites has been entered into the model. To create New, Copy, Rename, and Delete meteorological data set files, use the dedicated buttons in the upper pane as shown in the figure. Water Quality Cell Assignments Each water quality cell must be associated with a meteorological dataset. Choose either Nearest Meteorological station (assigns each water quality cell to the closest meteorological station), or Assign in table (allows specific water quality cells to be assigned to meteorological datasets within a user accessible table).
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Figure 19-24. Organizing meteorological data sets.
Meteorological Station Physical Description A handful of physical properties are required for each meteorological dataset. These are accessed by clicking on an individual meteorological station in the navigation bar. An example is shown in Figure 19-25.
Figure 19-25. Entering physical location information for a meteorological data set.
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Chapter 19 Water Quality Modeling Reference Elevation Although a time series of atmospheric pressure is preferred, if none is available, the local atmospheric pressure is estimated from the Reference Elevation entered in this subwindow. The reference elevation should correspond to an average land surface elevation. Latitude, Longitude and Standard Meridian Latitude, longitude, and standard meridian are required if a solar radiation time series is computed. If the weather station is located close to the river, the longitude and latitude entered here should be that of the location of the weather station. If the weather station is remote, or if weather data is compiled from multiple sources, choose a central longitude and latitude. If the system to be modeled is large it may be necessary to construct multiple meteorological data sets each with its own synthetic solar radiation time series. The Nearest Standard Meridian is the location corresponding with the local time zone. It is important that this time be the time zone in which all other time series (including flow and constituent boundary conditions) have been entered. Station Location Schematic coordinates of station location orients the meteorological station with the x-y coordinates of the river schematic. These are not geo-referenced coordinates, they are used only to determine the nearest meteorological station to each water quality cell if Nearest Meteorological Station is chosen in the water quality cell assignment window The easiest way to determine the coordinates of the meteorological station is to orient your mouse over the map provided in the lower window (Figure 1923) to the approximate location of the meteorological station. As you move the mouse, its location will be shown in the lower right hand corner. Write down the x- and y- coordinates that best approximate the location of the weather station. Enter these local x- and y-coordinates into the station location text box (Figure 19-24). These coordinates will only be used if Nearest Meteorological Station is selected under WQ Cell Assignment. Picture Filename Allows the user to assign a picture to be displayed in the window as shown in Figure 19-25.
Entering an Atmospheric Pressure Time Series Atmospheric or barometric pressure is the pressure exerted by all gases in moist air. Atmospheric pressure is a strong function of elevation and varies
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Chapter 19 Water Quality Modeling with local meteorology. It generally decreases with increasing altitude. sea level, observed values are on order of 1000mb.
At
Atmospheric pressure may be entered into the water quality model in units of millibars (mb), millimeters of mercury (mmHg), inches of mercury (inHg) or atmospheres (atm). The data will be converted to mb at simulation time. In a manner analogous to the way boundary condition information is managed and stored, weather time series may be referenced and or stored from a handful of sources. A time series may be entered directly into a table to be stored with the water quality model; a reference may be made to data residing in a DSS file; a reference may be made to data in an Excel Worksheet; a constant may be entered; or for the case of atmospheric pressure, an approximation may be made from the physical elevation of the site. An example is shown in Figure 19-26. In the figure, a constant value (1 atm) was entered, and since an elevation was entered for the site, the atmospheric pressure has also been estimated (at around 1000 mb). Because Plot all the defined Data Sources has been selected, both are plotted on the same graph. Note that they are both plotted in their original units.
Figure 19-26. Viewing and organizing atmospheric pressure data.
The data source selected in the pull down is the value that will be applied at the time of simulation. In this case, the value applied would be converted from 1 atm. Although it is preferable to obtain a time series of atmospheric pressure, this data is difficult to obtain and a constant (either entered or estimated) is often used instead. The model is much less sensitive to diurnal variation in atmospheric pressure than it is to diurnal variation in air temperature, humidity, wind speed, and solar radiation.
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Entering an Air Temperature Time Series A time series of air temperature is required input for the water temperature model. Air temperature may be entered in units of Celsius or Fahrenheit. Although a constant value is an available option, it is stressed that a time series is necessary if diurnal variations in water temperature are to be simulated. The constant value option is included for testing purposes. An example of the air temperature data entry window and an air temperature time series is shown in the Figure 19-27.
Figure 19-27. Viewing and organizing air temperature data.
Entering a Humidity Time Series A time series of humidity is required input for the water temperature model. Humidity may be expressed as relative humidity (%), wet-bulb temperature (ºC or ºF), dew-point temperature (ºC or ºF), or vapor pressure (mmHg, inHg, or mb). Although a constant value is an available option, it is stressed that a time series of humidity is necessary if diurnal variations in water temperature are to be simulated. An example of a humidity time series displayed as vapor pressure in mb is shown in Figure 19-28.
Figure 19-28. Viewing and organizing humidity data.
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Entering a Solar Radiation Time Series Measured solar radiation is often available from a local weather station. Satellite data is also available. The most common units for solar radiation are W/m2, cal/cm2/day and MJ/m2/day. Data can be entered in any of these units, internal calculations are performed in W/m2. If direct measurement cannot be obtained, solar radiation may be computed from the longitude and latitude of the site, the time of day, cloudiness, and a user supplied dust coefficient which represents local atmospheric attenuation and is often used as a calibration parameter. Solar radiation is the primary driver for the water temperature model.
Dust Coefficient The Dust Coefficient is entered in the shortwave radiation time series window (indicated by the red box in the figure below). The dust coefficient estimates attenuation of solar radiation by dust (due to scattering and absorption). The range of the dust coefficient is between 0 and 0.2 (the larger the dust coefficient, the greater the attenuation). Generally speaking, coefficients for urban areas tend toward 0.2; coefficients for rural areas toward 0. The dust coefficient is often treated as a calibration parameter. An example of a solar radiation time series generated from latitude, longitude, cloudiness, time of day, day of the year, and the dust coefficient is shown in the Figure 19-29.
Figure 19-29. Computing solar radiation from latitude, longitude, cloudiness, time of day, day of the year, and the user entered dust coefficient.
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Entering a Cloudiness Time Series Cloudiness is the fraction of sky covered with clouds and varies from 0 to 0.9. Cloudiness is a required parameter for both calculated solar radiation and downwelling longwave radiation. An increase in cloudiness leads to a decrease in computed solar radiation and an increase in computed downwelling longwave radiation. A rough guideline for cloudiness (Cl) is: Overcast skies Broken skies Scattered clouds Clear skies
0.9 0.5 - 0.9 0.1 - 0.5 0.1
An example time series is shown in Figure 19-30.
Figure 19-30. Entering cloudiness time series information.
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Entering a Wind Speed Time Series Wind is a necessary parameter for surface flux (latent and sensible heat) estimation. Common units for wind speed are meters per second, miles per hour, and feet per second. Data can be entered in any of these units; internal calculations are performed in meters per second.
Anemometer Height The standard height of a wind gage (anemometer) is two meters, and the surface flux formulations used in this model have been developed for wind measured at this standard height. However, anemometers are not always positioned at this height, particularly in urban areas. If the recording anemometer has been mounted at a non-standard height, select the NonStandard Height option (Figure 19-31), enter the elevation at which the anemometer is located and select the option for surface characterization that best describes the surrounding area. Enter the raw data recorded by the anemometer into the table data editor. It will be corrected for height and surface roughness. The plot window will show both the original and corrected data. The Figure 19-31 shows a time series of wind speed recorded by an instrument located at ten meters above the ground and the corrected wind speed at the standard height of two meters above the ground.
Figure 19-31. Entering wind speed time series information.
Wind Function Parameters The primary calibration parameters for the surface fluxes are the wind function parameters ‘a’, ‘b’, and ‘c’, and a flux partitioning coefficient (Kh/Kw). A Richardson’s number type stability adjustment is also provided and can be turned on or off using the Use Richardson number check box. See the Water Temperature Parameters section later in this chapter for a more detailed discussion of these parameters.
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Nutrient Parameters State variables for the nutrient model are: Dissolved Nitrite Nitrogen (NO2) Dissolved Nitrate Nitrogen (NO3) Dissolved Organic Nitrogen (OrgN) Dissolved Ammonium Nitrogen (NH4) Dissolved Organic Phosphorus (OrgP) Dissolved Orthophosphate (PO4) Algae (A) Carbonaceous Biological Oxygen Demand (CBOD) Dissolved Oxygen (DOX)
(mgN/L) (mgN/L) (mgN/L) (mgN/L) (mgP/L) (mgP/L) (mgA/L) (mgBOD/L) (mgDO/L)
Pathways between each of these state variables are controlled by user adjustable rate constants. These rate constants as well as other related parameters are set in the Nutrient Parameters subwindow, accessed via the navigation bar in the Water Quality data window, as shown in Figure 1932.
Figure 19-32. Entering Water Quality Nutrient Source and Sink Parameters.
Each of the Nutrient Parameters may be set by (1) typing in a cell in the table or (2) clicking on a symbol in the graphic to the right. Note that the pathways in the graphic are highlighted when the corresponding selection is made in the table. Each of the values has a suggested range that is displayed
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Chapter 19 Water Quality Modeling when the mouse is passed over the variable. Setting variables out of the suggested range is allowed in most cases; however, the field will be highlighted in yellow to indicate that the value falls outside of the expected range. Nutrient parameters include rate constants for physical and chemical reactions between algae, nitrogen, phosphorous, dissolved oxygen, CBOD, and sediment. These rate constants control the rates of the source / sink term (S) in the advection dispersion equation:
V Q x A x S t x x x V Φ Q Г A S
= = = = = =
(19-1)
volume of the water quality cell (m3) water temperature (C) or concentration (kg m-3) flow (m3 s-1) user-defined dispersion coefficient (m2 s-1) cross sectional area (m2) sources and sinks (kg s-1)
Source/Sink terms for each of the state variables as well as their user adjustable rate constants are discussed in this section of the manual.
Temperature Dependence of Rate Reactions Some water quality reactions are strongly influenced by temperature. Rate constants are specified at a reference temperature of 200C and are corrected to the local water temperature. The relationship between reaction rate and temperature is modeled with the Arrenius rate law (EPA 1985):
kT k20 T 20 kT k20 θ
(19-2) = = =
rate constant at temperature T rate constant at 200C temperature correction coefficient
Many of the rate constants used in the nutrient submodel are temperature dependant. The rate constant that is specified in the table is the rate at 200C. This rate constant is specified along with the empirical temperature correction coefficient (θ). The temperature correction coefficient for these water quality reactions is generally set to 1.024 for physical reactions and to 1.047 for chemical reactions with some exceptions. In the source sink equations that follow, an asterisk (*) after a rate constant indicates temperature dependence.
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Chapter 19 Water Quality Modeling
Algae The water quality model supports only phytoplankton algae which are freefloating and consume nutrients from the water column. Algal growth and respiration affects algal concentration (A), nutrient concentrations (NH4, NO3, PO4, OrgN and OrgP), and dissolved oxygen (DOX). During the day, algal photosynthesis produces dissolved oxygen and during the night respiration utilizes oxygen. Algae utilize phosphorus and nitrogen in their dissolved inorganic forms (NH4, NO3, and PO4). Algae are a source of the organic forms of nitrogen (OrgN) and phosphorus (OrgP). Sources and sinks as well as parameters that control their rates are described in detail in this section. User adjustable parameters for algae, default values and suggested ranges are summarized in Table 19-1, at the end of this section.
Chlorophyll-a (CHL) Chlorophyll-a is a commonly monitored water quality parameter; however it is not a modeled state variable. Chlorophyll-a concentrations (ug/L) are measures of the gross level of phytoplankton. Chlorophyll-a does not provide information on species levels, nor does it group algae into classes. Chlorophyll-a is a parameter in the non-linear formulation for algal growth light limitation (discussed later in the section). The linear relationship between chlorophyll-a and algal biomass is:
CHL 0 A CHL α0
(19-3) = =
chlorophyll-a concentration (µg Chl-a L -1) ratio of chlorophyll-a to algal biomass (µg Chl-a / mg A)
The chlorophyll-a: algal biomass ratio is user set. It is not temperature dependent. See Table 19-1.
Algal Biomass Concentration (A) The single internal source of algal biomass (A) is algal growth. Two sinks are simulated: algal respiration and settling. Sources and sinks of algae are computed as: A
source/sink
A *
=
- A * -
1* d
algal growth
(19-4)
algal respiration
A
algal settling
19-31
Chapter 19 Water Quality Modeling
*
=
algal local respiration rate (day-1)
The algal respiration rate combines the process of endogenous respiration of algae, conversion of algal phosphorus to organic phosphorus and conversion of algal nitrogen to organic nitrogen. The three processes are lumped and the range is user set. Temperature dependent. See Table 19-1. σ1*
=
algal settling rate (m day
-1
)
The algal settling rate is user set. See Table 19-1. Temperature dependent. d
=
average channel depth (m)
Water column depth is passed to the water quality model from the hydraulic model. μ
=
local growth rate for algae (day-1)
The local algal growth rate is a function of the user specified maximum algal growth rate and algal growth rate limitation functions. * max GL
μ
max*
=
(19-5) local maximum growth rate for algae.
Local maximum growth rate is a user defined parameter set in the nutrient parameter table. It is temperature dependent. See Table 19-1. GL
=
algal specific growth rate limitation
Algal growth limitation is a function of available nitrogen, phosphorus, and light. It is described in detail below.
Algal Specific Growth Rate Formulation The maximum algal growth rate is computed as a function of one of two growth rate limitation functions: Leiberg’s Limiting Nutrient formulation or the Multiplicative formulation. The limitation function is selected in the Nutrient Parameter window from the pulldown at the top of the parameter table as shown in the Figure 19-33.
19-32
Chapter 19 Water Quality Modeling
Figure 19-33. Choosing the algal local specific growth rate formulation.
Leiberg’s law of the minimum limiting nutrient formulation limits growth due to light availability and the nutrient that is least available. When this formulation is chosen, the nutrient in excess does not affect the growth rate.
GL FL min( FP, FN )
(19-6)
The multiplicative formulation limits growth due to light availability and the availability of both nutrients.
GL FL FP FN
(19-7)
Nutrient Limitation for Nitrogen (FN) The nutrient limitation for nitrogen (FN) is a function of user entered Michaelis-Menton nitrogen half-saturation constant (KN), and concentrations of the state variables ammonium (NH4), and nitrate (NO3). The Michaelis-Menton half-saturation constants determine the efficiency with which phytoplankton uptake nitrogen (and phosphorous) at low concentrations. The limitation factor for nitrogen is the unitless expression:
FN
Ne Ne KN
(19-8)
Ne is the effective local concentration of available inorganic nitrogen
Ne ( NH 4) ( NO3) KN
=
(19-9) half-saturation constant for nitrogen (mg N/L)
KN is a user adjustable parameter. It is not temperature dependent. See Table 19-1.
19-33
Chapter 19 Water Quality Modeling Nutrient Limitation for Phosphorous (FP) The nutrient limitation for phosphorus (FP) is a function of the user entered Michaelis-Menton phosphorus half-saturation constant (KP), and concentration of inorganic phosphorus. The limitation factor for phosphorus is the unitless expression:
FP
PO4 PO4 KP FP KP
= =
(19-10) limitation for phosphorous (unitless) half-saturation constant for phosphorus (mg P/L)
KP is a user adjustable parameter. It is not temperature dependent. See Table 19-1. Limitation for Light (FL) The limitation for light (FL) is a vertically averaged formulation that has been integrated over depth.
FL
1 KL I o ln d KL I o e d
Io d λ
= = =
(19-11)
surface light intensity (W m-2) average channel depth (m) light extinction coefficient (m-1)
The light saturation coefficient (KL) is the half saturation constant defining the light level at which algal growth is one-half the maximum rate. Because almost all radiation outside of the visible range is absorbed within the first meter below the surface (Orlob, 1977 as cited in Bowie, 1985), surface light intensity (Io) applied to lake models (and to this river model) is often estimated to be a fraction of the solar radiation flux density used in heat budget computations. The magnitude of the visible range is roughly half the computed (or observed) short-wave (solar) radiation (qsw). In the HEC-RAS code, an attenuation coefficient is used to adjust the computed (or observed) short-wave radiation (qsw) used in energy budget computations for use as surface light intensity in the light saturation coefficient computation:
I o a sw q sw qsw asw
(19-12) = =
short-wave (solar) radiation (W m-2) short-wave radiation attenuation coefficient (unitless)
The short-wave radiation attenuation coefficient (asw) has been set to 0.50 for this release of the HEC-RAS water quality code. The light extinction coefficient (λ) is coupled to the limitation for light (FL) using the equation:
19-34
Chapter 19 Water Quality Modeling
0 1 0 A 2 0 A2 / 3 λ0 λ1 λ2
= = =
(19-13)
non-algal portion of light extinction coefficient (m-1) linear algal self shading coefficient (ugChla)(m-1L-1) non-linear algal self shading coefficient (ugChla/L)-2/3(m1 )
All three of the above coefficients are user set. = 0 λ2 = For no algal self-shading set λ1 ≠ 0 λ2 = For linear algal self shading set λ1 For nonlinear self shading set λ1 and λ2 to appropriate values.
0 0
Table of Rate Constants and Parameters for Algae Suggested values and ranges for algae parameters are provided in the Table 19-1. Defaults shown in the table match those set in model code. For most coefficients, defaults are set to the lowest value in the range.
Table 19-1. Suggested range and default values for algae rate constants and parameters. 1
Units
QUAL2E Suggested Range
Default Value
Temperature Correction Coefficient (Θ)
µg Chla -1 mgA
10 - 100
10
no correction
-1
1.0 – 3.0
1.0
1.047
-1
0.05 – 0.5
0.05
1.047
4 - 20
4
no correction
0.01
no correction
Symbol
Variable
Description
α0
alpha0
Conversion algae → chlorophyll-a
µmax*
mu_max
Maximum algal growth rate
day
rho
Algal respiration rate
day
KL
KL
KN
KN
KP
KP
λ0
lambda0
λ1
lambda1
λ2
lambda2
σ 1*
sigma1
1
Michaelis-Menton half saturation constant (light) Michaelis-Menton half saturation constant (nitrogen) Michaelis-Menton half saturation constant (phosphorus) Light extinction coefficient (non-algal) Algal self shading coefficient (linear) Algal self shading coefficient (non-linear)
Settling rate (algae)
-
W m sec
-1
mg N/ liter
mg P/ liter
m
-1
0.01 – 0.30
0.001 – 0.05
0.001
≥ 0.03
0.03
no correction
0.007 – 0.07
0.007
no correction
variable
0.05
no correction
0.1 – 2.0
0.1
1.024
no correction
-1
m / µg Chla/L -1
m / (µg 2/3 Chla/L) m day
-1
Brown and Barnwell (1987)
19-35
Chapter 19 Water Quality Modeling
Nitrogen Parameters Nitrogen species most commonly found in river water are dissolved organic nitrogen (OrgN); dissolved ammonium nitrogen (NH4); dissolved nitrite nitrogen (NO2); dissolved nitrate nitrogen (NO3); and particulate organic nitrogen. The first four (dissolved) forms are HEC-RAS state variables. Particulate organic nitrogen is not included in this version of the water quality model. Within natural aerobic waters there is a constant transformation of organic nitrogen (OrgN) to ammonium (NH4), to nitrite (NO2), and finally to nitrate (NO3). Dissolved ammonium is assumed to be the form NH4. Although ammonia (NH3) and ammonium (NH4) may be present in surface water, the pH at which the transformation of ammonia to ammonium ion is half complete is 9.24, above the pH of most rivers (Hem, 1985). Measurements for nitrate and nitrite in river water are common largely because elevated concentrations are of concern for human and livestock health. Nitrite (NO2) is rarely abundant in rivers (Meybeck, 1982 and Hem, 1985). Nitrate (NO3) is more stable and commonly found in natural waters. Because organic nitrogen (OrgN) and nitrite (NO2) are generally unstable in aerated water, elevated concentrations of organic nitrogen and nitrite are potential indicators of a waste discharge nearby. Nitrate (NO3) and ammonium (NH4) are also indicators of waste discharge, but because these forms are more stable, elevated concentration of nitrate and ammonium suggest a waste source further upstream (Hem, 1985). Elevated concentrations of ammonium (NH4) are more commonly associated with urban waste, elevated concentrations of nitrate (NO3) are more commonly associated with agricultural runoff (Meybek, 1982). A constant stepwise process transforms organic nitrogen (OrgN) to ammonium nitrogen (NH4) then to nitrite (NO2) and nitrate (NO3). In addition to these (internal) sources and sinks, algal growth and decay also consumes and produces nitrogen. Finally the bed acts as a potential source and sink of nitrogen. Sources and sinks as well as parameters that control their rates are described in detail in this section. User adjustable parameters for the nitrogen cycle, default values and suggested ranges are summarized in Table 19-2, at the end of this section.
19-36
Chapter 19 Water Quality Modeling
Sources and Sinks of Dissolved Organic Nitrogen (OrgN) The only internal source of organic nitrogen (OrgN) in the model is algal respiration. Organic nitrogen sinks include settling to the bed and hydrolysis to form ammonium nitrogen (NH4). Sources and sinks for the organic nitrogen (OrgN) pool are: OrgNsource/sinks
+ 1 * A
=
*
- 3 OrgN *
- 4 OrgN
Algal Respiration
(19-14)
Hydrolysis (OrgN → NH4) Settling
= fraction of algal biomass that is nitrogen (mgN mgA-1) α1 User set parameter that describes the fraction of algae that is contributed to the nitrogen pool. See Table 19-2. ρ* = algal local respiration rate (day-1) See algae section, and Table 19-1. = rate constant: hydrolysis of OrgN to ammonium (day-1) β3* User set parameter. Temperature dependent. See Table 19-2. = rate constant: organic N settling rate (day-1) σ 4* User set parameter. Temperature dependent. See Table 19-2.
Sources and Sinks of Ammonium Nitrogen (NH4) Internal sources of ammonium (NH4) include hydrolysis of organic nitrogen (OrgN), and uptake (diffusion) from the benthos. Internal sinks include oxidation of ammonium to form nitrite (NO2) and algal uptake. Sources and sinks for the ammonium nitrogen (NH4) pool are: *
NH4source/sinks = 3 OrgN +
3* d
- 1 (1 exp KNR DOX ) NH 4 *
F11A
Hydrolysis (OrgN → NH4)
(19-15)
Diffusion from benthos Oxidation (NH4 → NO2)
Algal uptake
19-37
Chapter 19 Water Quality Modeling β 3* = rate constant: hydrolysis of OrgN to ammonium (day-1) See organic nitrogen section and Table 19-2. = rate constant: oxidation of ammonium to nitrite (day-1) β1* The NH4 oxidation rate is user set. Temperature dependent. See Table 19-2. = benthos source rate: ammonium (mgN m-2 day-1) σ3* The NH4 benthos source rate is user set. See Table 19-2. d = average channel depth (m) Water column depth is passed to the water quality model from the hydraulic model. μ
=
local growth rate for algae (day-1)
See algae section. = fraction of algal biomass that is nitrogen (mgN mgA-1) α1 See organic nitrogen section and Table 19-2. KNR = first order nitrification inhabitation coefficient (mgO-1 L) User set parameter, generally set to 0.6. See Table 19-2. GL = growth limitation for algae (unitless) Computed value. See algae section for discussion. F1
=
fraction of algal uptake from ammonium pool (unitless)
The fraction algal uptake from the ammonia pool (F1) is a computed value. It is a function of the user entered nitrogen preference factor (PN) and the state variables ammonium (NH4) and nitrate (NO3):
F1
PN NH 4 PN NH 4 1 PN NO3
(19-16)
The preference factor for ammonia (PN) varies between zero and one. Setting the value to 1 indicates algal preference for ammonium (NH4). Setting the value to 0 indicates algal preference for nitrate (NO3). The default for this parameter is 1.0 (exclusive preference for ammonium). See Table 19-2.
19-38
Chapter 19 Water Quality Modeling
Sources and Sinks of Nitrite Nitrogen (NO2) The internal source of nitrite (NO2) is oxidation of ammonium (NH4) to nitrite (NO2). The only modeled sink is oxidation of nitrite (NO2) to nitrate (NO3). Sources and sinks for the nitrite pool are: NO2
Sources/Sinks
=
1 * (1 exp KNR DOX ) NH 4
Oxidation (NH4 → NO2)
- 2 * (1 exp KNR DOX ) NO 2
Oxidation (NO2 → NO3)
(19-17)
= rate constant: oxidation of ammonium to nitrite (day-1) β 1* See ammonium section and Table 19-2. = rate constant: oxidation of nitrite to nitrate (day-1) β2* Progresses rapidly. Temperature dependent. See Table 19-2. KNR = first order nitrification inhabitation coefficient (mgO-1 L) See Table 19-2.
Sources and Sinks of Nitrate Nitrogen (NO3) The only internal source of nitrate nitrogen is oxidation of nitrite (NO2) to nitrate (NO3). The only modeled sink is algal uptake. Sources and sinks for the nitrate pool are: NO3
Sources/Sinks
=
2 * (1 exp KNR DOX ) NO 2 -
1 F1 1A
Oxidation (NO2 → NO3) (19-18) Algal uptake
= rate constant: oxidation of nitrite to nitrate (day-1) β 2* See nitrite nitrogen section and Table 19-2. KNR = first order nitrification inhabitation coefficient (mgO-1 L) See ammonium nitrogen section and Table 19-2. = fraction of algal biomass that is nitrogen (mgN mgA-1) α1 See organic nitrogen section and Table 19-2. μ
=
local growth rate for algae (day-1)
See algae section. F1 = fraction of algal uptake from ammonium pool (unitless) See Table 19-2.
19-39
Chapter 19 Water Quality Modeling
Table of Rate Constants and Parameters for Nitrogen Suggested values and ranges for nitrogen parameters are provided in Table 19-2. Defaults shown in the table match those set in model code. For most coefficients, defaults are set to the lowest value in the range.
Table 19-2. Parameters for nitrogen sources and sinks. Default Value
Temperature Correction Coefficient (Θ)
Symbol
Variable
Description
Units
β 3*
beta3
Rate constant: OrgN → NH4
day
-1
0.02 – 0.4
0.02
1.047
β 1*
beta1
Rate constant: NH4 → NO2
day
-1
0.10 – 1.0
0.1
1.083
β 2*
beta2
Rate constant: N02 → NO3
day
-1
0.20 – 2.0
0.2
1.047
σ 4*
sigma4
Settling rate (Organic N)
day
-1
0.001 – 0.1
0.001
1.024
sigma3
Benthos source rate (dissolved N)
mg N m -1 day
Variable
0.0
1.074
KNR
KNR
Nitrification inhabitation coefficient
unitless
0.6 - 0.7
0.6
no correction
PN
PN
unitless
1 = NH4 0 = NO3
1
α1
alpha1
mg N -1 mgA
0.07 – 0.09
0.07
σ 3*
19-40
QUAL2E Suggested Range
Algal preference factor for ammonia Fraction algal biomass that is nitrogen
-2
no correction
no correction
Chapter 19 Water Quality Modeling
Phosphorus Two phosphorus species are modeled: dissolved organic phosphorus (OrgP), and dissolved inorganic orthophosphate (PO4). In the natural environment, phosphorus is obtained from dissolution of rocks and minerals with low solubility. Soil erosion in agricultural areas is a significant source of (particulate) phosphorus. Although phosphorus enters rivers primarily as particulate matter, particulate organic phosphorus is not included in this version of the water quality model. It is planned for future versions. Animal metabolic waste is another source of phosphorus. Phosphorus is present in sewage in the dissolved inorganic form (PO4) (Brown and Barnwell 1987). Sources and sinks as well as parameters that control their rates are described in detail in this section. User adjustable parameters for the phosphorus cycle, default values and suggested ranges are summarized in Table 19-3, at the end of this section.
Sources and Sinks of Organic Phosphorus (OrgP) The only internal source of organic phosphorus (OrgP) is algal respiration. Internal sinks for OrgP are decay of organic phosphorus (OrgP) to form orthophosphate (PO4), and settling to the bed. Sources and sinks for the organic phosphorus pool are:
OrgP
Sources/Sinks
= 2 * A
4*OrgP 5* OrgP
Algal respiration
(19-19)
Decay (OrgP → PO4) Org P settling
= rate constant: oxidation of OrgP to PO4 (day-1) β 4* User set. Temperature dependent. See Table 19-3. = settling rate: organic phosphorus (OrgP) (day-1) σ 5* User set. Temperature dependent. See Table 19-3. ρ* = algal local respiration rate (day-1) See algae section, and Table 19-1. = fraction of algal biomass that is phosphorus (mgP mgA-1) α2 User set parameter that describes the fraction of algae that is contributed to the phosphorus pool. See Table 19-3.
19-41
Chapter 19 Water Quality Modeling
Sources and Sinks of Orthophosphate (PO4) There are two internal sources of dissolved orthophosphate (PO4): decay of organic phosphorus (OrgP); and diffusion from benthos. The only internal sink of orthophosphate (PO4) is algal uptake. The differential equation that governs the orthophosphate pool is: PO4
Sources/Sinks
= 4OrgP +
Decay (OrgP → PO4)
2* d
- 2 A
(19-20)
Diffusion from benthos Algal uptake
σ 2* = benthos source rate: orthophosphate (PO4) (mgP m-2 day-1) Set by the user. QUAL2e does not provide a suggested range. Temperature dependent. See Table 19-3. = fraction of algal biomass that is phosphorus (mgP mgA-1) α2 See Table 19-3. μ
=
local growth rate for algae (day-1)
See algae section. d = average channel depth (m) Water column depth is passed to the water quality model from the hydraulic model.
Table of Rate Constants and Parameters for Phosphorus Suggested values and ranges for phosphorus parameters are provided in Table 19-3. Defaults shown in the table match those set in model code. For most coefficients, defaults are set to the lowest value in the range. Table 19-3. Parameters for phosphorus sources and sinks.
Symbol
β4
Temperature Correction Coefficient (Θ)
Description
Units
*
beta4
Rate constant OrgP → PO4
day
-1
0.01 – 0.7
0.01
1.047
*
sigma5
Settling rate (Organic P)
day
-1
0.001 – 0.1
0.001
1.024
*
sigma2
Benthos source rate (dissolved P)
mg P m -1 day
Variable
0.0
1.074
alpha2
Fraction algal biomass that is phosphorus
mg P mgA
0.01 – 0.02
0.01
no correction
σ2
19-42
Default Value
Variable
σ5
α2
QUAL2E Suggested Range
-2
-1
Chapter 19 Water Quality Modeling
Carbonaceous Biological Oxygen Demand (CBOD) Carbonaceous biological oxygen demand (CBOD) is a state variable. A first order reaction describes oxidation of CBOD. The carbon cycle is not modeled in this version of the model. Losses of CBOD include settling and decay via oxidation: CBOD Sources/Sinks
=
K1CBOD
(oxidation)
K 3CBOD
(settling)
(19-21)
= deoxygenation rate coefficient (day-1) K1* User set parameter. Temperature dependent. See Table 19-4. K3*
=
rate of loss of carbonaceous BOD from settling (day-1)
User set parameter. Temperature dependent. See Table 19-4.
Table of Rate Constants and Parameters for CBOD Suggested values and ranges for CBOD parameters are provided in Table 194. Defaults shown in the table match those set in model code. For most coefficients, defaults are set to the lowest value in the range.
Table 19-4. Parameters for CBOD sources and sinks. QUAL2E Suggested Range
Default Value
Temperature Correction Coefficient (Θ)
-1
0.02 – 3.4
0.02
1.047
-1
-0.36 – 0.36
1.024
Symbol
Description
Units
K1 *
Deoxygenation rate (CBOD)
day
K3 *
Settling rate (CBOD)
day
19-43
Chapter 19 Water Quality Modeling
Dissolved Oxygen (DOX) Dissolved oxygen sources are atmospheric reaeration and algal photosynthesis. Generally speaking, dissolved oxygen concentrations are less than saturation; however, photosynthesis can result in dissolved oxygen concentrations exceeding saturation. Losses include algal respiration, sediment oxygen demand, carbonaceous biological demand (CBOD), and oxidation of ammonium and nitrite. Sources and sinks of dissolved oxygen are: DOX
Source/Sink
=
K 2* Osat DOX reaeration
(19-22)
A 3 4 photosynthesis and respiration - K1CBOD -
K4 d
CBOD demand sediment demand
- 5 1 NH 4
ammonium oxidation
- 6 2 NO 2
nitrite oxidation
Osat = dissolved oxygen concentration at saturation (mgO L Computed value. Function of water temperature. = O2 production per unit algal growth (mgO mgA α3 User entered value. See Table 19-5. = O2 uptake per unit algae respired (mgO mgA α4 User entered value. See Table 19-5. = O2 uptake per unit NH4 oxidized (mgO mgN α5 User entered value. See Table 19-5.
=
carbonaceous BOD deoxygenation rate (day
)
)
)
-1
)
= O2 uptake per unit NO2 oxidized (mgO mgN α6 User entered value. See Table 19-5. K1*
-1
-1
-1
-1
)
-1
)
See CBOD section, and Table 19-4. K2* = reaeration transfer rate (day -1) Reaeration is the process of oxygen exchange between the water and the atmosphere across the air-water interface. In this model, reaeration is simulated as a flux gradient process, the product of a reaeration rate constant (K2) and the difference between the actual and saturated dissolved oxygen
19-44
Chapter 19 Water Quality Modeling concentration. This parameter is user set and temperature dependent. See Table 19-5. K4* = sediment oxygen demand rate (mg m2 day -1) This parameter is user set and temperature dependent. See Table 19-5. β 1*
=
rate of ammonia oxidation (day
-1
)
See ammonium nitrogen section and Table 19-2. β2*
=
rate of nitrite oxidation (day
-1
)
See nitrite nitrogen section and Table 19-2. d
=
average channel depth (m)
Table of Rate Constants and Parameters for DOX Suggested values and ranges for dissolved oxygen parameters are provided in Table 19-5. Defaults shown in the table match those set in model code. For most coefficients, defaults are set to the lowest value in the range.
Table 19-5. Parameters for dissolved oxygen sources and sinks. QUAL2E Suggested Range
Default Value
Temperature Correction Coefficient (Θ)
-1
1.4 – 1.8
1.4
no correction
-1
1.6 – 2.3
1.6
no correction
mg O mgN
-1
3.0 – 4.0
3.0
no correction
O2 uptake per unit of NO2 oxidized
mg O mgN
-1
1.0 – 1.14
1.0
no correction
K2
Reaeration rate
day
0 – 100
1.024
K4
SOD oxygen uptake
mgO -2 -1 m day
variable
1.060
Symbol
Variable
Description
Units
α3
alpha3
O2 production per unit algal growth
mg O mgA
α4
alpha4
O2 uptake per unit algae respired
mg O mgA
α5
alpha5
O2 uptake per unit NH4 oxidized
α6
alpha6
*
*
K2
K4
-1
19-45
Chapter 19 Water Quality Modeling
Arbitrary Constituent Parameters Running a simple simulation with a single arbitrary constituent with no decay is a prudent first step that should be taken before attempting to simulate water temperature or other more complicated water quality constituents. Arbitrary constituents are enabled by checking the Arbitrary Constituents box in the Water Quality Data submenu, as shown in Figure 19-33. To add an arbitrary constituent or edit an existing one, use the three buttons to the right of the arbitrary constituent list. The Add… and Edit… buttons will bring up the Arbitrary Constituent Information subwindow as shown in the figure. This subwindow allows the user to set the first order decay constant if desired. The differential equation that governs non-conservative arbitrary constituents is: Arbitrary constituent
Source/Sink
=
KC
(19-23 )
C
=
concentration of arbitrary constituent (state variable) (mg L-1)
K
=
rate constant (day-1)
Note that the user must explicitly enter a negative sign for this reaction to simulate decay. Figure 19-34 shows the water quality data window and the arbitrary constituent subwindow. Note that the K value is set to a negative number, for first order decay.
Figure 19-34. Creating and editing arbitrary constituents.
19-46
Chapter 19 Water Quality Modeling
Mass Injection HEC-RAS provides a method to introduce a quantity of mass rather than concentration into the model. This feature is useful for simulation of spills and for dye studies, when the problem requires the introduction of mass rather than concentration of a particular constituent. If you have not done so already, create an arbitrary constituent using the method described in the previous section. To introduce into the system, choose Mass Injection in the navigation bar of the Water Quality Data Entry Window as shown in Figure 19-35. Mass will be instantaneously introduced into the water quality cell just downstream of the river station selected using the Add RS location(s) to the Table… button. Use the Constituent pulldown to assign the mass injection to the selected constituent. Mass will be added during the water quality time step bounding the specified injection time. Multiple mass injections may be specified. If a time series is desired at a particular location it must be entered as a collection of instantaneous injections.
Figure 19-35. Creating an instantaneous mass injection point.
Water Temperature Parameters For heat transport the source/sink term is Heat
Source/Sink
=
q net As w C pw V
(19-24)
qnet
=
net heat flux at the air water interface (W m-2)
ρw
=
density of water (kg m-3)
Cpw
=
specific heat of water (J kg-1 C-1)
As
=
surface area of water quality cell (m2)
V
=
volume of water quality cell (m3)
19-47
Chapter 19 Water Quality Modeling
Net Heat Flux Net heat flux is computed as the sum of individual heat budget components:
qnet qsw qatm qb qh q qsw qatm qb qh ql
=
= = = =
(19-25)
solar radiation (W m-2) atmospheric (downwelling) longwave radiation back (upwelling) longwave radiation (W m-2) sensible heat (W m-2) latent heat (W m-2)
(W m-2)
Note: the water quality module of HEC-RAS does not simulate ice formation and ice decay, and the energy associated with these physical processes is not included in this energy budget. Once heat loss has progressed to the extent that water temperature reaches freezing, temperatures will be reported at 0˚C. When surface heating increases, no energy is lost to ice decay.
Solar Radiation Solar radiation is computed as:
q sw q o at (1 R )(1 0.65C 2 )
qo
Qo sin sin cos cos cosh r2
=
latitude (rad)
=
declination (rad)
h
=
local hour angle (rad)
Qo
=
the solar constant (1360 W m-2)
r
=
radius vector (unitless)
at
=
atmospheric attenuation
(19-26) (19-27)
Computed value is a function of: cloudiness, site elevation, air temperature, vapor pressure, dust coefficient. R
=
reflectivity of the water surface
Computed value is a function of: solar altitude, cloud cover. Cl
=
percent sky covered with clouds
User entered cloudiness time series.
19-48
Chapter 19 Water Quality Modeling See the technical reference manual for more details on solar radiation computations.
Atmospheric Longwave (Downwelling) Radiation Upwelling (or back) longwave radiation is computed as:
q atm a Tak4
(19-28)
εa = emissivity of air (unitless) Computed value f(air temperature, cloudiness) σ = Stefan Boltzman constant (W m-2 K-1) Physical constant. = air temperature (K) Tak Air temperature is a user entered time series value.
Back Longwave (Upwelling) Radiation Upwelling (or back) longwave radiation is computed as: 4 qb w TwK
(19-29)
= emissivity of water (unitless) εw Assumed constant (set to 0.97) Twk = water temperature (K) Water temperature is a state variable. step is used.
Computed value from previous time
Surface Fluxes The surface fluxes (latent and sensible heat) are closely related in their formulation. Both are flux gradient approximations. Both fluxes include an empirical wind function that is adjustable using the ‘a’, ‘b’ and ‘c’ coefficients.
Latent Heat
q
0.622 L w (es ea) f (U ) P
(19-30)
P = atmospheric pressure (mb) User entered time series
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Chapter 19 Water Quality Modeling L = latent heat of vaporization (J kg-1) Computed as function of water temperature ρw
=
density of water (kg m-3)
Computed as function of water temperature es
=
saturated vapor pressure at water temperature (mb)
Computed as function of water temperature ea = vapor pressure of overlying air (mb) User entered time series f(U) = the wind function (m s-1) User entered time series adjusted with user entered parameters
Sensible Heat
qh (
Kh )C p w (Ta Tw) f (U ) Kw
(19-31)
= Cp Constant
specific heat of air at constant pressure (J kg-1 C-1)
Ta
air temperature (C)
=
User entered time series Tw = water surface temperature (C) Water temperature is a state variable. Computed value from previous time step is used. f(U) = wind function (m s-1) Same as wind function in latent heat formulation Kh/Kw =
diffusivity ratio (unitless)
The diffusivity ratio (Kh/Kw) is a parameter that allows the user to partition flux between latent and sensible heat. It is generally set to unity but is allowed by the software to range between 0.5 and 1.5. A range of 0.9 to 1.1 is recommended. The wind function is:
f (U ) R a bU C
19-50
(19-32)
a=
user entered calibration coefficient on order of 10-6
b=
user entered calibration coefficient on order of 10-6
c=
user entered coefficient on order of one
R=
a function of the Richardson number set by the software to one unless the Use Richardson Number… box has been checked.
Chapter 19 Water Quality Modeling R is a function of air temperature, water temperature, and wind speed, varying from .03 under very stable conditions to 12.3 under unstable conditions. The Richardson number is a measure of atmospheric stability. Without the Richardson number included in the wind function, the function tends to underestimate mixing processes under unstable atmospheric conditions, under predicting the surface fluxes. The converse is also true. Without the Richardson number, the function tends to over predict the surface fluxes under stable conditions. The Richardson number is computed as:
Ri
g air sat z
(19-33)
air u 2
G = gravity (9.806 m s-2) ρair = density of moist air (at air temperature) (kg m-3) ρsat = density of saturated air (at water temperature) (kg m-3) z
= elevation of the recording station (m)
User entered physical description of the meteorological data set u = wind speed (m s-1) User entered time series The Richardson number is positive for stable atmospheric conditions, negative for unstable, and near zero for neutral conditions. The multiplier included in the wind function is set to unity unless the Use Richardson number… box is checked. If the box is checked, the multiplier is computed as a function of the Richardson number. For an unstable atmosphere (ρair > ρsat):
R 12.3
for
1 Ri
R (1 22 Ri ) 0.80
for
0.01 Ri 1
For a neutral atmosphere:
R 1
for
0.01 Ri 0.01
For a stable atmosphere (ρair < ρsat):
R (1 34 Ri ) 0.80
for
0.01 Ri 2
R 0.03
for
2 Ri
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Chapter 19 Water Quality Modeling Latent and sensible heat are difficult fluxes to estimate. The parameters provided are appropriate for many cases, but it important to keep in mind that some combinations can result in flux estimates that are not physically possible. The model will report individual energy budget terms in both time series format and spatial plots. Review of the magnitudes of latent and sensible heat fluxes is an important step in water temperature modeling that should not be overlooked. The ‘a’, ‘b’ and ‘c’ coefficients and the diffusivity ratio are entered in the wind speed submenu in the Water Quality Data window (Figure 19-36). Checking the Use Richardson number box includes the Richardson number in the wind function. Leaving this box unchecked sets the Richardson number to one.
Figure 19-36. Adjusting the wind function calibration parameters.
Entering Observed Data Time series of observed data may be entered into HEC–RAS to enable comparison with model results. Each observed data time series is associated with a particular cross-section. To enter an observed data time series, click on the Observed Data icon in the navigation bar as shown in Figure 19-37. Observed data time series may be moved, copied, deleted or created using the tools in the top pane of the data window.
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Chapter 19 Water Quality Modeling
Figure 19-37. Entering and adjusting observed data.
Choosing New… brings up a submenu that allows the user to select the cross section to which the new data will be assigned. A time series of observed data may be entered for any state variable at any location. These data will be available for plotting with model output. Once a new dataset has been created at a particular location, clicking on the new observed data location in the navigation bar brings up a submenu that allows a time series or constant value to be entered. Time series may be entered directly as Table Data, from a DSS file, or from an Excel spreadsheet. The Downstream distance form RS to gage option allows the user to enter a distance downstream of the associated river station to spatially locate the observed data time series on the WQ spatial data plot. Note that this adjustment is associated with spatial placement only. The observed time series will not be adjusted for the travel time associated with this downstream distance.
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Chapter 19 Water Quality Modeling
Figure 19-38. The water quality observed data entry window.
The observed dataset Type pulldown allows the user to choose between a continuous record and grab sample data. These options affect how data is displayed in the WQ spatial data plot. The continuous record option is preferred for regularly spaced time series data. The grab sample option is a convenient option for data that has been sampled infrequently. The applicable duration pulldown allows the user to choose the duration over which each of the entered points will be displayed on the WQ spatial data plot.
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Chapter 19 Water Quality Modeling
Water Quality Analysis Water quality data simulations are run through the Water Quality Analysis Window. This window is accessed from the main HEC-RAS window either through the menu bar by selecting Run… Water Quality Analysis or by selecting the Water Quality Analysis Icon.
Referencing the Hydraulics Plan Before a water quality simulation is run, a calibrated steady or unsteady hydraulic model must be in place. The water quality analysis window organizes water quality input files: the hydraulics plan; and the water quality data file. If the hydraulics plan is from a steady flow analysis, a profile must also be specified. The Water Quality Analysis window is shown in Figure 19-39.
Figure 19-39. The water quality analysis window with an unsteady flow plan selected.
Simulation Time Window The water quality simulation period is specified in this section. At runtime, two data requirements must be met. There must be necessary time series information for all selected water quality constituents that either coincide exactly with (or encompass) the water quality simulation period. These include time series information at flow boundaries, and meteorological information (if water temperature is selected). In addition to this water quality data requirement, a hydrodynamics flow field must also be available for the entire water quality simulation period. A water quality analysis may be performed using results from a steady flow or an unsteady flow simulation. If an unsteady flow simulation has been specified, the specified start and end times for the water quality simulation
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Chapter 19 Water Quality Modeling must fall within (or exactly on) the start and end times of the unsteady flow simulation. The two do not need to coincide exactly, but the water quality simulation period may not begin prior to or conclude after the simulation period of the unsteady flow simulation. If a steady flow simulation has been specified, the start and end times of the water quality simulation period are not constrained by the hydraulics solution.
Water Quality Simulation Options The simulation options subwindow is opened by selecting Simulation Options… under Options in the Water Quality Analysis menu bar as shown in Figure 19-40.
Figure 19-40. Opening the simulation options window.
Resolution of Hydrodynamic Continuity Error Because the hydraulics model and the water quality model do not solve continuity in exactly the same way, a small continuity error (in water volume) is sometimes encountered in the water quality model. The preserve concentration option adds or subtracts this small difference in volume, resolving the hydrodynamic continuity error. The constituent concentration associated with this (water) volume is assumed to be the concentration of the cell at the previous time step. Thus adding (water) volume to satisfy continuity adds both water and constituent mass to the system, and removing volume removes constituent mass. This method resolves any hydrodynamic continuity errors; however, it does so at the cost of conservation of constituent mass.
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Chapter 19 Water Quality Modeling The conserve mass option does nothing to correct for a (water) volume continuity error. That is, if continuity of water volume is not met, no water volume is added or subtracted (and no associated constituent mass is added or removed). This method conserves total system constituent mass; however because losses or gains in (water) volume associated with hydrodynamic continuity errors are not resolved, this option will result in a corresponding increase or decrease in constituent concentration. This method may result in irregular concentrations in some systems.
ULTIMATE Limiter Advection problems sometimes present challenges that can result in nonphysical oscillations. The universal limiter (ULTIMATE) developed by Leonard (1991) for the QUICKEST scheme has been included in the code. When ULTIMATE is on, computed cell concentrations are first tested for non-physical oscillation and monotonicity. If the solution reveals that either of these two conditions are not met, the computed solution is not reported, and the concentration of the cell face is used instead. Even when ULTIMATE is on, it is selective and it is active only when the cell face solution is found to be nonmonotonic.
Upper Limit on Computational Time Step Unlike the hydraulic model, which allows the user to set and adjust the time step, the time step used by the water quality computation engine is recalculated dynamically at each computation interval. This ensures model stability by determining a time step that satisfies the local Courant and Peclet constraint and minimizes run time by selecting the largest time step that satisfies these constraints. In order to enhance model stability, the water quality model constantly adjusts the model time step to ensure a Courant condition less than 0.9, and a Peclet number less than 0.4. The Courant number is a face property and is a function of the (previously computed) velocity, the time step, and the water quality cell length. The Peclet number is also a face property and is a function of the (user entered) dispersion coefficient and the water quality cell length. The time step is chosen to satisfy both the Courant (C) and Peclet (α) constraints:
C us u us
t 0.9 x
Cus uus ∆x ∆t
us us
t x 2
αus
= = = =
(19-34) local Courant number (dimensionless) velocity at water quality cell face (m s-1) length of water quality cell (m) time step (s)
0 .4 =
(19-35) local Peclet number (dimensionless)
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Chapter 19 Water Quality Modeling Γus ∆x ∆t
= = =
dispersion coefficient at water quality cell face (m2 s-1) length of water quality cell (m) time step (s)
Both the local Courant number and the local Peclet numbers are available as model output. To compute the optimal time step, the model code selects the smallest of three values: the maximum time step that satisfies the Courant constraint; the maximum time step that satisfies the Peclet constraint; and the user entered Maximum Allowable Time Step. The latter is selected in the Water Quality Computation Options sub-menu. This menu is obtained by selecting Run… Water Quality Analysis in the menu bar of the main RAS icon) followed by selecting Options… window (or by selecting the Simulation Options in the Water Quality Analysis window as shown in Figure 19-37. In some cases, the water quality computational time step may be longer than desired. For example, an hourly time step is desirable for water temperature simulation results. Setting the Maximum Allowable Time Step to one hour ensures that the time step will never be greater, even in cases of very slow velocities and long cell lengths. Note that the Courant and Peclet constraints can force a very short time step if water quality cell lengths are small. Small time steps lead to long computation time and large output files. For these reasons, it is recommended that small water quality cells be combined using the Water Quality Cell tool in the Data Entry Window.
Post Processing Daily maximum, minimum and average values are available as post processed information, and if selected they appear in an additional special output file. In order to create this special file, the Write max, min, mean and daily range to output file selection must be checked for the file to be created.
Output Options and Additional Output Variables In addition to state variable concentrations which are always available as model output, water quality sources and sinks and other incremental computations are also available as optional model output. These additional output variables must be selected in the Additional Output Variables section of the Water Quality Output Computation window, obtained by selecting Simulation Options… under the Options tab in the Water Quality Analysis window. Most of these additional output variables are component parts of the difference equation for advection diffusion. The equation is:
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Chapter 19 Water Quality Modeling
V n 1 n 1 * * * * Qdn up dn Adn up Aup V n n t Qup up SS t x dn x up t
(19-36)
where
n 1
=
Concentration at present time step (kg m-3)
n
=
Concentration at previous time step (kg m-3)
* up
=
QUICKEST concentration at upstream face (kg m-3)
* x up
=
QUICKEST derivative at upstream face (kg m-4)
up
=
upstream face dispersion coefficient (m2 s-1)
Vn+1
=
volume of the water quality cell at next time step (m3)
Vn
=
volume of the water quality cell at current time step (m3)
Qup
=
upstream face flow (m3 s-1)
Aup
=
(cross sectional) upstream face area (m2)
t SS
=
cell energy budget terms (C m-3s-1)
or
cell nutrient terms (kg m-3s-1)
Note that the subscript dn indicates the downstream face. Because faces are shared (the downstream face of an upstream cell is the upstream face of the adjacent downstream cell) only upstream faces are available for output. Water quality model output is available at two time intervals. Output on an even time interval, or Output on a selected number of time steps. Output on Even Time Interval The first option, Output on an even time interval allows the user to select an even time increment (e.g. 1 hour, 15 minute) for model output. Select the time interval using the pull-down menu. Because the water quality model time step is not evenly spaced, this option requires interpolation of model results. Water quality state variables are linearly interpolated between their values at the two bounding computational time steps. Some other output variables, such as energy budget terms and cell nutrient terms, are output in a stepwise manner. In this (stepwise) case, the value from the last (water quality) computational time step is reported for all output times until the next computational time step is reached. Table 19-6 summarizes these special
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Chapter 19 Water Quality Modeling output variables and indicates the manner in which they have been interpolated. Output on a Selected Number of Time Steps This option writes output at a computational time step. No interpolation is necessary. Cell Energy Budget or Cell Nutrient Terms Multiple variables are output when Cell nutrient terms or Cell energy budget terms are selected. These variables include individual sources and sinks and limitation factors. All are available as special output when selected. See Table 19-7 for details.
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Chapter 19 Water Quality Modeling
Table 19-6. Output variables and interpolation method for output on even time intervals. Output Variable
Symbol
Description of variable and source
Reporting Method for WQ Output at User Defined Intervals
Units
Flow at upstream face Upstream face flow
Qus
Upstream face area
Aus
Hydraulic model output at (hydraulic) computational time step.
3
Stepwise
m s
Stepwise
m
Stepwise
ms
Not time varying
m s
Stepwise
m
3
m
2
Running total
m
3
-1
Interpolated linearly to obtain value for A/D difference equation at (WQ) computational time step. Cross sectional flow area at upstream face Computed from channel average depth Computed at each (WQ) computational time step from Q and A
Upstream face velocity
uus
Upstream face dispersion
Γus
Geometric property of water quality cells
Cell volume
V
V = 0.5 * ( V
Cell surface area
As
Computed from hydraulic output average width and WQ cell length
2
-1
uus = Qus / Aus
n+1
n
+ V )
Stepwise
2
-1
As = average width * cell length Cell continuity error
qe
qe = V
Upstream face avg conc
Φ*
QUICKEST computed time-average concentration at upstream cell face
Linear Interpolation
kg m
-3
Upstream face avg d(conc)/dx
dΦ/dx *
QUICKEST computed spatial derivative of time-average concentration at upstream cell face
Linear Interpolation
kg m
-4
Upstream face advection mass
Madv
Madv = Φ* Δt Qus
Linear Interpolation
kg
Upstream face dispersion mass
Mdisp
Mdisp = dΦ/dx * Δt Γus Aus
Linear Interpolation
kg
Cell mass
VΦ
Cell concentration ( Φ ) is the computed result of the A/D difference equation
Linear Interpolation
kg
Local Courant
Cus
Cus = uus * ( Δt / cell length)
Stepwise
unitless
αus
2
Stepwise
unitless
Stepwise
Wm
Stepwise
kg m s
Local Peclet
n+1
n
- V + Δt (Qup - Qdn )
αus = Γus * ( Δt / cell length ) qnet, qsw, qatm, qb, qh, ql
Cell energy budget terms
Cell nutrient terms
See water temperature source sink section for details.
Nitrogen, phosphorus, algae, DO, and BOD sources and sinks.
-2
-3
-1
See table 19-7
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Chapter 19 Water Quality Modeling
Table 19-7. Nutrient model special output variables. Variable
19-62
Mathematical Expression
Description Rate of hydrolysis of OrgN to NH4 Rate of loss of NH4 (or gain of NO2) via oxidation
Units mgN/L/day
N_NOrg_NH4_Hydrolysis
β3*OrgN
N_NH4_NO2_Oxidation
β1*NH4* Nitrification_Inhibition
N_NO2_NO3_Oxidation
β2*NO2
Rate of loss of NO2 (or gain of NO3) via oxidation
mgN/L/day
N_Algae_OrgN
α1*ρ*A
Rate of gain of organic N from algal growth
mgN/L/day
N_OrgN_Settling
σ4*OrgN
Rate of loss due to settling of OrgN
mgN/L/day
N_NH4_Benthos
σ3/depth
F1
______PN*NH4_______ [PN*NH4+(1-PN)NO3]
N_NH4_Algal_Uptake
F1* α1*μ*A
N_NO3_Algal_Uptake
(1-F1) α1*μ*A
N_Limitation_Factor
NPool/(KN+NPool)
Nitrification_Inhibition
1-exp
P_Limitation_Factor
PO4/(KP+PO4)
P_OrgP_PO4_Decay
β 4*OrgP
PO4AlgalUptake
α2*μ*A
O2_NH4_Oxidation
α 5* β 1*NH4
O2_NO2_Oxidation
α 6* β 2*NO2
O2_CBOD_oxidation
K1 *CBOD
O2_Sediment_Demand
K4 / depth
O2_Reaeration_Diffusion
K2 (Osat-O)
O2_Photosythesis
α3*μ*A
O2_Respiration
α4* ρ *A
A_Respiration
ρ *A
A_Growth A_Settling
μ*A A*σ3/depth
CBOD_Decay
K1*CBOD
CBOD_Settling
K3*CBOD
(-KNF*DOX)
mgN/L/day
Rate of gain of NH4 via diffusion from benthos
mgN/L/day
Fraction of algal uptake from the ammonium pool
unitless
Loss of NH4 to algal uptake Loss of NO3 to algal uptake Nitrogen limitation factor for algal growth Nitrification inhibition coefficient for low DO Phosphorus limitation factor for algal growth Rate of decay of OrgP to Orthophosphate (PO4) Loss of PO4 to algal uptake Rate of loss of O2 via oxidation of NH3 to NO2 Rate of loss of O2 via oxidation of NO2 to NO3 Rate of loss of O2 via oxidation of CBOD Rate of loss of O2 via sediment demand Rate of flux of O2 to/from atmosphere Rate of oxygen production from photosynthesis Rate of oxygen loss via respiration Rate of loss of algal to respiration Rate of algal growth Rate of loss of algae to settling Rate of loss of CBOD to oxidation Rate of loss of CBOD to settling
mgN/L/day mgN/L/day unitless unitless unitless mgP/L/ day mgP/L/ day mgO/L/ day mgO/L/ day mgO/L/ day mgO/L/ day mgO/L/ day mgO/L/ day mgO/L/ day mgA/L/ day mgA/L /day mgA/L /day mgBOD/L/ day mgBOD/L /day
Chapter 19 Water Quality Modeling
Restart Files A restart file saves a snapshot of model output at all model locations. Once created, a restart file can be used to specify initial conditions for subsequent model runs. Restart files are written at a user specified time. This time may be specified as either a number of hours from the start of the simulation or at a particular date and time (Figure 19-41). To generate a restart file, the Write Water Quality Initial Condition File option must be selected in the Water Quality Computation Options window prior to performing a water quality simulation. The water quality initial condition file is named by default as the current plan appended with “.wqrst”.
Figure 19-41. Writing a water quality initial conditions file.
Detailed Log Output Checking Detailed Log Output writes a system summary to the water quality computations log file, identified with the extension “.wqo”.
Temperature Override Checking Override computed temperatures replaces the computed output temperature with the value entered in the text box to the right. Energy
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Chapter 19 Water Quality Modeling budget computations still proceed; however, the output temperature is overwritten. Note that even with this option checked, solar radiation time series values are passed through to the NSM model. This allows algal growth and respiration to respond to day and night conditions. This option is not recommended except for testing purposes.
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Chapter 19 Water Quality Modeling
Performing a Water Quality Simulation In order to perform a water quality simulation, a completed, calibrated hydraulic model (steady or unsteady) must be available and water quality input data assembled. This section outlines the steps required to assemble and run a water quality model.
Preparing the Water Quality Input Requirements for All Water Quality Models
Standard output files for a calibrated steady flow model
or
Computation level output file for a calibrated unsteady flow model
Requirements for Water Temperature Modeling
Water temperature time series at all hydraulic boundaries
At least one initial condition value in each reach or a restart file
Meteorological Time Series o
Solar radiation (or site latitude and longitude)
o
Air Temperature
o
Relative Humidity (or vapor pressure, dew point, wet bulb)
o
Wind speed
o
Cloudiness
o
Atmospheric Pressure (or estimation of site elevation)
Requirements for Nutrient Modeling
A complete set of water temperature model input as outlined above
Time series of constituent concentrations at all hydraulic boundaries
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Chapter 19 Water Quality Modeling
At least one initial condition value for all constituents in each reach or a restart file
Requirements for Arbitrary Constituent Modeling
Time series of constituent concentration at all hydraulic boundaries
At least one initial condition value for the constituents in each reach or a restart file
Estimated rate constant(s) for the constituent(s) to be modeled
Running the Water Quality Model Selecting Compute brings up a status window. An example is shown in Figure 19-42. If the water quality model runs without error, a message is displayed along with the total computation time.
Figure 19-42. Successful completion of a water quality simulation.
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Chapter 19 Water Quality Modeling
Viewing and Interpreting Results
Water Quality Output Files Model output is made available in two output files. The default water quality output file (*.wqxx) is always written and contains output at all water quality cells at the time interval specified in the Water Quality Computation window. The secondary output file (*.daily.wqxx) contains computed daily maximum, mean, and minimum temperatures. The default water quality file is always written; the secondary output file is written only when the appropriate check box in the Water Quality Computation Options Window has been selected.
Water Quality Spatial Plots Water quality spatial plots are available from the main HEC-RAS menu under the View menu heading as shown in Figure 19-43.
Figure 19-43. Opening a water quality spatial plot from the main HEC-RAS window.
Individual profiles may be selected using the Profiles… button which allows the user to select one or multiple times for which profiles are drawn. The animation tools at the right of the menu bar allow animation of a complete time series. A profile plot of water temperature is shown in Figure 19-44. Observed data, if available, is displayed on the profile plot if the Plot Observed Data check box is selected.
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Chapter 19 Water Quality Modeling
Figure 19-44. Water temperature profile plot showing model simulation and observed data.
Line weights and colors can be changed by the user by right clicking on the legend and choosing Lines and Symbols…. If additional output variables have been specified, they may also be viewed in a spatial format. For example, a profile plot of the radiation terms is shown in Figure 19-45. This plot was generated by selecting solar radiation, upwelling long wave radiation and downwelling long wave radiation from the Variables… button.
Figure 19-45. Heat flux profile plot.
In addition to profile plots for particular simulation times, profile plots of daily mean, maximum, and minimum are also available by selecting the *.daily.wqxx file from the file menu. Note that this file is only available if the Write max, min, mean and daily range to output file check box in the Water Quality Computation Window has been selected. If observed data is available, daily maximum, minimum and mean will be computed from the observed data and it will be available for plotting along with model output. Figure 19-46 is a profile plot of the daily mean, maximum, and minimum for September 7. Vertical lines indicate the daily range computed from observed. Diamonds located on each vertical line indicate observed daily mean values.
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Chapter 19 Water Quality Modeling
Figure 19-46. Daily maximum, minimum, and mean water temperature profile with observed data.
Adding Reference Values Reference values are useful when model output is to be compared to a target or critical value. Reference values may be added for one or all constituents and will appear on both spatial and time series plots. A single value or a maximum minimum pair (range) may be entered. Choose Reference Values in the navigation bar of the water quality data submenu to activate this feature as shown in Figure 19-47.
Figure 19-47. Adding Reference Values.
Schematic Plots Schematic plots display results in the form of a color-coded map. Water temperature is plotted in the example shown in Figure 19-48. Color may be set by the user with the legend to the far right.
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Chapter 19 Water Quality Modeling
Figure 19-48. Schematic plot of water temperature.
There are several default color scales available to choose from in the Color Scale Selector/Editor where users can also create their own scales. The default color scales use the “Dynamic Scale.” This option adjusts the values associated with the colors to cover the range at that particular time step. Fixed Scales set the values so that the meaning of the color does not change from time step to time step. The schematic spatial plot uses a custom color scale with the Fixed Scale option with smooth color transitions. With smooth color transitions, the cells with temperatures between the set values are interpolated. To open the Color Scale Selector/Editor, right click on the Legend and choose Color Scale… or choose the option from the View menu as shown in Figure 19-49.
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Chapter 19 Water Quality Modeling
Figure 19-49. Using the color scale tool with a schematic plot.
Finally, all profile information is available in table form for copying and pasting into spreadsheets by choosing the Table tab.
Water Quality Time Series Plots Time series plots display model results and observed data at particular locations. Water quality time series plots are available from the main HECRAS menu under the View menu heading as shown in Figure 19-50.
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Chapter 19 Water Quality Modeling
Figure 19-50. Opening a water quality time series plot from the main HEC-RAS window.
Simulated results may be viewed at any of the water quality cells by selecting the desired cell in the River Station menu bar at the left of the screen (Figure 19-51). This pulldown menu also provides a facility for displaying observed data. These observed data are indicated with a special icon. If the observed data check box is selected, observed data will be displayed when available.
Figure 19-51. Water Quality Time Series Plot.
Selecting the Table tab presents time series data in table format enabling copying and pasting into Excel or other programs. In addition to water temperature data, energy budget components are also available for plotting. In the example shown in Figure 19-52, net heat flux and solar radiation have been plotted together.
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Chapter 19 Water Quality Modeling
Figure 19-52. Time series plot of energy budget components.
Detailed Log Output File The Computations Log File is available through the Options pull down in the water quality analysis window as shown in Figure 19-53. This file records the simulation options settings as well as detailed continuity error information for water volume and conservative arbitrary constituents. Continuity error information is not available for non-conservative arbitrary constituents, nutrient sub-model constituents or water temperature. Each time step is listed along with the volume of both water and constituent mass at the start of each time step. In addition to system volume, total system inflow and outflow at external reaches and lateral inflows are reported along with the associated continuity error. This information is provided for troubleshooting purposes and allows the user to locate periods in the simulation when there are instabilities in the hydrodynamics field.
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Chapter 19 Water Quality Modeling
Figure 19-53. Opening the Computations Log File.
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Chapter 20 RAS Mapper
CHAPTER
20
RAS Mapper New geospatial capabilities have been introduced in HEC-RAS 4.1 to assist the hydraulic engineer to more efficiently create and refine the geometry of the hydraulic model. Using the RAS Mapper, HEC-RAS users can quickly analyze model results through the geospatial visualization of simulation results with of base geometric data to more readily identify hydraulic model deficiencies and make improvements. This chapter will discuss how to use the RAS Mapper for performing floodplain mapping of HEC-RAS output and viewing results.
Contents
Getting Started
Floodplain Mapping
Data Visualization and Management
Getting Started The HEC-RAS Mapper module is an interface accessed from the HEC-RAS program. The RAS Mapper is intended to provide visualization of HEC-RAS simulation results along with pertinent geospatial data to assist users in improving hydraulic models. The HEC-RAS Mapper module is accessed using the GIS Tools | RAS Mapper menu item on the main HEC-RAS program interface, shown in Figure 20-1.
Figure 20-1. Access to the RAS Mapper from the GIS Tools menu.
20-1
Chapter 20 RAS Mapper
The RAS Mapper interface, shown in Figure 20-2, is comprised of a data layers window, display window, and status window.
Figure 20-2. RAS Mapper module interface.
Data Layers Window The Data Layers Window provides a list of the layers available for display and is organized with a tree structure based on the HEC-RAS data structure – there is a “Geometry” node and a “Results” node. Listed under the Geometry node, you will find the name of the RAS geometry files with sub-nodes for each major feature set in the geometry (i.e. river, cross sections, levees, storage areas, etc.). The Results node provides a list of HEC-RAS Plan names for which the user has processed results data (i.e. floodplain boundary, depth grids, etc.). A layer is the representation of a data source that will be used by the RAS Mapper. Currently, the RAS Mapper supports a vector data source in the Shapefile format. The raster data source type that is supported is the binary raster floating-point (.flt) format. The order in which layers are listed indicates the order in which they are drawn to the display: layers on the top are displayed on top of the layers on the bottom. To change the display order of a layer, right click on a layer and choose from the Move Layer options. Options available for each layer are summarized in Table 20-1.
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Table 20-1. Layer options available through from a right-click. Option
Description
Remove Layer
Remove a layer from the Layers Window.
Move Layer
Used to reorder the layer list and by moving the layer: To the Top, Up one Level, Down one Level, or To the Bottom of the layer list.
Zoom to Layer
Zooms the Display Window to the extents of the selected layer.
Add File to Layer
Adds the supported file type to the layer, using the same symbology for all of the features.
Save Layer As
Saves the layer file to a Shapefile.
Layer Properties
Provides access to the underlying feature table and display symbology.
The text for the highlighted layer will be drawn in the selection color. This layer will be the active layer for selecting features in the Display Window using the Select tool.
Display Window The Display Window is used for displaying the geospatial component (features) of a data layer. It is intended that the display will provide visualization of HEC-RAS results along with the geometric data that was used in performing the analysis. Tools, summarized in Table 20-2, provide the user interaction with the display to change the viewable extents and query data. Table 20-2. Display Window tools provide interaction with the display extent and data. Tool
Description
Select
Selects the feature of interest. The feature will change to the highlight color. If a raster layer is selected the value for the selected grid cell is reported.
Pan
Interactively drag the display window to view a new location.
Zoom In
Zooms into the region identified with a bounding box.
Zoom Out
Zooms out from a selected location to double the viewable extent.
Zoom In (Fixed)
Zooms into the center of the viewable extent reducing the viewable area by half.
Zoom Out (Fixed)
Zooms out from the center of the viewable extent to double the viewable extent.
Zoom to Entire Extent
Zooms to the maximum viewable extent of all the loaded data layers.
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Status Window Messages that keep the user informed of the actions performed by the RAS Mapper are displayed in the Status Window. Using the messages in the status window the user can gage how long it will take to complete the inundation mapping process.
Floodplain Mapping Mapping of HEC-RAS results is performed in the RAS Mapper from the Tools | Floodplain Mapping menu item. To perform mapping for a given HEC-RAS Plan, output results must exist – in other words, a steady flow or unsteady flow simulation must have been previously run. The Floodplain Mapping dialog, shown in Figure 20-3, is used to manage the RAS Plan, Geometry, Profiles, and Variables to be used for mapping RAS results.
Figure 20-3. RAS Mapping dialog.
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Chapter 20 RAS Mapper
RAS Plan The RAS Plan list is used to pick the plan of interest. By default, the plan chosen in the main HEC-RAS interface will be selected. (The output file that RAS will use for the analysis will be listed next to the selected plan.) Selecting a plan will in turn select the corresponding HEC-RAS Geometry. Also, the default output location for the mapping results will be created based on the RAS project location and plan name (you may choose to select a new directory location).
HEC-RAS Geometry The geometry used to perform the model simulation will automatically be populated when the RAS Plan is selected. The shape of the river centerline, cross section layout, and location of the bank stations are used for creating a transition lines layer. The transitions layer is then used to create regions between cross sections over which interpolation of simulation results can occur. It is intended that the user be able to edit the transition lines layer to allow for refining of the floodplain delineation. At this time the RAS Mapper does not provide editing tools, however, the transition lines layer may be added to a GIS for editing. If the transition lines layer exists from a previous mapping effort, check the “Use Existing Transition Lines” and the interpolation surface will be computed directly from the transition lines and cross sections to generate the interpolation regions. This option is beneficial when hydraulic results have changed but the river and cross section layout has not or if you have manually edited the transition lines layer.
Terrain A ground surface layer in the binary floating-point raster format (.flt) is required to perform mapping of HEC-RAS results. The terrain layer is used as the template for writing/storing output results. The terrain layer may be specified as a single terrain dataset or a layer of multiple terrain data sources. To specify a new terrain dataset, select the New Terrain button. In the browse dialog provided, select the terrain data files. A new dialog will then request you provide a name for the terrain layer, shown in Figure 20-4. A new terrain layer is created after selecting .flt files and providing a layer name.. The new terrain layer will be added to the Layers Window. As the terrain data is added to the RAS Mapper, it will be examined to derive information about the elevation values to create a color ramp; therefore, if the data is large there may be a short wait (but the Status Window will provide updates as to the progress). Additional terrain datasets can be added to the terrain layer using the Add Files button.
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Figure 20-4. A new terrain layer is created after selecting .flt files and providing a layer name.
The terrain layer will be used in the inundation mapping process (discussed later) to compare the computed water surface to the ground surface. Depending on the expanse and resolution of the terrain data, users may choose to tile the terrain data. This is often a successful way to reduce file size and improve computational efficiency using raster (gridded) datasets. If using multiple terrain tiles, the tiles may be organized as desired by the user: tiles may overlap or can have gaps between them and individual cross sections (or rivers or storage areas) can span one or more terrain tiles. Read access of the floating-point raster files has been specifically designed so that there are not file size limitations.
Profiles Water surface profile results available for analysis are listed in the Profiles data frame. Results can be mapped for individual or multiple profiles.
Variables Results for hydraulic variables are available for processing based on what is available in the HEC-RAS output file. At this time, Water Surface Elevation (depth and floodplain boundary), Velocity, Shear Stress, Stream Power, and Ice Thickness are options available for mapping.
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Generate Layers Once the requisite data have been provided in the RAS Mapping dialog, pressing the Generate Layers button will process the HEC-RAS results for each of the water surface profiles for each of the variables selected. The Water Surface Elevation variable must be processed prior to any of the other variables, because the inundation depth grid provides a template for the processed output. As layers are processed, status messages are displayed and the layers are added to the Data Layers window, as shown in Figure 20-5.
Figure 20-5. Inundation mapping layers are created and added to the Results node of the layer list.
Transition Lines and Interpolation Surface The first step in performing the geospatial analysis of HEC-RAS results is to pre-process the geometric data to create transition lines from cross section to cross section over the area of interest. After pressing the Generate Layers button, the RAS Mapper will create transition lines for an interpolation surface using the cross section locations, shape of the stream centerline, and bank stations. Note that the transition line generation process requires that each cross section intersect with a river centerline. Four interpolation “regions” are created using the transition lines: (1) between the left edge of the cross section and the left bank station line, (2) between the left bank station line and stream centerline, (3) between the stream centerline and right bank station line, and (4) between the right bank station line and the right edge of the cross section. To compute and view the interpolation surface separately, use the “Compute Interpolation Surface”
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Chapter 20 RAS Mapper button on the Floodplain Mapping window. The RAS Mapper uses the resulting interpolation surface to determine how to distribute computation results from cross section to cross section for each region. While the transition lines are being generated, the RAS Mapper status message will advise “Creating Transition Lines …”. If the transition lines exist from a previous processing run, you can select the layer and check the “Use Existing Transition Lines” option. During the generation of the interpolation surface, the RAS Mapper status message will display “Creating Interpolation Surface …”. The time it takes to create the transition lines and interpolation surface will be rapid in comparison to the floodplain delineation process.
Water Surface Elevation Water surface elevations are mapped by evaluating the difference in the water surface elevation and the ground surface. Where the water surface elevation is higher than the ground surface a positive depth is computed (conversely, where the ground surface is higher than the water surface, a negative depth is computed). The resulting surface is referred to as the water surface depth grid. Floodplain Depths. The water surface depth grid is evaluated by identifying all of the grid cells having positive depths that are connected wetted portions of the cross section as computed in HEC-RAS (cells that are contiguous). The contiguous cells and the positive depths due to storage areas are added to create the depth grid. A floodplain depth grid in the float format is created for each terrain tile provided in the analysis. The file name convention will be “Depth#.flt” where “#” indicates a counter based on the number of terrain tiles. An example inundation depth grid is shown in Figure 20-6. Floodplain Boundary. The floodplain boundary is identified by contouring the water surface depth grid at zero-depth. This results in a smooth and more accurate floodplain boundary; however, the floodplain boundary will not exactly match the corresponding depth grid. The floodplain boundary is stored in the Shapefile format with the name “Floodmap.shp” with a feature for each terrain tile used in the analysis. If you wish to designate the projection for the floodplain boundary dataset to be included in the creation of the “Floodpmap.shp”, a tool is provided by selecting Tools | Define Projection. The Define Projection dialog allows you to select an ESRI projection file (.prj) that will be used in creating the floodplain shapefile. The projection file is not used for the creation of the depth grid (.flt) files.
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Figure 20-6. Example depth grid generated using the RAS Mapper.
Velocity Velocity results are mapped using the interpolation surface created during the geometric preprocessing. Interpolation of velocity data is done within each interpolation region, therefore, values in the overbanks are not affected by values within the banks. This methodology prevents extreme velocity areas from influencing the distribution of parameter values (low velocity values in the overbank being affected by high velocity values in the channel).
Shear Stress Shear stress results are mapped using the interpolation surface created during the geometric preprocessing. Interpolation of data is done within each interpolation region, therefore, values in the overbanks are not affected by values within the banks. This methodology prevents extreme values from influencing the distribution of parameter values.
Stream Power Stream power results are mapped using the interpolation surface created during the geometric preprocessing. Interpolation of data is done within each interpolation region, therefore, values in the overbanks are not affected by values within the banks. This methodology prevents extreme values from influencing the distribution of parameter values.
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Ice Thickness Ice thickness results are mapped using the interpolation surface created during the geometric preprocessing. Interpolation of ice data is done within each interpolation region, however, there is an transition in thickness from the main channel to overbank thickness using a 1:1 slope from the bank stations into the overbank area.
Data Visualization and Management The RAS Mapper supports visualization of data using data “Layers”. Data ldLayers provide a spatial component that can be visualized in the display window and have an underlying data table that provides descriptive information. Currently, the RAS Mapper provides support for the Shapefile format and binary raster floating-point (FLT) files. Data can be added to the RAS Mapper using the File | Load Layer menu option or by dragging and dropping the file onto the RAS Mapper from Windows Explorer. Further, a dataset may be added to an existing data layer by right-clicking on the layer and selecting the Add File to Layer option. Data layers are managed using use a tree structure that automatically organizes RAS data by “Geometry” and “Results”. Data layers are further grouped based on the title of the geometry or by the plan name. Data layers may be reorganized by dragging and dropping the layers from one part of the tree structure to another part. The order in which layers are drawn to the Display Window is dependent on the layer tree – the bottom layer is drawn first while the top layer is drawn last. Use the checkbox next to each layer to toggle on whether it is visible (or not) and use the plus/minus object to expand/contract the layer tree. Layer order can also be organized by clicking on a layer and dragging it into a new position in the tree. The display of layer features can be customized by right-clicking on a data layer and choosing Layer Properties (or double-clicking the layer). This will bring up the Layer Properties dialog and allow you to choose the symbols with which to draw the layer. For point, line, and polygon features you have access to symbol and style pallets to customize the display of vector features. For raster datasets, you have access to the style dialog for rendering a surface (shown in Figure 20-7) that will allow for selecting from a set of predefined color ramps. All symbols have a transparency property that may be set. You can also set the line symbol for raster datasets. This color will be used to draw the raster extents when zoomed out beyond the default RAS Mapper drawing resolution.
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Figure 20-7. Color ramp style selector for raster datasets.
The floodplain boundary layer is created in the Shapefile format. The Shapefile can be used by most GIS packages. The shapefile will not have a projection file associated with it unless you define the projection using the Tools | Define Projection option. At this time, the projection definition dialog, shown in Figure 20-8, will allow you to browse to an ESRI projection file for the desired coordinate system. (For example, ArcGIS coordinate systems are stored at by default in the “C:\Program Files\ArcGIS\Coordinate Systems” directory.) When the floodplain boundary shapefile is created, the projection file will be copied to it.
Figure 20-8. An ESRI projection file may be used to specify a coordinate system for the floodplain boundary shapefile.
All output raster datasets created by the RAS Mapper are stored in the FLT format. Along with the binary .flt a second ASCII header file (.hdr) will be created. Both files must be used together to define a raster dataset. This file format can be compressed for transmission of smaller file sizes. It is also a format that is widely supported and can be imported to other GIS for analysis. In ArcGIS, for instance, there are utilities to import and export the FLT format. The conversion utilities can be found in ArcToolbox |
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Chapter 20 RAS Mapper Conversion Tools. To Raster | Float to Raster will import the FLT format into the ESRI grid format and From Raster | Raster to Float will export the ESRI grid format to the FLT format (for use in RAS Mapper). Conversion from one file format to the next is not time consuming.
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Appendix A - References
APPENDIX
A
References Barkau, Robert L., 1992. UNET, One-Dimensional Unsteady Flow Through a Full Network of Open Channels, Computer Program, St. Louis, MO. Binkley, D., G.G. Ice, J. Kaye, and C.A. Williams, 2004. Nitrogen and Phosphorus Concentrations in Forest Streams of the United States. Journal of the American Water Resources Association, 40(5), 2004, pp. 1277-1291. Brown and Barnwell, 1987. The Enhanced Stream Water Quality Models QUAL2E and QUAL2E-UNCAS: Documentation and Users Manual. Environmental Research Laboratory. Office of Research and Development. U.S. Environmental Protection Agency, Athens Georgia 30613. Bureau of Public Roads (BPR), 1965. Hydraulic Charts for the Selection of Highway Culverts, Hydraulic Engineering Circular No. 5, U.S. Department of Commerce. Bureau of Reclamation, 1977. Design of Small Dams, Water Resources Technical Publication, Washington D.C.. Bureau of Reclamation, 1998. Prediction of Embankment Dam Breach Parameters, Dam Safety Research Report DSO-98-004, Dam Safety Office, U.S. Department of the Interior. Carpenter, S.R., N.F. Caraco, D.L. Correll, R. W. Howarth, A.N. Sharpley, and V.H. Smith (1998) Nonpoint Pollution of Surface Waters With Phosphorus and Nitrogen Ecological Applications, 8(3), 1998, pp. 559-568. Federal Emergency Management Agency, 1985. Flood Insurance Study Guidelines and Specifications for Study Contractors, FEMA 37, Washington D.C., September 1985. Federal Highway Administration, 1978. Hydraulics of Bridge Waterways, Hydraulic Design Series No. 1, by Joseph N. Bradley, U.S. Department of Transportation, Second Edition, revised March 1978, Washington D.C.. Federal Highway Administration, 1985. Hydraulic Design of Highway Culverts, Hydraulic Design Series No. 5, U.S. Department of Transportation, September 1985, Washington D.C.. Fread, D.L., 1988 (revised 1991). BREACH: An Erosion Model For Earth Dam Failures, National Weather Service, NOAA, Silver Springs Maryland.
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Appendix A - References Fread, D.L., Ming Jin and Janice M. Lewis, 1996. An LPI Numerical Solution for Unsteady Mixed Flow Simulation, North American Water Congress, Anehiem, CA June 22-28, 1996, American Society of Civil Engineers. FHWA, 1996. Evaluating Scour at Bridges, Federal Highway Administration, HEC No. 18, Publication No. FHWA-IP-90-017, 2nd Edition, April 1993, Washington D.C. Froehlich, D.C., 1988. Analysis of Onsite Measurements of Scour at Piers, Proceedings of the ASCE National Hydraulic Engineering Conference, Colorado Springs, CO. Froehlich, D.C., 1989. Local Scour at Bridge Abutments, Proceedings of the 1989 National Conference on Hydraulic Engineering, ASCE, New Orleans, LA, pp. 13-18. Froehlich, D.C., 1995. Embankment Dam Breach Parameters Revisited, Water Resources Engineering, Proceedings of the 1995 ASCE Conference on Water Resources Engineering, San Antonio, Texas, August 14-18, 1995. Hem, John D. (1985) Study and Interpretation of the Chemical Characteristics of Natural Water. Third Edition. U.s. Geological Survey Water-Supply Paper 2254. Hydrologic Engineering Center, 1991. HEC-2, Water Surface Profiles, User's Manual, U.S. Army Corps of Engineers, Davis CA. Hydrologic Engineering Center, 1993. UNET, One-Dimensional Unsteady Flow Through a Full Network of Open Channels, User's Manual, U.S. Army Corps of Engineers, Davis, CA. Hydrologic Engineering Center, 1994. HECDSS, User's Guide and Utility Programs Manual, U.S. Army Corps of Engineers, Davis CA. Hydrologic Engineering Center, 1995. RD-41, A Comparison of the OneDimensional Bridge Hydraulic Routines from: HEC-RAS, HEC-2, and WSPRO, U.S. Army Corps of Engineers, Davis CA., September 1995 Hydrologic Engineering Center, 1995. RD-42, Flow Transitions in Bridge Backwater Analysis, U.S. Army Corps of Engineers, Davis CA., September 1995 Johnson, Billy, and T. Gerald, 2006. Development of a Distributed nutrient Sub-Model (NSM Version 1.0) for Watersheds – Kinetic Process Descriptions. Environmental Laboratory. U.S. Army Engineer Research and Development Center. Vicksburg, MS. 39180. ERDC/EL TR-06-X September 2006 Laursen, E.M., 1960. Scour at Bridge Crossings, ASCE Journal of Hydraulic Engineering, Vol. 89, No. HY 3. Laursen, E.M., 1963. An Analysis of Relief Bridges, ASCE Journal of Hydraulic Engineering, Vol. 92, No. HY 3.
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Appendix A - References Leonard, B. P., 1991. The ULTIMATE Conservative Difference Scheme Applied to Unsteady One-Dimensional Advection, Computer Methods in Applied Mechanics and Engineering, vol 88, pp 17-74. Leonard, B.P., 1979. A Stable and Accurate convective Modelling Procedure Based on Quadratic Upstream Interpolation, Computer Methods in Applied Mechanics and Engineering, vol 19, pp 59-98. Martin, J.L., and S. C. McCutcheon, 1999. Hydrodynamics and Transport for Water Quality Modeling. CRC Press, Boca Raton, Florida. McDonald, Thomas C. and Jennifer Langridge-Monopolis, 1984. Breaching Characteristics of Dam Failures, Journal of Hydraulic Engineering, vol. 110, No. 5, pp 567-586. Meybek, Michel, 1982. Carbon, Nitrogen, and Phosphorus Transport by World Rivers. American Journal of Science, Vol. 282, April, 1982, P. 401-450. Microsoft Corporation, 2007. Microsoft Windows Vista, User's Manual, Redmond WA. Richardson, E.V., D.B. Simons and P. Julien, 1990. Highways in the River Environment, FHWA-HI-90-016, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C. U.S. Army Corps of Engineers, 1965. Hydraulic Design of Spillways, EM 1110-2-1603, Plate 33. Wunderlich, W.O. 1972. Heat and mass transfer between a water surface and the atmosphere, TN Report 14, Tennessee Valley Authority Water Resources Research Engineering Laboratory, Norris, TN.
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data
APPENDIX
B
HEC-RAS Data Exchange At Version 2.0, HEC-RAS introduced a geospatial component to the geometry for the description of river networks and cross sections. This capability makes it possible to import channel geometry from CADD or GIS programs though automated data extraction procedures. Similarly, water surface elevations and other HEC-RAS results can be exported to CADD and GIS where they can be used to created model water surfaces for inundation mapping. The spatial data the HEC-RAS can import and export are evolving - each new version of the software results in additional capabilities. HEC-RAS Version 3.1.3 will import and export data using in a spatial data format in an ASCII text file. Data import options include:
The structure of the river network, as represented by a series of interconnected reaches.
The location and geometric description of cross sections for elevation data, bank positions, downstream reach lengths, Manning’s n values data, levee positions and elevations (limited to one per bank), ineffective flow area positions and elevations.
Bridge deck information for top-of-weir profile, deck width, and distance to the upstream cross section.
Lateral and inline structure information top-of-weir profile, deck width, and distance to the upstream cross section for inline structures.
Storage area elevation-volume information.
Data export options include:
Cross section locations and elevation data.
Water surface elevations at each cross section.
Bounding polygon information for each water surface profile.
Cross-sectional properties.
Spatial Data Format HEC-RAS Version 3.1.3 will import and export data using a formatted ASCII text file. In general, the spatial data format consists of records, keywords and values. This section provides the general rules for constructing the and HEC-RAS import and export file.
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data This file format is evolving in that additional data types will be added and existing one may be modified for future versions. If you are writing software to read and write to the HEC-RAS spatial data format, keep in mind that you may need to modify your software to remain compatible with future versions of HEC-RAS.
Records The spatial data format is composed of records, which are composed of keywords and values. All records must begin with a keyword. A record can also contain a value or a set of values following the keyword. Spaces, tabs, or line ends can be used as delimiters within a record. A record that contains a keyword and no value marks the beginning or end of a group of related records. For example, the record “BEGIN HEADER:” MARKS the beginning of the header section of the file. A record that contains a keyword and a value assigns that value to the part of the model being named by the keyword.
Keywords Keywords are used to identify that values unique to the part of the model being named by the keyword will follow. Keywords must end with a colon separating the keyword and the values. All keywords will have the spaces removed up to the colon and the letters capitalized. The keywords “Begin Header:”, “Begin header:”, and “Be GiNH eadEr:” are all equivalent. For readability, keywords named in this document will contain internal spaces.
Values A record can assign a single value to a single variable or multiple values in an array. Values can be integers, floating point numbers, text strings, or locations (X, Y, Z, label). A single value in an array of values is called an “element” of that array. A numerical value cannot contain internal blanks. A floating point number can contain a decimal point; an integer cannot. Elements in an array can be separated by commas, blanks, tabs, or line ends. A text string can contain internal blanks, tabs, and commas, but cannot contain internal line ends. A location consists of three coordinate values and a label (X, Y, Z, label). The first two coordinates are planar and the third is elevation. The coordinate values are floating point numbers and the label can by any type of value. In certain contexts, the elevation value or the label may not be required. If a label is used, all three coordinate values must be given; the value of “NULL” is valid for the elevation coordinate only. The coordinate values and the label
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data can be separated by commas, blanks, or tabs, but a location cannot contain internal line ends.
Data Groups Records in the data file can be collected in two types of groups: objects and file sections. An object is a group of records that combine to describe an entity within the model – a cross section, for example. A file section is a logical or functional grouping of data. The file header, for example, is a section that contains a description of the entire file. Objects and file sections begin and end with records that contain keywords but no values. A file section stats with a record containing the a keyword composed of the word “BEGIN” followed by the section name and a colon and ends with a keyword composed of the word “END” followed by the section name and a colon. For example, records containing only the keywords “BEGIN HEADER:” and “END HEADER:” are used to start and end the header section of a file. An object starts with a record containing a keyword naming an object type and “END:” only. For example, a cross-section object begins and ends with records containing the keywords “CROSS-SECTION:” and “END:” only.
Comments Hash characters (#) are used to identify comments. When a hash character is encountered in the file all data from the hash to the next line end is ignored. A line that begins with a hash is equivalent to a blank line.
RAS GIS Import File (RASImport.sdf) HEC-RAS reads channel geometry from a text file composed of several sections. A discussion of the sections in the import file is provided. And example RAS GIS import File is provided at the end of this appendix.
Header The header is bounded by the records “BEGIN HEADER:” and “END HEADER:” and should contain a record to identify the units system used in the imported data set. The units system can be “US CUSTOMARY” or “METRIC”. A summary of record that may be used in the Header section are provided in Table B-1.
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data
Table B-1. Header options for the spatial data file. Keyword
Value Type
Value
UNITS:
String
US CUSTOMARY or METRIC
DTM TYPE:
String
Type of terrain model (TIN or GRID)
DTM:
String
Name of terrain model
STREAM LAYER:
String
Name of Stream Centerline layer used in the CADD or GIS.
NUMBER OF REACHES:
Integer
Number of hydraulic reaches in the SDF file.
CROSS-SECTION LAYER:
String
Name of the Cross-Sectional Cut Lines layer used in the CADD or GIS.
NUMBER OF CROSSSECTIONS:
Integer
Number of cross sections in the SDF file.
MAP PROJECTION:
String
Projection (coordinate) system used (e.g. Stateplane)
PROJECTION ZONE:
String
Projection zone (if applicable, e.g. 5101)
DATUM:
String
Reference datum for planar coordinates.
VERTICAL DATUM:
String
Reference datum for vertical coordinates.
BEGIN SPATIAL EXTENT:
None
None. Begin of Spatial Extents object.
Xmin:
Float
Minimum easting of geospatial data.
Ymin:
Float
Minimum northing of geospatial data.
Xmax:
Float
Maximum easting of geospatial data.
Ymax:
Float
Maximum northing of geospatial data.
END SPATIAL EXTENT:
None
None. End of Spatial Extents object.
NUMBER OF PROFILES:
Integer
Number of profile exported from HECRAS. RAS GIS Export File only.
PROFILE NAMES:
String array
Water surface profile names exported from HEC-RAS. RAS GIS Export File only.
River Network The river network section is bounded by the records “BEGIN STREAM NETWORK:” and “END STREAM NETWORK:” and contains records describing
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data reaches and reach endpoints. At a minimum, the stream network section must contain at least two endpoints and one reach. A reach endpoint is represented by a record containing the keyword “ENDPOINT:” followed by four comma-delimited values containing the endpoint’s X, Y, Z coordinates and an integer ID. A reach consists of a multi-record object that begins with a record containing only the keyword “REACH:” and ends with a record only containing the keyword “END:”. At a minimum, a reach object must contain records setting values for a Stream ID, a Reach ID, a FROM point, and a TO point. A reach’s FROM and TO point IDs must match IDs for endpoints listed before the reach object in the file. The reach object must also contain an array of locations defining the stream centerline. This array begins with a record containing only the keyword "CENTERLINE:" and ends when any keyword is encountered. A location element in the array contains the X, Y, and Z coordinates of a point on the stream centerline, and the point’s river station. In HEC-RAS, elevation and stationing are optional in the stream network definition. If a location element includes a station value, it must occupy the fourth field in the element. If the elevation is not known, the word "NULL" must take its place. Stationing is used for indexing locations along reaches, and is not used to precisely locate objects in the model. A summary of record that may be used in the River Network section are provided in Table B-2. Table B-2. River network options for the spatial data file. Keyword
Value Type
Value
ENDPOINT:
Location
X, Y, Z coordinates and integer ID.
REACH:
None
Marks beginning of Reach object.
END:
None
Marks end of Reach object.
The following records are required for a Reach object. STREAM ID:
String
River identifier to include reach.
REACH ID:
String
Unique ID for reach within river.
FROM POINT:
String
Integer reference to upstream endpoint.
TO POINT:
String
Integer reference to downstream endpoint.
CENTERLINE:
Location array
Array elements contain coordinates and station values.
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data
Cross Sections The cross-sectional data section begins with a record containing the only the keyword "BEGIN CROSS-SECTIONS:" and ends with a record containing the only the keyword "END CROSS-SECTIONS:". A cross section is represented by multi-record object beginning with a record containing only the keyword "CROSS-SECTION:" and ending with a record containing only the keyword "END:." A cross-sectional object must include records identifying the Stream ID, Reach ID, and Station value of the cross-section, a 2D cut line, and a series of 3D locations on the cross section. Stationing is given in miles for data sets with plane units of feet and in kilometers for data sets with plane units of meters. A cut line is composed of the label "CUT LINE:" followed by an array of 2D locations. A cross-sectional polyline consists of the label "SURFACE LINE:" plus 3D coordinates written as comma-delimited X, Y, Z real-number triples, one triple to a line. A summary of record that may be used in the River Network section are provided in Table B-3. Table B-3. Cross-sectional data section options for the spatial data format. Keyword
Value Type
Value
CROSS-SECTION:
None
Marks beginning of Cross Section object.
END:
None
Marks end of a Cross Section object.
The following records are required for a Cross Section object. STREAM ID:
String
Identifier for the River on which the cross section resides.
REACH ID:
String
Identifier for the Reach on which the cross section resides.
STATION:
Float
Relative position of the cross section on the river reach.
CUT LINE:
Location array
Array elements contain planar coordinates of cross section strike line.
SURFACE LINE:
Location array
Array elements contain 3D coordinates of cross section.
The following records are optional for a Cross Section object.
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NODE NAME:
String
Description of cross section.
BANK POSITIONS:
Float
Fraction of length along cut line where main channel bank stations are located. (Left, Right)
REACH LENGTHS:
Float
Distance along left overbank, main channel and right overbank flow paths to next cross
Appendix B - HEC-RAS Import/Export Files for Geospatial Data
Keyword
Value Type
Value section downstream. (Left, Channel, Right)
N VALUES:
Float
Manning’s n values expressed as a fraction along cut line to start of n value. (fraction, n value)
LEVEE POSITIONS:
String, Float
Levee positions expressed as a fraction along cut line to position with elevation. (ID, fraction, elevation)
INEFFECTIVE POSITIONS:
String, Float
Ineffective flow areas expressed as a fraction along cut line to beginning and end positions with trigger elevation. (ID, begin fraction, end fraction, elevation)
BLOCKED POSTITIONS:
Float
Blocked flow areas expressed as a fraction along cut line to beginning and end positions with trigger elevation. (ID, begin fraction, end fraction, elevation)
WATER ELEVATION:
String array
Water surface profile names exported from HEC-RAS. RAS GIS Export File only.
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data
Additional Cross Section Properties Geospatial data used for display purposes in HEC-RAS for levees, ineffective flow areas, are blocked obstructions are stored outside of the Cross Section block of information. A summary of additional cross section properties is summarized in Table B-4. Table B-4. Addition cross section properties options for the spatial data file. Keyword
Value Type
Value
BEGIN LEVEES:
None
Marks beginning of Levees object.
LEVEE ID:
String
Levee identifier. Corresponds to ID in LEVEE POSITIONS object on cross section.
SURFACE LINE:
Location array
Array elements contain 3D coordinates of levee profile points. Array concludes with END:
END LEVEES:
None
Marks end of Levees object.
Levee records
Ineffective flow area records BEGIN INEFFECTIVE AREAS:
None
Marks beginning of Ineffective Areas object.
INEFFECTIVE ID:
String
Ineffective area identifier. Corresponds to ID in INEFFECTIVE POSITIONS object on cross section. Concludes with an “END:”.
POLYGON:
Location array
Array elements contain 2D coordinates of ineffective area polygon points.
END INEFFECTIVE AREAS:
None
Marks end of Ineffective Areas object.
Blocked obstruction records
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BEGIN BLOCKED AREAS:
None
Marks beginning Blocked Obstructions object.
BLOCKED ID:
String
Blocked obstructions identifier. Corresponds to ID in BLOCKED POSITIONS object on cross section.
POLYGON:
Location array
Array elements contain 2D coordinates of ineffective area polygon points.
END BLOCKED AREAS:
None
Marks end of Blocked Obstructions object.
Appendix B - HEC-RAS Import/Export Files for Geospatial Data
Bridge/Culverts The bridge/culvert data section begins with a record containing the only the keyword "BEGIN BRIDGE/CULVERTS:" and ends with a record containing the only the keyword "END BRIDGE/CULVERTS:". A bridge is represented by multi-record object beginning with a record containing only the keyword "BRIDGE/CULVERT:" and ending with a record containing only the keyword "END:." Bridges/Culverts have the same required records as the Cross Sections object, but have other optional records. A summary of Bridge/Culvert records is provided in Table B-5. Table B-5. Bridge/Culvert options in the spatial data format file. Keyword
Value Type
Value
BRIDGE/CULVERT:
None
Marks beginning of Bridge/Culvert object.
END:
None
Marks end of a Bridge/Culvert object.
The following records are required for a Briodge/Culvert object. STREAM ID:
String
Identifier for the River on which the bridge/culvert resides.
REACH ID:
String
Identifier for the Reach on which the bridge/culvert resides.
STATION:
Float
Relative position of the bridge on the river reach.
CUT LINE:
Location array
Array elements contain planar coordinates of bridge location.
SURFACE LINE:
Location array
Array elements contain 3D coordinates of bridge deck.
The following records are optional (but recommend) for a Bridge/Culvert object. NODE NAME:
String
Description of cross section.
US DISTANCE:
Float
Distance to upstream cross section.
TOP WIDTH:
Float
Top width of bridge deck.
Inline Structures The inline structures data section begins with a record containing the only the keyword "BEGIN INLINE STRUCTURES:" and ends with a record containing the only the keyword "END INLINE STRUCTURES:". An inline structure is represented by multi-record object beginning with a record containing only
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data the keyword "INLINE STRUCTURES:" and ending with a record containing only the keyword "END:." Inline structures have the same required records as the Bridge/Culvert object. A summary of Inline Structures records is provided in Table B-6. Table B-6. Inline structure options in the spatial data format file. Keyword
Value Type
Value
INLINE STRUCTURES:
None
Marks beginning of Inline Structure object.
END:
None
Marks end of a Inline Structure object.
The following records are required for a Inline Structure object. STREAM ID:
String
Identifier for the River on which the inline structure resides.
REACH ID:
String
Identifier for the Reach on which the inline structure resides.
STATION:
Float
Relative position of the inline structure on the river reach.
CUT LINE:
Location array
Array elements contain planar coordinates of inline structure location.
SURFACE LINE:
Location array
Array elements contain 3D coordinates of inline weir profile.
The following records are optional (but recommend) for an Inline Structure object. NODE NAME:
String
Description of inline structure.
US DISTANCE:
Float
Distance to upstream cross section.
TOP WIDTH:
Float
Top width of inline weir.
Lateral Structures The inline structures data section begins with a record containing the only the keyword "BEGIN LATERAL STRUCTURES:" and ends with a record containing the only the keyword "END INLINE STRUCTURES:". A lateral structure is represented by multi-record object beginning with a record containing only the keyword "LATERAL STRUCTURES:" and ending with a record containing only the keyword "END:." Lateral structures have the same required records as the inline structures object. A summary of Lateral Structures records is provided in Table B-7.
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data
Table B-7. Lateral structure options in the spatial data format file. Keyword
Value Type
Value
LATERAL STRUCTURES:
None
Marks beginning of Lateral Structures object.
END:
None
Marks end of Lateral Structures object.
The following records are required for a Lateral Structure object. STREAM ID:
String
Identifier for the River on which the lateral structure resides.
REACH ID:
String
Identifier for the Reach on which the lateral structure resides.
STATION:
Float
Relative position of the lateral structure on the river reach.
CUT LINE:
Location array
Array elements contain planar coordinates of lateral structure location.
SURFACE LINE:
Location array
Array elements contain 3D coordinates of weir profile.
The following records are optional (but recommend) for a Lateral Structure object. NODE NAME:
String
Description of lateral structure.
US DISTANCE:
Float
Distance to upstream cross section.
TOP WIDTH:
Float
Top width of weir.
Storage Areas The storage areas data section begins with a record containing the only the keyword "BEGIN STORAGE AREAS:" and ends with a record containing the only the keyword "END STORAGE STRUCTURES:". The keyword “SA ID:” identifies a storage area object. A summary of Lateral Structures records is provided in
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data Table B-8.
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data
Table B-8. Storage area options in the spatial data format file. Keyword
Value Type
Value
SA ID:
String
Storage area identifier.
POLYGON:
Location array
Array elements contain 2D coordinates of storage area boundary. Concludes with an “END:”
ELEVATION-VOLUME:
Float array
Elevation volume information for storage area. (Elevation, Volume) Concludes with an “END:”
The following records are optional for a Storage Area object. TERRAIN:
Float array
X,Y,Z coordinates for terrain data within storage area. Concludes with an “END:”.
Storage Area Connections The storage areas data section begins with a record containing the only the keyword "BEGIN SA CONNECTIONS:" and ends with a record containing the only the keyword "END SA CONNECTIONS:". An inline structure is represented by multi-record object beginning with a record containing only the keyword "SA CONNECTION:" and ending with a record containing only the keyword "END:." A summary of Storage Area Connection records is provided in Table B-9. Table B-9. Storage area connection options in the spatial data format file. Keyword
Value Type
Value
SACONNID:
String
Storage area connection identifier.
USSA:
String
Identifier of upstream storage area (SA ID).
DSSA:
String
Identifier of downstream storage area (SA ID).
CUT LINE:
Location array
Array elements contain planar coordinates of storage area connection location.
SURFACE LINE:
Location array
Array elements contain 3D coordinates of weir profile.
The following records are optional for a Storage Area Connection object. NODE NAME:
String
Description of storage area connection.
TOP WIDTH:
Float
Top width of weir.
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data
RAS GIS Export File (RASExport.sdf) HEC-RAS exports model results to a text file using the same spatial data format as the data import file. The contents of the file, however, are not identical. An example HEC-RAS model export file is shown at the end of this appendix. A summary of model elements for data export from HEC-RAS that differs from the import file is provided in Table B-10. Table B-10. HEC-RAS export options in the spatial data format file Keyword
Value Type
Value
The following records are required for Header section of the RAS GIS Export File NUMBER OF PROFILES:
Integer
Number of profile exported from HECRAS. Required if greater than 1.
PROFILE NAMES:
String array
Water surface profile names exported from HEC-RAS. Required if number of profiles is greater than 1.
The following records area required in the Cross Section portion of the Export File WATER ELEVATION:
Float array
Elevation of water surface at the cross section. The array must contain a value for each profile.
PROFILE ID:
String array
Water surface profile name(s). This must match the name(s) in the Profile Names record.
The following records area optional in the Cross Section portion of the Export File VELOCITIES:
Float, paired array
Fraction along cut line and value of velocity (fraction, value). Velocity records must follow Profile ID record.
WATER SURFACE EXTENTS:
Location array
A series of 2D locations marking the limits of a water surface on the cross section.
The following records make up a section defining Storage Areas in the Export File
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BEGIN STORAGE AREAS:
None
Marks beginning of Storage Area object.
END STORAGE AREAS:
None
Marks end Storage Area object.
SA ID:
String
Storage area identifier.
WATER ELEVATION:
Float array
Elevation of water surface at the storage area. The array must contain a value for each profile.
Appendix B - HEC-RAS Import/Export Files for Geospatial Data
Keyword
Value Type
Value
POLYGON:
Location array
Array elements contain 2D coordinates of storage area limits.
The following records make up a section defining Bounding Polygons for the water surface limits in the Export File BEGIN BOUNDARIES:
None
Marks start of boundaries section.
END BOUNDARIES:
None
Marks end of boundaries section.
PROFILE LIMITS:
None
Marks start of an object defining the limits of a single water surface profile. Concludes with and “END:”
PROFILE ID:
String
Name of the profile. This must match a name in the Profile Names record in the header.
POLYGON:
Location array
Array elements contain 2D coordinates of water surface limits. A single profile limit can be merged from multiple polygons.
Water Surface Bounding Polygon In addition to a water surface elevation at each cross section (one for each profile), the HEC-RAS program sends a bounding polygon for each hydraulic reach in the model (the program outputs a new set of bounding polygons for each profile computed). The bounding polygon is used as an additional tool to assist the GIS (or CADD) software to figure out the boundary of the water surface on top of the terrain. In most cases, the bounding polygon will represent the outer limits of the cross section data, and the actual intersection of the water surface with the terrain will be inside of the polygon. In this case, the GIS software will use the water surface elevations at each cross section and create a surface that extends out to the edges of the bounding polygon. That surface is then intersected with the terrain data, and the actual water limits are found as the location where the water depth is zero. However, is some cases, the bounding polygon may not represent the extents of the cross-section data. For example, if there are levees represented in the HEC-RAS model, which limit the flow of water, then the bounding polygon will only extend out to the levees at each cross section. By doing this, when the information is sent to the GIS, the bounding polygon will prevent the GIS system from allowing water to show up on both sides of the levees. In addition to levees, the bounding polygon is also used at hydraulic structures such as bridges, culverts, weirs, and spillways. For example, if all of the flow is going under a bridge, the bounding polygon is brought into the edges of the bridge opening along the road embankment on the upstream
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data side, and then back out to the extent of the cross-section data on the downstream side. By doing this, the GIS will be able to show the contraction and expansion of the flow through the hydraulic structures, even if the hydraulic structures are not geometrically represented in the GIS. Another application of the bounding polygon is in FEMA floodway studies. When a floodway study is done, the first profile represents the existing conditions of the floodplain. The second and subsequent profiles are run by encroaching on the floodplain until some target increase in water surface elevation is met. When the encroached profile is sent to the GIS, the bounding polygon is set to the limits of the encroachment for each cross section. This will allow the GIS to display the encroached water surface (floodway) over the terrain, even though the water surface does not intersect the ground.
Import/Export Guidelines The following rules apply to channel and cross-section import/export data.
Defining the River Network
B-16
The stream network is represented by a set of interconnected reaches. A stream is a set of one or more connected reaches that share a common Stream ID.
A stream is composed of one or more reaches with the same Stream ID, and each reach in a stream must have a unique Reach ID. Every reach must be identified by a unique combination of stream and reach IDs.
Stream IDs and Reach IDs are alphanumeric strings. Reach endpoint IDs are integers.
Streams cannot contain parallel flow paths. (If three reaches connect at a node, only two can have the same Stream ID.) This prevents ambiguity in stationing along a stream.
A reach is represented by an ordered series of 3D coordinates, and identified by a Stream ID, a Reach ID, and IDs for its endpoints.
A reach endpoint is represented by its 3D coordinates and identified by an integer ID.
Reaches are not allowed to cross, but can be connected at their endpoints (junctions) to form a network.
The normal direction of flow on a reach is indicated by the order of its endpoints. One point marks the upstream or "from" end of the reach, the other marks the downstream or "to" end of the reach.
Appendix B - HEC-RAS Import/Export Files for Geospatial Data
Defining Cross Sections
Each cross section is defined by a series of 3D coordinates, and identified by a stream name and reach name (which must refer to an existing stream and reach) and a station, indicating the distance from the crosssection to the downstream end of the stream.
A cross-section line can cross a reach line exactly once, and cannot cross another cross-section line.
Results of a water surface calculation are exported in a file that contains cross-section locations in plane (2D) coordinates, water-surface elevations for the cross-sections, and boundary polygons for the reaches.
Water Surface Export Data Rules
A cross-section is represented by a water surface elevation and a series of 2D coordinates on the cross-section cut line. The full width of the crosssection is included.
One bounding polygon is created for each reach in the stream network, and for each profile.
A reach’s bounding polygon is made up of the most upstream crosssection on the reach, the endpoints of all cross-sections on the reach, and the most upstream cross-sections of reaches downstream of the reach.
For purposes of defining bounding polygons only, the endpoints of a crosssection are adjusted to the edge of the water surface at the cross-section if the cross-section is part of a floodway, a leveed section of the reach, or the water extent is controlled by a hydraulic structure. This allows calculated water surfaces that are higher than the land surface to be reported back to the CADD or GIS program.
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data
Sample RAS GIS Import File #This file is generated by HEC-GeoRAS for ArcGIS BEGIN HEADER: DTM TYPE: TIN DTM: C:\Examples\Baxter\baxter_tin STREAM LAYER: C:\Examples\Baxter\baxter.mdb\River NUMBER OF REACHES: 3 CROSS-SECTION LAYER: C:\Examples\Baxter\baxter.mdb\XSCutLines NUMBER OF CROSS-SECTIONS: 173 MAP PROJECTION: STATEPLANE PROJECTION ZONE: DATUM: NAD83 VERTICAL DATUM: BEGIN SPATIAL EXTENT: XMIN: 6366478.85990533 YMIN: 2010839.52690533 XMAX: 6468128.45990533 YMAX: 2112489.12690533 END SPATIAL EXTENT: UNITS: FEET END HEADER: BEGIN STREAM NETWORK: ENDPOINT: ENDPOINT: ENDPOINT: ENDPOINT:
6453740, 6421541, 6387438, 6426447,
2051685, 2051194, 2035323, 2059280,
60, 1 34, 2 32.95776, 3 52.14808, 4
REACH: STREAM ID: Baxter River REACH ID: Upper Reach FROM POINT: 1 TO POINT: 2 CENTERLINE: 6453739.98997957, 2051684.77998051, 59.99999997, 89378.4140625 --- many lines omitted --6421540.44998505, 2051194.18999834, 34.00000001, 48157.06640625 END: REACH: STREAM ID: Baxter River REACH ID: Lower Reach FROM POINT: 2 TO POINT: 3 CENTERLINE: 6421540.44998505, 2051194.18999834, 34.00000001, 48157.06640625 --- many lines omitted --6387438.24001357, 2035323.14001705, 32.95775604, 0 END: REACH: STREAM ID: Tule Creek REACH ID: Tributary FROM POINT: 4 TO POINT: 2 CENTERLINE: 6426446.76000561, 2059279.84000069, 52.14807890, 12551.4970703125 --- many lines omitted ---
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data
6421540.44998505, 2051194.18999834, 34.00000001, 0 END: END STREAM NETWORK:
BEGIN CROSS-SECTIONS: CROSS-SECTION: STREAM ID: Baxter River REACH ID: Upper Reach STATION: 84815.69 NODE NAME: BANK POSITIONS: 0.5417204, 0.6313727 REACH LENGTHS: 343.447, 815.2449, 627.6476 NVALUES: 0, 0.06 0.2595427, 0.035 0.6867172, 0.06 LEVEE POSITIONS: INEFFECTIVE POSITIONS: BLOCKED POSITIONS: CUT LINE: 6451252.61043617, 2049658.48075948 6450473.97548097, 2050754.33739816 6449753.01716107, 2051480.10208855 SURFACE LINE: 6451252.61043617, 2049658.48075948, 125.00000002 --- many lines omitted --6449753.01716107, 2051480.10208855, 110.31235503 END: CROSS-SECTION: STREAM ID: Baxter River REACH ID: Upper Reach STATION: 77909.16 NODE NAME: BANK POSITIONS: 0.4635276, 0.572924 REACH LENGTHS: 223.1558, 229.2013, 233.3537 NVALUES: 0, 0.06 0.4353712, 0.035 0.6486487, 0.06 LEVEE POSITIONS: INEFFECTIVE POSITIONS: 354, 0, 0.3630761, 93.26781 355, 0.6235623, 1, 105.4026 BLOCKED POSITIONS: 379, 0.37786, 0.9548786, 79.19141 CUT LINE: 6446531.40685930, 2048445.67038340 6446341.91498890, 2048655.03933954 6446207.54346581, 2049102.94440073 6446140.35770426, 2049409.01289628 6446028.38145080, 2049909.17358660 6445838.02350501, 2050713.98307530 SURFACE LINE: 6446531.40685930, 2048445.67038340, 93.26781466 --- many lines omitted --6445838.02350501, 2050713.98307530, 105.40263370 END: --- many Cross Sections omitted ---
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data
CROSS-SECTION: STREAM ID: Baxter River REACH ID: Lower Reach STATION: 34251.78 NODE NAME: BANK POSITIONS: 0.2088515, 0.2746628 REACH LENGTHS: 678.4368, 652.6373, 592.5861 NVALUES: 0, 0.06 0.2023585, 0.035 0.5760272, 0.05 LEVEE POSITIONS: 380, 0.5949767, 72.00802 INEFFECTIVE POSITIONS: BLOCKED POSITIONS: CUT LINE: 6412787.19596798, 2042663.48848210 6412627.43755387, 2043633.45026854 6412056.87180271, 2047399.18430193 SURFACE LINE: 6412787.19596798, 2042663.48848210, 80.15862274 --- many lines omitted --6412056.87180271, 2047399.18430193, 77.57256318 END: END CROSS-SECTIONS:
BEGIN BRIDGES/CULVERTS: BRIDGE/CULVERT: STREAM ID: Tule Creek REACH ID: Tributary STATION: 4514.028 NODE NAME: Yosemite Street US DISTANCE: 100 TOP WIDTH: 96 CUT LINE: 6422221.24109452, 2055203.79594125 6421766.89378999, 2055127.22052519 6421302.33643314, 2054958.75468559 6421128.76554372, 2054912.80947382 6420924.56454467, 2054892.38936919 SURFACE LINE: 6422221.24109452, 2055203.79594125, 88.73309329 --- many lines omitted --6420924.56454467, 2054892.38936919, 83.88871764 END: --- many Bridges/Culverts omitted --END BRIDGES/CULVERTS:
BEGIN LEVEES: LEVEE ID: 380 SURFACE LINE: 6416224.46794023, 2048201.03890064, 80.30300144 --- many lines omitted --6408127.91921907, 2047348.05802148, 73.83999635 END: END LEVEES:
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data
BEGIN INEFFECTIVE AREAS: INEFFECTIVE ID: 354 POLYGON: 6446126.65267778, 2049275.06766575 6446347.63945516, 2049062.58037434 6446466.63230616, 2048960.58649530 --- many lines omitted --6446126.65267778, 2049275.06766575 END: INEFFECTIVE ID: 355 POLYGON: 6446009.40721919, 2049877.88188569 6445816.78229256, 2050758.82118551 --- many lines omitted --6446009.40721919, 2049877.88188569 END: --- many Ineffective Areas omitted --END INEFFECTIVE AREAS:
BEGIN BLOCKED OBSTRUCTIONS: BLOCKED ID: 379 POLYGON: 6422107.09773554, 6423542.24950153, 6422076.43212521, 6422107.09773554, END:
2052558.24567028 2052503.04750541 2052184.12491178 2052558.24567028
END BLOCKED OBSTRUCTIONS:
BEGIN LATERAL STRUCTURES: LATERAL STRUCTURE: STREAM ID: Baxter River REACH ID: Lower Reach STATION: 27469.68 NODE NAME: North LS US DISTANCE: 0 TOP WIDTH: 20 CUT LINE: 6407389.53497197, 2047168.40301990 6406371.11447597, 2046886.24321303 --- many lines omitted --6402363.56369299, 2045153.60574580 SURFACE LINE: 6407389.53497197, 2047168.40301990, 69.83999637 --- many lines omitted --6402363.56369299, 2045153.60574580, 65.27986148 END: END LATERAL STRUCTURES:
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data
BEGIN STORAGE AREAS: SA ID: 369 POLYGON: 6402631.96981374, 2045430.51958869 --- many lines omitted --6402631.96981374, 2045430.51958869 END: ELEVATION-VOLUME: 63.34, 0 64.59, 272682.8 65.84, 2102153 67.09, 1.130536E+07 68.34, 2.241535E+07 69.59, 3.505853E+07 70.84, 4.921408E+07 72.09, 6.477892E+07 73.34, 8.095226E+07 74.59, 9.734569E+07 75.84, 1.142249E+08 END: TERRAIN: END: END STORAGE AREAS: BEGIN SA CONNECTIONS: SA CONNECTION: SACONN ID: 444 NODE NAME: US SA: 369 DS SA: 371 TOP WIDTH: 20 CUT LINE: 6407389.53497197, 2047168.40301990 6406371.11447597, 2046886.24321303 --- many lines omitted --6402363.56369299, 2045153.60574580 SURFACE LINE: 6407389.53497197, 2047168.40301990, 69.83999637 --- many lines omitted --6402363.56369299, 2045153.60574580, 65.27986148 END: END SA CONNECTIONS:
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data
Sample RAS GIS Export File # RAS export file created on DAY DAYMONTHYEAR TIME # by HEC-RAS Version 3.1.3 BEGIN HEADER: UNITS: DTM TYPE: TIN DTM: C:\Examples\Baxter\baxter_tin STREAM LAYER: C:\Examples\Baxter\baxter.mdb\River CROSS-SECTION LAYER: C:\Examples\Baxter\baxter.mdb\XSCutLines MAP PROJECTION: STATEPLANE PROJECTION ZONE: DATUM: NAD83 VERTICAL DATUM: BEGIN SPATIALEXTENT: Xmin: 6386768.00418383 Ymin: 2029042.52107352 Xmax: 6454403.07894787 Ymax: 2059837.49270508 END SPATIALEXTENT: NUMBER OF PROFILES: 3 PROFILE NAMES: 50yr 100yr 500yr NUMBER OF REACHES: 3 NUMBER OF CROSS-SECTIONS: 179 END HEADER: BEGINSTREAMNETWORK: ENDPOINT:6421540.50,2051194.25, ENDPOINT:6453739.99,2051684.78, ENDPOINT:6387438.24,2035323.14, ENDPOINT:6426446.76,2059279.84,
, , , ,
1 2 3 4
REACH: STREAM ID: Baxter River REACH ID: Upper Reach FROM POINT: 2 TO POINT: 1 CENTERLINE: 6453739.99, 2051684.78, , 6421540.45, 2051194.19, , END: REACH: STREAM ID: Baxter River REACH ID: Lower Reach FROM POINT: 1 TO POINT: 3 CENTERLINE: 6421540.45, 2051194.19, , 6387438.24, 2035323.14, , END: REACH: STREAM ID: Tule Creek REACH ID: Tributary
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Appendix B - HEC-RAS Import/Export Files for Geospatial Data
FROM POINT: 4 TO POINT: 1 CENTERLINE: 6426446.76, 6421540.45, END:
2059279.84, , 2051194.19, ,
ENDSTREAMNETWORK:
BEGIN CROSS-SECTIONS: CROSS-SECTION: STREAM ID:Baxter River REACH ID:Upper Reach STATION:84815.69 NODE NAME: CUT LINE: 6451252.6104362 , 2049658.4807595 6450473.975481 , 2050754.3373982 6449753.0171611 , 2051480.1020886 REACH LENGTHS:826.24,806.49,525.17 BANK POSITIONS:0.45159,0.51309 LEVEE POSITIONS: 380,0.93260,79.95625 WATER ELEVATION:70.39427,76.72782,86.74971 WATER SURFACE EXTENTS: 6450877.21, 2050186.83, 6450289.15, 6450896.85, 2050159.18, 6450262.99, 6450912.28, 2050137.47, 6450189.98, PROFILE ID:50yr VELOCITIES: 0.32733, 1.558 0.46174, 2.381 0.55094, 3.764 0.56925, 4.280 0.58721, 6.164 0.60317, 5.713 0.62166, 3.942 0.64436, 1.926 PROFILE ID:100yr VELOCITIES: 0.31866, 2.972 0.45698, 3.829 0.55086, 5.019 0.56908, 5.459 0.58709, 7.245 0.60341, 6.737 0.62189, 5.168 0.65404, 3.202 PROFILE ID:500yr VELOCITIES: 0.31332, 4.739 0.45464, 5.533 0.55081, 6.526 0.56894, 6.860 0.58698, 8.456 0.60365, 7.890 0.62206, 6.635 0.66467, 4.272 SURFACE LINE: 6451252.61, 2049658.48, 125.00 --- many lines omitted ---
B-24
2050940.40 2050966.73 2051040.23
Appendix B - HEC-RAS Import/Export Files for Geospatial Data
449753.02, 2051480.10, 110.31 END: CROSS-SECTION: STREAM ID:Tule Creek REACH ID:Tributary STATION:1595.102 NODE NAME: CUT LINE: 6422369.1971783 , 2052943.6596315 6421588.0439919 , 2052573.50648 --- many lines omitted --6420275.0509832 , 2052670.3666247 WATER ELEVATION:62.67044,69.44948,78.49661 WATER SURFACE EXTENTS: 6421432.49, 2052554.70, 6420609.83, 6421570.89, 2052571.43, 6420459.69, 6422048.65, 2052791.77, 6420316.40, PROFILE ID:50yr VELOCITIES: 0.47364, 0.016 0.65126, 0.056 0.74604, 0.171 0.75411, 0.221 0.76221, 0.247 0.77030, 0.207 0.77842, 0.151 0.79265, 0.059 PROFILE ID:100yr VELOCITIES: 0.44844, 0.116 0.62783, 0.185 0.74591, 0.383 0.75406, 0.466 0.76221, 0.514 0.77035, 0.444 0.77857, 0.350 0.81222, 0.177 0.86985, 0.096 PROFILE ID:500yr VELOCITIES: 0.21051, 0.019 0.42227, 0.092 0.62301, 0.192 0.74582, 0.350 0.75403, 0.407 0.76221, 0.444 0.77039, 0.393 0.77866, 0.327 0.81602, 0.232 0.88706, 0.146 0.94874, 0.075 SURFACE LINE: 6422369.20, 2052943.66, 80.22 --- many lines omitted --6420275.05, 2052670.37, 85.26 END:
2052432.00 2052510.35 2052634.53
END CROSS-SECTIONS:
B-25
Appendix B - HEC-RAS Import/Export Files for Geospatial Data
BEGIN STORAGE AREAS: SA ID: 369 WATER ELEVATION:65,65,65 POLYGON: 6402631.9698137 , 2045430.5195887 6402648.7543614 , 2046009.5857725 --- many lines omitted --6402631.9698137 , 2045430.5195887 END:
, , ,
SA ID: 370 WATER ELEVATION:65,65,65 POLYGON: 6411089.902679 , 2043584.9518455 , 6411100.24735 , 2041762.9675571 , --- many lines omitted --6411089.902679 , 2043584.9518455 , END: END STORAGE AREAS:
BEGIN BOUNDS: PROFILE LIMITS: PROFILE ID:50yr POLYGON: 6449753.02,2051480.10,70.39 --- many lines omitted --6449462.09,2051308.23,70.35 POLYGON: 6424775.60,2059535.58,62.69 --- many lines omitted --6424246.32,2059434.43,62.69 POLYGON: 6420221.24,2052718.80,62.36 --- many lines omitted --6420143.38,2052744.69,62.32 END: PROFILE LIMITS: PROFILE ID:100yr POLYGON: 6449753.02,2051480.10,76.73 --- many lines omitted --6449462.09,2051308.23,76.73 POLYGON: 6424775.60,2059535.58,69.51 --- many lines omitted --6424246.32,2059434.43,69.52 POLYGON: 6420221.24,2052718.80,69.19 --- many lines omitted --6420143.38,2052744.69,69.17 END: PROFILE LIMITS: PROFILE ID:500yr POLYGON: 6449753.02,2051480.10,86.75 --- many lines omitted --6449462.09,2051308.23,86.83
B-26
Appendix B - HEC-RAS Import/Export Files for Geospatial Data
POLYGON: 6424775.60,2059535.58,78.54 --- many lines omitted --6424246.32,2059434.43,78.54 POLYGON: 6420221.24,2052718.80,78.18 --- many lines omitted --6420143.38,2052744.69,78.16 END: END BOUNDS:
B-27
Appendix C- HEC-RAS Output Variables
APPENDIX
C
HEC-RAS Output Variables Hydraulic Output Variables Variable Name # Barrels Alpha Area
Units # sq ft
Area Channel Area Left Area Right Base WS
sq ft sq ft sq ft ft
Beta BR Open Area BR Open Vel
sq ft ft/s
Br Sel Mthd Breach CL Breach WD Breach Bottom El Breach Top El Breach SSL Breach SSR C & E Loss Center Station Ch Sta L Ch Sta R Clv EG No Wr
ft ft ft ft ft ft ft ft ft ft ft
Coef of Q Conv. Chnl Conv. Left Conv. Ratio
cfs cfs -
Conv. Right Conv. Total Crit Depth
cfs cfs ft
Description Number of barrels in a culvert. Alpha - energy weighting coefficient. Flow area of the entire cross section including ineffective flow. Flow area of the main channel including ineffective flow. Flow area of the left overbank including ineffective flow. Flow area of the right overbank including ineffective flow. Water surface for first profile (used in comparison to encroachment profiles). Beta - momentum weighting coefficient. Total area of the entire bridge opening. Average velocity inside the bridge opening (Maximum of BU and BD). Selected bridge hydraulic modeling method. Center line of weir breach. Bottom width of weir breach. Bottom Elevation of weir breach. Top Elevation of weir breach. Left side slope of weir breach. Right side slope of weir breach. Contraction or expansion loss between two cross sections. Stationing of the center of the main channel. Left station of main channel. Right station of main channel. Energy grade elevation at the culvert when calculated without the weir. WSPRO bridge method coefficient of discharge. Conveyance of main channel. Conveyance of left overbank. Ratio of the conveyance of the current cross section to the conveyance of the downstream cross section. Conveyance of right overbank. Conveyance of total cross section. Critical depth. Corresponds to critical water surface.
C-1
Appendix C- HEC-RAS Output Variables
Crit E.G.
ft
Crit Enrgy 1 Crit Enrgy 2 Crit Enrgy 3 Crit Num Crit W.S.
ft ft ft # ft
Crit W.S. 1 Crit W.S. 2 Crit W.S. 3 Culv Crt Depth Culv Depth Blocked Culv EG In Culv EG Out Culv Ent Lss Culv Ext Lss Culv Frctn Ls Culv Ful Lngh Culv Inlet Mann n Culv Inv El Dn Culv Inv El Up Culv Length Culv Nml Depth Culv Outlet Mann n Culv Q Culv Vel DS Culv Vel US Culv WS In Culv WS Out Cum Ch Len Deck Width
ft ft ft ft ft ft ft ft ft ft ft
Delta EG Delta WS Dist Center L Dist Center R E.G. DS E.G. Elev E.G. IC E.G. OC E.G. Slope E.G. US.
ft ft ft ft ft ft ft ft ft/ft ft
Enc Method Enc Sta L Enc Sta R Enc Val 1 Enc Val 2
ft ft ft ft
C-2
ft ft ft ft cfs ft/s ft/s ft ft ft ft
Critical energy elevation. Minimum energy on the energy versus depth curve. Energy associated with first critical depth. Energy associated with second critical depth. Energy associated with third critical depth. Number of critical depths found. Critical water surface elevation. Water surface corresponding to the minimum energy on the energy versus depth curve. Water surface elevation of first critical depth. Water surface elevation of second critical depth. Water surface elevation of third critical depth. Critical depth inside the culvert. Depth of fill in a culvert. Energy gradeline inside the culvert at the inlet. Energy gradeline inside the culvert at the outlet. Culvert entrance loss (energy loss due only to entrance). Culvert exit loss (energy loss due to exit). Friction loss through the culvert barrel. The length that the culvert flows full. The composite n value at the culvert inlet. Culvert inside invert elevation downstream. Culvert inside invert elevation upstream. Length of the culvert barrel. Normal depth for this culvert (and flow). The composite n value at the culvert outlet. Flow through all barrels in a culvert group. Velocity inside of culvert at inlet. Velocity inside of culvert at outlet. Water surface elevation inside the culvert at the inlet. Water surface elevation inside the culvert at the outlet. Cumulative Channel Length. Width of bridge/culvert Deck (top of embankment), in direction of flow. Change in energy grade line through culvert(s) and bridge(s). Change in water surface through culvert(s) and bridge(s). Distance from center of channel to left encroachment. Distance from center of channel to right encroachment. Energy grade elevation at downsteam end of bridge or culvert. Energy gradeline for calculated WS Elev. Upstream energy gradeline at culvert based on inlet control. Upstream energy gradeline at culvert based on outlet control. Slope of the energy grade line. Energy grade elevation at upstream end of bridge or culvert (final answer). Encroachment method used at this cross section. Left station of encroachment. Right station of encroachment. Target for encroachment analysis. Second target for encroachment analysis.
Appendix C- HEC-RAS Output Variables
Encr WD Energy EG
ft ft
Energy WS
ft
Energy/Wr EG
ft
Energy/Wr WS
ft
Flow Area Flow Area Ch Flow Area L Flow Area R Frctn Loss Frctn Slope Frctn Slp Md Froude # Chl Froude # XS Gate #Open Gate Area Gate Group Q Gate Invert Gate Open Ht Gate Submerg
sq ft sq ft sq ft sq ft ft ft/ft # sq ft cfs ft ft -
Headloss Hydr Depth
ft ft
Hydr Depth C
ft
Hydr Depth L
ft
Hydr Depth R
ft
Ice Btm Chan Ice Btm LOB Ice Btm ROB Ice Err Ice Thick Chan Ice Thick LOB Ice Thick ROB Ice Top Chan Ice Top LOB Ice Top ROB Ice Vol Total Ice Vol. Chan Ice Vol. LOB Ice Vol. ROB
ft ft ft ft ft ft ft ft ft ft cu ft cu ft cu ft cu ft
Top width between encroachments. Energy grade elevation upstream of bridge for energy only method. Water surface elevation upstream of bridge for energy only method. Energy grade elevation upstream of bridge for low energy and weir method. Water surface elevation upstream of bridge for low flow energy method and weir flow. Total area of cross section active flow. Area of main channel active flow. Area of left overbank active flow. Area of right overbank active flow. Friction loss between two cross sections. Representative friction slope between two cross sections. Friction slope averaging method used. Froude number for the main channel. Froude number for the entire cross section. The number of gates opened in the current group. The flow area in an opened gate. Flow through all gate openings in a gate group. Gate spillway invert elevation. Height of gate opening. Degree of gate submergence. The ratio of the downstream depth above the gate to the upstream depth above the gate. Total energy loss between two cross sections. Hydraulic depth for cross section (Area/Topwidth of active flow). Hydraulic depth in channel (channel flow area/topwidth of channel flow). Hydraulic depth in left overbank (left overbank flow area/topwidth of left overbank flow). Hydraulic depth for right over bank (right overbank flow area/topwidth of right overbank flow). The bottom elevation of ice in the main channel. The bottom elevation of ice in the left overbank. The bottom elevation of ice in the right overbank. Convergence error in ice thickness for dynamic ice jam. Ice thickness in the main channel. Ice thickness in the left overbank. Ice thickness in the right overbank. The top elevation of ice in the main channel. The top elevation of ice in the left overbank. The top elevation of ice in the right overbank. Cumulative volume of ice in an ice jam. Cumulative volume of ice in the main channel for an ice jam. Cumulative volume of ice in the left overbank for an ice jam. Cumulative volume of ice in the right overbank for an ice jam. C-3
Appendix C- HEC-RAS Output Variables
Ice WS Err Ineff El Left Ineff El Right Inflow Invert Slope
ft ft ft cfs ft/ft
IW Gate Flow
cfs
K Perc L K Perc R L. Freeboard
ft ft ft
L. Levee Frbrd Left Sta Eff Length Chnl Length Left Length Rght Length Wtd.
ft ft ft ft ft ft
Levee El Left Levee El Right LOB Elev Mann Comp Mann Wtd Chnl Mann Wtd Chnl Mann Wtd Rght Mann Wtd Total Max Chl Dpth Min Ch El Min El Min El Prs Min Error
ft ft ft -
ft ft ft ft ft
Min El Weir Flow Min Weir El Momen. EG
ft ft ft
Momen. WS
ft
Net Flux Num Trials
cfs #
Obs WS Outflow Perc Q Leaving Piping Flow Power Chan
ft cfs
C-4
ft lb/ft s
Convergence error in water surface for dynamic ice jam. The elevation of the left ineffective area. The elevation of the right ineffective area. Net inflow into a storage area. The slope from the invert of this cross section to the next cross section downstream. Total flow through all of the gate groups of an inline weir/spillway. Conveyance reduction from left encroachment. Conveyance reduction from right encroachment. The freeboard in the main channel at the left bank (left bank elevation minus water surface elevation). The freeboard before the left levee is over-topped. Furthest left station where there is effective flow. Downstream reach length of the main channel. Downstream reach length of the left overbank. Downstream reach length of the right overbank. Weighted cross section reach length, based on flow distribution, in left bank, channel, and right bank. The elevation of the left levee. The elevation of the right levee. The ground elevation at the left bank of the main channel. Composite Manning’s n value for main channel. Conveyance weighted Manning's n for the main channel. Conveyance weighted Manning's n for the left overbank. Conveyance weighted Manning's n for the right overbank. Manning’s n value for the total main cross section. Maximum main channel depth. Minimum main channel elevation. Minimum overall section elevation. Elevation at the bridge when pressure flow begins. The minimum error, between the calculated and assumed water surfaces when balancing the energy equation. Elevation where weir flow begins. Minimum elevation of a weir. Energy grade elevation upstream of bridge for momentum method. Water surface elevation upstream of bridge for momentum method. Net inflow - outflow for a storage area. Current number (or final number) of trials attempted before the energy equation is balanced. Observed water surface elevation. Net outflow into a storage area. Percentage of flow leaving through a lateral weir. Flow from piping weir failure. Total stream power in main channel (main channel shear stress times main channel average velocity). Used in Yang’s and other sediment transport equations.
Appendix C- HEC-RAS Output Variables
Power LOB
lb/ft s
Power ROB
lb/ft s
Power Total
lb/ft s
Prof Delta EG
ft
Prof Delta WS
ft
Profile Prs O EG
# ft
Prs O WS
ft
Prs/Wr EG
ft
Prs/Wr WS
ft
Pumping Head Q Barrel Q Bridge Q Channel Q Culv Q DS Q Lat RC Q Leaving Total Q Left Q Perc Chan Q Perc L Q Perc R Q Pump Group Q Pump Station Q Right Q Total Q US Q Weir R. Freeboard
ft cfs cfs cfs cfs cfs cfs cfs cfs ft ft ft cfs cfs cfs cfs cfs cfs ft
R. Levee Frbrd Rght Sta Eff ROB Elev SA Area SA Chan
ft ft ft acres acres
Total stream power in left overbank (left overbank shear stress times left overbank average velocity). Used in Yang’s and other sediment transport equations. Total stream power in right overbank (right overbank shear stress times right overbank average velocity). Used in Yang’s and other sediment transport equations. Total stream power (total cross section shear stress times total cross section average velocity). Used in Yang’s and other sediment transport equations. Difference in EG between current profile and EG for first profile. Difference in WS between current profile and WS for first profile. Profile number. Energy grade elevation upstream of bridge for pressure only method. Water surface elevation upstream of bridge for pressure only method. Energy grade elevation upstream of bridge for pressure and/or weir method. Water surface elevation upstream of bridge for pressure and/or weir method. Pumping head for the pump station. Flow through one barrel in a culvert group. Flow through the bridge opening. Flow in main channel. Total flow in all culvert groups. Flow in cross section downstream of lateral weir. Lateral rating curve flow. Total flow leaving in a lateral weir including all gates. Flow in left overbank. Percent of flow in main overbank. Percent of flow in left overbank. Percent of flow in right overbank. Pump group flow. Total flow in all pump groups in a pump station. Flow in right overbank. Total flow in cross section. Flow in cross section upstream of a lateral weir. Flow over the weir. The freeboard in the main channel at the right bank (right bank elevation minus water surface elevation). The freeboard before the right levee is over-topped. Furthest right station that still has effective flow. The ground elevation at the right bank of the main channel. Surface area of a storage area. Cumulative surface area for main channel from the bottom of the reach.
C-5
Appendix C- HEC-RAS Output Variables
SA Left
acres
SA Min El SA Right
ft acres
SA Total
acres
SA Volume Shear Chan Shear LOB Shear ROB Shear Total Spc Force PR
acre-ft lb/sq ft lb/sq ft lb/sq ft lb/sq ft cu ft
Specif Force
cu ft
Sta W.S. Lft Sta W.S. Rgt Std Stp Case
ft ft #
Top W Act Chan
ft
Top W Act Left
ft
Top W Act Right
ft
Top W Chnl
ft
Top W Left
ft
Top W Right
ft
Top Wdth Act
ft
Top Width Total Gate Flow
ft cfs
Trvl Tme Avg
hrs
Trvl Tme Chl
hrs
Vel Chnl Vel Head Vel Left Vel Right Vel Total Vol Chan
ft/s ft ft/s ft/s ft/s acre-ft
C-6
Cumulative surface area for left overbank from the bottom of the reach. Minimum elevation of a storage area. Cumulative surface area for right overbank from the bottom of the reach. Cumulative surface area for entire cross section from the bottom of the reach. Storage volume of a storage area. Shear stress in main channel (γRCH Sf). Shear stress in left overbank ( γRLOB Sf). Shear stress in right overbank ( γRROB Sf). Shear stress in total section ( γRT Sf). Specific force prime. For mixed flow, the specific force at this cross section for the flow regime that does not control. The specific force for this cross section at the computed water surface elevation. SF = ATYcent + (Q2)/(gAact) Left station where water intersects the ground. Right station where water intersects the ground. Standard step method used to determine WSEL (1 = successful convergence, 2 = minimum error, 3 = resorted to critical depth). Top width of the wetted channel, not including ineffective flow. Top width of the wetted left bank, not including ineffective flow. Top width of the wetted right bank, not including ineffective flow. Top width of the main channel. Does not include 'islands', but it does include ineffective flow. Top width of the left overbank. Does not include 'islands', but it does include ineffective flow. Top width of the right overbank. Does not include 'islands', but it does include ineffective flow. Top width of the wetted cross section, not including ineffective flow. Top width of the wetted cross section. Total flow through all of the gate groups of an inline/lateral weir. Cumulative travel time based on the average velocity of the entire cross section, per reach. Cumulative travel time based on the average velocity of the main channel, per reach. Average velocity of flow in main channel. Velocity head. Average velocity of flow in left overbank. Average velocity of flow in right overbank. Average velocity of flow in total cross section. Cumulative volume of water in the channel (including ineffective flow).
Appendix C- HEC-RAS Output Variables
Vol Left
acre-ft
Vol Right
acre-ft
Volume
acre-ft
W.P. Channel W.P. Left W.P. Right W.P. Total W.S. DS W.S. Elev WS Inlet WS Outlet W.S. Prime
ft ft ft ft ft ft ft ft ft
W.S. US. Weir Avg Depth Weir Max Depth Weir Sta DS Weir Sta Lft Weir Sta Rgt Weir Sta US Weir Submerg
ft ft ft ft ft ft ft -
Wr Flw Area Wr Top Wdth WS Air Entr. WSPRO EG
sq ft ft ft ft
WSPRO WS
ft
Wtd. n Chnl Wtd. n Left Wtd. n Right XS Delta EG
ft
XS Delta WS
ft
Yarnell EG
ft
Yarnell WS
ft
Cumulative volume of water in the left overbank (including ineffective flow). Cumulative volume of water in the right overbank (including ineffective flow). Cumulative volume of water in the direction of computations (including ineffective flow). Wetted perimeter of main channel. Wetted perimeter of left overbank. Wetted perimeter of right overbank. Wetted perimeter of total cross section. Water surface downstream of a bridge, culvert, or weir. Calculated water surface from energy equation. WS at the inlet of a pump station. WS at the outlet of a pump station. Water surface prime. For mixed flow, the water surface of the flow regime that does not control. Water surface elevation upstream of bridge or culvert. The average depth of flow over the weir. The maximum depth of flow over the weir. Downstream station where weir flow ends. Station where flow starts on the left side of weir. Station where flow ends on the right side of weir. Upstream station for weir flow starts. The ratio of the downstream depth above the weir to the upstream depth above the weir. Area of the flow going over the weir. Top width of water over the weir. Water surface elevation accounting for air entrainment. Energy grade elevation upstream of bridge for the WSPRO method. Water surface elevation upstream of bridge for the WSPRO method. Conveyance weighted Manning's n for the main channel. Conveyance weighted Manning's n for the left overbank. Conveyance weighted Manning's n for the right overbank. Change in energy gradeline between current section and next one downstream. Change in water surface between current section and next one downstream. Energy grade elevation upstream of bridge for Yarnell method. Water surface elevation upstream of bridge for Yarnell method.
C-7
Appendix C- HEC-RAS Output Variables
Sediment Transport Output Variables
Variable Name
Units
Description
Ch Invert El step. Wsel Observed Data Invert Change Mass Out: All
ft
Minimum elevation of the main channel at each output time
Elevation of the water surface at each output time step. Observed elevation of main channel bed, entered by the user. Delta change in the minimum elevation of the main channel. Total sediment mass, for all grain size classes, going out of the sediment control volume, per individual computational time step. Mass Out: Class 1-20 tons Sediment mass leaving the sediment control volume per grain size fraction, per computational time step. Mass In: All tons Total sediment mass, for all grain size classes, coming into the sediment control volume, per individual computational time step. Mass In: Class 1-20 tons Sediment mass entering the sediment control volume per grain size fraction, per computational time step Flow cfs Total flow at the cross section for each output time step. Velocity ft/s Average velocity of the movable portion of the bed at each time step. Shear Stress lb/sq ft Average shear stress of the movable portion of the bed at each time step. EG Slope ft/ft Slope of the energy gradeline at each output time step. This can be a point value at the cross section or an average value between cross sections. Mass Bed Change Cum: All (tons) Cumulative mass of the change in the bed elevation over time. Mass Bed Change Cum: class 1-20 (tons) Cumulative mass of the change in bed elevation over time, per grain size fraction (Bins 1 – 20). This only displays the size fraction bins that are being used. Mass Bed Change: All tons Incremental total mass change in the bed for the current computational time step. Mass Bed Change: Class 1–20 (tons) Incremental mass change in the bed for the current time step, by individual grain size fraction. Mass Out Cum: All tons Cumulative total sediment mass leaving the sediment control volume for a specific cross section, per individual computational time step. Mass Out Cum: Class 1-20 (tons) Cumulative sediment mass leaving the sediment control volume per grain size fraction, at a cross section, per computational time step. Mass In Cum: All tons Cumulative total sediment mass entering the sediment control volume for a specific cross section, per individual computational time step.
C-8
ft ft ft tons
Appendix C- HEC-RAS Output Variables
Mass In Cum: Class 1-20 (tons)
Cumulative sediment mass entering the sediment control volume per grain size fraction, at a cross section, per computational time step. Mass Capacity: All tons/day Transport capacity in total mass at the current computational time step. Mass Capacity: Class 1-20 (tons/day) Transport capacity in mass, by grain size fraction, at the current computational time step. Mean Eff Ch Invert ft Average channel invert elevation computed by subtracting the effective depth of the main channel from the water surface elevation. Mean Eff Ch Invert Change (ft) Change in the average channel invert elevation, which is computed by subtracting the effective depth of the main channel from the water surface elevation. Long. Cum Mass change (tons) Total change in bed mass, cumulative in space and time. Spatial accumulation is from the current cross section to the upstream end of the river reach in which this cross section resides. d50 Cover mm d50 of the cover layer at the end of the computational increment. Used in the Exner 5 bed sorting and armoring routine. d50 Subsurface mm d50 of the surface layer material at the end of the computational time step. Used in the Exner 5 bed sorting and armoring routine. d50 Active mm d50 of the active layer of the simple active layer bed sorting and armoring routine. d50 Inactive mm d50 of the inactive layer at the end of each computational time step. Used in the Exner 5 and simple active layer bed sorting and armoring routine. Cover Thickness ft Thickness of the cover layer at the end of each computational time step. Used in the Exner 5 bed sorting and armoring routine. Subsurface Thickness ft Thickness of the surface layer at the end of each computational time step. Used in the Exner 5 and simple active layer bed sorting and armoring routine. Active Thickness ft Thickness of the active layer at the start of each computational time step. Used in the simple active layer bed sorting and armoring routine. Mass Cover: All tons Total tons of material in the cover layer at the end of each computational time step. Used in the Exner 5 bed sorting and armoring routine. Mass Cover: Class 1-20 (tons) Tons of material in the cover later at the end of each computational time step, by individual grain size fraction. Used in the Exner 5 bed sorting and armoring routine. Mass Subsurface: All tons Total tons of material in the surface layer at the end of each computational time step. Mass Subsurface: Class 1-20 (tons) Tons of material in the surface layer at the end of each computational time step, by individual grain size fraction. Mass Inactive: All tons Total tons of material in the inactive layer at the end of each computational time increment. C-9
Appendix C- HEC-RAS Output Variables
Mass Inactive: Class 1-20 (tons)
Tons of material in the inactive layer at the end of each computational increment, by individual grain size fraction. Armor Reduction: All (fraction) Fraction that the total sediment transport capacity is reduce to, based on the concepts of a cover layer computation. Armor Reduction: Class 1-20 (fraction) Fraction for each individual grain size, that the transport capacity is reduce to, based on the concepts of a cover layer computation. Sediment Discharge tons/day Total sediment discharge in tons/day going out of the sediment control volume for a specific cross section, per individual computational time step. Sediment Concentration (mg/l) Total sediment concentration in mg/liter going out of the sediment control volume at the end of the computational time step. Eff Depth ft Effective depth of the water in the mobile portion of the cross section, at the end of the computational time step. Eff Width ft Effective width of the water in the mobile portion of the cross section, at the end of the computational time step. Ch Manning n Main channel manning’s n value. Ch Froude Num Main channel Froude number at the end of the current computational time step. Shear Velocity u* ft/s Shear velocity. Used in Shields diagram and several sediment transport potential equations. d90 Cover mm d90 of the cover layer at the end of the computational increment. Used in the Exner 5 bed sorting and armoring routine. d90 Subsurface mm d90 of the surface layer material at the end of the computational time step. Used in the Exner 5 bed sorting and armoring routine. d90 Active mm d90 of the active layer of the simple active layer bed sorting and armoring routine. d90 Inactive mm d90 of the inactive layer at the end of each computational time step. Used in the Exner 5 and simple active layer bed sorting and armoring routine. Dredge Vol Cum ft3 Total volume of sediment removed from each cross section by the dredging routines.
C-10
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