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.

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.

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

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

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 the steps below to draw a storage area:

1. From the Input ribbon menu, click the Storage Areas menu item and choose the Draw Storage Areas command.
   

2. The Draw Storage Areas dialog box will be displayed.
   
3. 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. Note that the user can use the Create curvilinear polygon checkbox option to draw the polygon using curvilinear segments.
   
4. The Draw Storage Areas dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next.
5. Draw the storage area on the Map View. 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.
6. When finished, press the [Enter] key or right-click and choose Done from the displayed context menu.
7. The Draw Storage Areas dialog box will be redisplayed and the status of the Storage area polygon read-only field will be changed from Not Drawn to Drawn.
   
8. Enter a unique name for the storage area in the Storage area ID entry field.
9. Click 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 the steps below to assign a storage area:

1. From the Input ribbon menu, click the Storage Areas menu item and choose the Assign Storage Areas command.
   

2. The Assign Storage Areas dialog box will be displayed.
   
3. Click the [Pick] button adjacent to the Storage area polyline/polygon field.
   
4. The Assign Storage Areas dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next.
5. Select the storage area polygon/polyline on the Map View.
6. The Draw Storage Areas dialog box will be redisplayed and the status of the Storage area polyline/polygon read-only field will be changed from Not Selected to Selected.
   
7. Enter a unique name for the storage area in the Storage area ID entry field.
8. Click the [Apply] button and the software will treat the selected polyline or polygon as a storage area.

Georeferencing HEC-RAS Storage Areas

The Georeference Storage Areas command is used to manually georeference each of the storage areas to the background base map displayed in the Map View. Refer to this article in our knowledge base to learn how to use the Georeference Storage Areas command.

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.