The Flow Roughness Change Factors command allows the user to adjust roughness coefficients with changes in flow. It enables the user to adjust roughness coefficients up or down individually as well as in a group to calibrate a HEC-RAS model to observed data.
This feature is very useful for calibrating an unsteady flow model for flows that vary from low to high. Note that roughness generally decreases with increases in flow and depth. In other words, for shallow, lower flows the Manning’s roughness can be increased to a higher value, but for deep, higher flows the Manning’s roughness will be reduced to a lower value.
For example, agricultural areas might become flooded during a large storm event. Depending upon the amount of discharge and corresponding flood depth, the surface roughness can vary. If the flood depth is shallow, the Manning’s roughness would be high. But, as discharge increases, and the corresponding flood depth becomes deeper, the agricultural crops would flatten, and the Manning’s roughness would be reduced.
Assigning Flow Roughness Change Factors
Follow the steps below to use the Flow Roughness Change Factors command:
- From the Input ribbon menu, select the Manning’s Roughness menu item and then select the Flow Roughness Change Factors command.
- The Flow Roughness Change Factors dialog box will be displayed.
The following sections describe the Flow Roughness Change Factors command and how to interact with the above dialog box.
Selecting River Reach
The Select River Reach section enables the user to select the desired river reach. Click the [Pick] button adjacent to the Reach name entry. The dialog box will temporarily disappear, and a prompt is displayed on the status line, informing the user what to do next. Select the river by clicking on it in the Map View.
Alternatively, the user can click the dropdown combo boxes adjacent to the River name and the Reach name entries and select the desired river reach.
Roughness Factor Data
This section displays a table listing different flow data and their respective roughness coefficients corresponding to their respective cross sections. The user can export the table as a PDF file or a Microsoft Excel file, or copy the table to the clipboard.
Roughness Factor Editing
This section enables the user to automatically generate multiple flow data as well as batch edit the flow and roughness factor data. It contains two tabs:
- Selected Cell Editing
- Auto Generate Flows
Selected Cell Editing Tab
The Selected Cell Editing section enables the user to select the desired cells from the Roughness Factor Data table and then edit the flow and roughness coefficients either individually or in a batch. The user can edit the coefficients using:
- Add constant
- Multiply by a factor
- Apply value
To edit the cell’s coefficient, select the cell by clicking on it while pressing the [Ctrl] or [Shift] key; choose the desired cell-editing option; enter the change coefficient, and then click the [Apply Change] button.
Auto Generate Flow Tab
The Auto Generate Flow section enables the user to automatically generate the river flow based on user-desired options. Simply select the upstream and the downstream cross sections from the respective dropdown combo box and enter the starting flow value. Then, select the flow growth type. It can be either uniform spacing or exponential.
Enter the uniform increase value or the increase factor coefficient, and then select the number of flows to generate. Next, click the [Apply] button.