Creating a terrain model for floodplain mapping

A geographic information system (GIS) based approach is presented for the development of a terrain model based on stream channel representation of the U. S. Army Corps of Engineers Hydrologic Engineering Center's River Analysis System (HEC-RAS) hydraulic model. As input, the approach requires a completed HEC-RAS simulation, a digital elevation model (DEM), and a GIS representation of the stream thalweg. The process begins with export of the channel data from HEC-RAS to GIS, followed by data conversion from hydraulic model coordinates to geographic coordinates. A digital terrain model is subsequently synthesized by merging HEC-RAS data for the stream channel with comparatively lower-resolution DEM data for the floodplain. The resulting surface model provides a good representation of the general landscape and contains additional detail within the stream channel. The accuracy of the terrain model is comparable to high-resolution terrain data acquired through aerial photogrammetry and can be used in conjunction with the new generation of river hydraulic models capable of accepting GIS terrain data. An example application to Waller Creek in Austin, Texas, is presented.