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Compute Slopes Command

Some of the stormwater runoff methods that HEC-HMS provides require that the subbasin slope be defined. Since GeoHECHMS works with elevation grids, it is straightforward to compute the subbasin slope and store this as a subbasin property.

Follow these steps to use the Compute Slopes command:

  1. From the Watershed ribbon menu, select the Compute Slopes command.
  2. The Compute Slopes dialog box will be displayed.

The following sections describe how to interact with the above dialog box.

Selecting Subbasins

This section includes a table that lists all the subbasins contained within the current scenario.

This section is used to manually select subbasins from the Map View. If a subbasin is already selected on the Map View prior to running this command, the same subbasin will be shown selected within the table.

Alternatively, click the [Pick] button to interactively select subbasins from the Map View. Clicking on the [Pick] button will cause the dialog box to temporarily disappear, allowing the user to select HEC-HMS subbasins from the Map View. Upon returning to the Compute Slopes dialog box, the total number of selected subbasins will be displayed in the Total selected entry.

Selecting Terrain Surface

The Terrain surface dropdown combo box lists all the elevation grids (i.e., DEMs) loaded in the project. Select the elevation grid to be used in computing the subbasin slope.

Computing Subbasin Slopes

Once the data has been defined in the Compute Slopes dialog box, click the [Compute] button. The software will then look at each subbasin selected and compute the average overland flow slope for each subbasin.

After the slopes are computed, the values will be shown in the Computed Slope (ft/ft) editable column, as shown below. The software also allows the user to modify these computed values.

Once done, click the [OK] button, and all HEC-HMS stormwater runoff methods that require a subbasin slope will have the data populated into the appropriate fields.

About the Author Chris Maeder

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