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Hydraulic Parameters – Cross Sections Command

In GeoHECRAS, the Hydraulic Parameters-Cross Sections command is used to define the hydraulic parameters that are used for building cross section rating curves. During unsteady flow simulations, GeoHECRAS converts the geometry of cross sections into a set of rating curves defining relationships between hydraulic parameters and stage. When running a hydraulic model, the software picks values from these curves rather than computing hydraulic parameters for each time step, which speeds up the unsteady flow computations. This data is only used for unsteady flow simulations and is ignored for steady flow simulations.

Follow the steps below to use the Hydraulic Parameters – Cross Sections command:

  1. From the Analysis ribbon menu, expand the Rating Curves – Hydraulic Parameters dropdown combo box and select the Hydraulic Parameters – Cross Sections command.
    Hydraulic Parameters – Cross Sections Command
  2. The Hydraulic Parameters – Cross Sections dialog box will be displayed.
    Hydraulic Parameters – Cross Sections Dialog Box

The following sections describe the Hydraulic Parameters – Cross Sections command and how to interact with the above dialog box.

Rating Curve Parameters

This section contains three editable columns within a data grid in which the user can enter minimum elevation, increment height, and the number of points for the rating curve. The remaining columns are read-only. The first time the user opens the dialog box, all the columns are automatically filled.
Rating Curve Parameters

The data in the following columns are editable:

  • Rating Curve (RC) Minimum Elevation
    By default, these column entries are automatically filled with an elevation equal to 0.5 ft (0.15 meters) higher than the channel minimum invert. However, the user can change these values as per specific modeling requirements.
  • Rating Curve (RC) Increment Height
    By default, these column entries are filled with a value of 1.0 foot (1.0 meter). However, if this default value causes any cross section to have less than 20 rating curve points (because the cross section is not tall enough), the offending cross section entry is automatically filled with a value equal to the cross section height (maximum – minimum elevation) divided by 20. However, the user can change this value to any positive non-zero value as per the user’s modeling requirements.
  • Number of Rating Curve (RC) Points
    By default, these column entries are set to a value that will allow the table to extend to the top of the cross section. However, the user can adjust the number of points between 10 and 500. More points mean that HEC-RAS can interpolate between more values. Therefore, it is a good idea to increase the number of points when you are having stability problems.

Note that the River Name and Reach Name columns can be filtered, allowing the user to select specific rivers and reaches for review.

In addition, the user can use the cut, copy, and paste commands (using standard Windows function keys) to move data to and from the Windows clipboard.

Selected Cells Group Editing

This section allows the user to manipulate the cell values for the above-described column entries. The user can select a range of cells (or an entire data column) and use the following group editing radio button options:

  • Add constant
    Use this radio button option to add a constant value to the selected cells’ value.
  • Multiply by factor
    Use this radio button option to multiply the selected cells’ value by a factor.
  • Apply value
    Use this radio button option to enter a pre-defined value in all the selected cells.
  • Apply invert
    Use this radio button option to set your Rating Curve Minimum Elevation value to be the same as your Channel Minimum Elevation value. This will cause the software to start small computation increments from the channel invert, not 1 ft (0.3 m) above the channel invert.
    Selected Cells Group Editing

Once the required group editing radio button option is selected, the user can click the [Apply Change] button to complete the editing process.

Note that the [Apply Change] button is only available when a group editing radio button option other than the default No Change option is selected.

If cells are selected in the data grid, then clicking the [Defaults] button resets the selected cells back to their default values. If no cells are selected, then clicking the [Defaults] button resets the entire table back to its default values.

Cross Section Plot

The Cross Section Plot section displays the cross section that is selected in the Rating Curve Parameters table, along with lines representing the corresponding rating curve elevation values.
Cross Section Plot

After entering the appropriate information, the user can click the [OK] button.

About the Author Chris Maeder

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