It is essential to have a detailed and accurate terrain model in order to create a detailed and accurate hydraulics model. The quality of the terrain data can be a limiting factor in the quality of the hydraulics model the user can create. Terrain data comes from many different sources, formats, and levels of detail. Currently HEC-RAS uses gridded data for terrain modeling. It is up to the user to gather data from multiple sources, then convert/export it into a gridded data format that can be read in by HEC-RAS.

It is necessary to create a terrain model in HEC-RAS Mapper before the user can perform any model computations that contain two-dimensional (2D) flow areas, or before the user can visualize any one-dimensional (1D), 2D, or combined 1D/2D mapping results.  This section of the document describes how to create a terrain model in RAS Mapper, as well as other optional geospatial layers.  The user can develop one or more terrain models that can then be associated with a specific geometry input file, or a specific results output file.  Additionally, users have the option in RAS Mapper to create a Landcover data layer, Soils layer, and an Infiltration layer.  Once a Landcover layer is created, users can associate Manning’s n values with each of the landcover types.  Optionally users can specify a percent impervious for each landcover type (% impervious is only necessary if the user is modeling precipitation and infiltration within HEC-RAS).  Soils data can be imported from gridded Soil Survey Geographic (gSSURGO) database files or the information can be imported from existing shape files. Soils data can be specified as either Soil Conservation Service (SCS) hydrologic soil groups (HSG) or more detailed soil texture classes.  An Infiltration layer can be derived from a landcover and Soils layer, just landcover, or just a soils layer.  In RAS Mapper, users have the option of three different infiltration methods for creating an infiltration layer: Deficit-Constant, SCS Curve Number, or Green and Ampt.


For more details on creating terrain models and other geospatial layers with HEC-RAS Mapper, please review the HEC-RAS Mapper User’s manual.