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  4. Defining HEC-RAS Storage Areas

Defining HEC-RAS Storage Areas

Upstream and downstream boundaries of a river reach can be connected to a reservoir, lake, or other types of large water bodies. These water bodies are called storage areas in HEC-RAS.

HEC-RAS Storage Areas

In addition, off-channel ponding areas that exchange water with the adjacent river can be modeled as storage areas. A lateral structure is required to exchange water between the river and the storage area.

Off-channel Ponding Storage Areas

The following diagram represents the three different situations where a storage area can be connected to a river reach.

Scenarios to Connect SA with River Reach

HEC-RAS storage areas can be defined by either drawing or assigning the boundary using the following commands:

  • Draw Storage Areas
  • Assign Storage Areas

Existing storage areas can also be georeferenced to a more accurate outline representation of the water body assigned.

Drawing HEC-RAS Storage Areas

HEC-RAS storage areas can be defined by drawing the boundary of the reservoir, lake, or ponded area using the Draw Storage Areas command. Follow these steps to draw a storage area:

  1. From the Input ribbon menu, click the Storage Areas menu item and choose the Draw Storage Areas command.
  1. The Draw Storage Areas dialog box will be displayed.
  1. Click the [Draw] button adjacent to the Storage area polygon field. By default, the software allows you to draw the storage area polygon using linear segments. Use the Create curvilinear polygon checkbox option to draw the polygon using curvilinear segments. While drawing elements, the user can use the [Ctrl] key to switch between the curvilinear and the linear digitizing. To learn more about drawing elements on the Map View, refer to this article in our knowledge base.
  2. The Draw Storage Areas dialog box will temporarily disappear, and a prompt will be displayed on the status line informing what to do next.
  3. Draw the storage area on the Map View, and then right-click and choose Done from the displayed context menu.
  4. The Draw Storage Areas dialog box will reappear with the storage area drawn.
  5. Provide a name for the storage area in the Storage area ID entry field.
  6. Click on the [Apply] button and the storage area will be created.

Assigning HEC-RAS Storage Areas

HEC-RAS storage areas can be defined by assigning an already drawn polyline or polygon as the boundary of the reservoir, lake, or ponded area using the Assign Storage Areas command. Follow these steps to assign a storage area:

  1. From the Input ribbon menu, click the Storage Areas menu item and choose the Assign Storage Areas command.
  1. The Assign Storage Areas dialog box will be displayed.
  1. Click the [Pick] button adjacent to the Storage area polyline/polygon field.
  2. The Assign Storage Areas dialog box will temporarily disappear, and a prompt will be displayed on the status line informing what to do next.
  3. From the Map View, click on a polyline or polygon representing the storage area boundary.
  4. The Draw Storage Areas dialog box will reappear with the storage area selected.
  5. Provide a name for the storage area in the Storage area ID entry field.
  6. Click on the [Apply] button and the storage area will be created.

Georeferencing HEC-RAS Storage Areas

Existing storage areas can be snapped to a Map View reference polyline or polygon. Follow these steps to georeference an existing storage area:

  1. From the Input ribbon menu, click the Storage Areas menu item and choose the Georeference Storage Areas command.
  1. The Georeference Storage Areas dialog box will be displayed.
  1. Click the [Pick] button adjacent to the Storage area ID dropdown combo box.
  1. The Georeference Storage Areas dialog box will temporarily disappear, and a prompt will be displayed on the status line informing what to do next.
  2. Select the storage area on the Map View.
  3. The Georeference Storage Areas dialog box will reappear with the storage area selected.

Alternatively, choose the storage area from the dropdown combo box adjacent to the Storage area ID entry.

Once the storage area has been selected, the user can choose between the following options to georeference the storage area:

  • Snap to Alignment Polyline/Polygon
  • Draw on Map View

Snap to Alignment Polyline/Polygon

This option allows the reference polyline or polygon to be interactively selected from the Map View:

  1. Choose the Snap to Alignment Polyline/Polygon radio button option and click the [Pick] button.
  2. The Georeference Storage Areas dialog box will temporarily disappear, and a prompt will be displayed on the status line informing what to do next.
  3. Select the alignment polyline or polygon on the Map View.
  4. The Georeference Storage Areas dialog box will reappear.
  5. Click the [Snap] button.
  6. The software will snap the storage area to the selected alignment polyline or polygon.

Draw on Map View

This option allows the user to draw an alignment polygon on the Map View:

  1. Choose the Draw on Map View radio button option and click the [Draw] button.
  2. The Georeference Storage Areas dialog box will temporarily disappear, and a prompt will be displayed on the status line informing what to do next.
  3. Draw the polygon on the Map View representing the storage area, and then right-click and choose Done from the displayed context menu.
  4. The Georeference Storage Areas dialog box will reappear.
  5. The software will automatically update the storage area accordingly.

Revising HEC-RAS Storage Area Boundary

Existing storage area boundaries can be revised using the Reshape Polygon command. Refer to this article in our knowledge base to learn how to use the Reshape Polygon command.

Extracting Storage Area Volume Curve

GeoHECRAS software can be used to calculate how much storage area a site has and what volume it holds. After defining the storage area boundary, the software automatically computes the fixed area “footprint” that the storage area occupies to define the storage volume. This method works well in situations when the storage area does not change with depth. However, defining the storage volume in this manner results in an inaccurate representation of the storage volume for the regions where the storage area varies considerably with the elevation change. For regions where a more accurate representation of the storage volume is required, the user can extract the storage volume from the terrain model if the bathymetry of the storage area is contained in the terrain model.

Refer to this article in our knowledge base to learn how to extract storage area volume from the terrain model.

About the Author Chris Maeder

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