HEC-RAS Flood Result Raster Grids

While GeoHECRAS constructs a flood map representing the area of inundation, it can simultaneously generate an elevation raster grid (or elevation grid) file representing the water surface elevation for the mapped area. This raster grid file allows the flood map to be utilized in other software programs, such as ESRI ArcMap and ArcGIS. Each grid cell within the elevation raster contains a single value representing the computed water surface elevation. Areas that are not flooding are marked as “NODATA” in their grid cells.

HEC-RAS Flood Map Types

In addition to creating a flood map elevation raster grid, the software can generate raster grid files for any of the available flood maps that GeoHECRAS can produce, including:

It is a simple process to direct the software to generate a corresponding raster grid file while generating a flood map. The following section details how this is done.

Creating a Water Surface Elevation Raster Grid

This section describes the process used to generate a water surface elevation raster while generating the flood map. Take note that this same process can be used to generate raster grids for other flood results, such as maximum velocity flood maps.

For the flood map and corresponding water surface elevation raster grid to be generated, the HEC-RAS model analysis results must be available. If not, first perform a HEC-RAS flow analysis for the defined model.

Follow the steps below to create the water surface elevation raster grid:

1. From the Results ribbon menu, select the Flood Map command.
   
2. The Flood Map dialog box will be displayed.
   
3. From the General Specifications section, click on the Flood map type dropdown combo box, and then select the Water Surface Elevation option.
   
4. Then click on the Flood map computation dropdown combo box and select either the Detailed Intersection or Time Series Animation option.
   
5. Now, click the [Options…] button adjacent to the Flood map computation dropdown combo box.
   
6. The Flood Map Computation Options dialog box will be displayed.
   
7. Select the Create Water Surface Elevation Grid checkbox to enable the content of this section. To generate a raster grid of the flood map in the Map Data Layers panel, select any of the following options:
   • Maximum water surface elevation grid: This will create a raster grid of maximum water surface elevation for the selected flood map type.
   • Last time step water surface elevation grid: This will create a raster grid of water surface elevation at the last time step.
   • Water surface elevation grid for the time step: This will create a raster grid of water surface elevation at any manually selected time step. On selecting this option, the user needs to select the desired time step from the dropdown combo box provided next to this option.
8. Define the name of the raster grid layer in the Layer name input field.
9. Check the Delete previous flood grid checkbox to allow the software to delete any previously generated flood map raster grid.
10. Once finished, click the [OK] button and the software will then generate a water surface elevation grid that corresponds to the flood results from HEC-RAS.

Displaying the Water Surface Elevation Raster Grid

Once the elevation raster grid has been created, it will be automatically added to the Map Data Layers panel. If desired, the user can change the way the computed water surface elevation raster grid is displayed on the Map View.

1. From the Map Data Layers panel, click on the […] Properties button adjacent to the raster grid layer.
   
2. The Elevation Grid Properties dialog box will be displayed.
   
3. Turn on the Elevation Color Fill checkbox and select the desired elevation color fill option.
   
4. Then, click the [OK] button.

If the elevation raster is not displayed, turn off the display of other raster layers or online map layers that lie above the elevation raster in the Map Data Layers panel. The order of the layers listed in the Map Data Layers panel determines the display order of each layer, and there might be another layer that overlays the elevation raster layer. Alternatively, drag the elevation raster above the other layers listed in the Map Data Layers panel so that the elevation raster can be seen on the Map View.