DIGITAL FLOOD INUNDATION MODELS GENERATED FROM LIDAR DATA AND HEC-RAS SOFTWARE – TETON RIVER, MADISON COUNTY, IDAHO

LiDAR data, in conjunction with geographic information system (GIS) software, are successfully used to model flood inundation along theTeton River, Idaho. The model predicts inundation for a 100-year flood as a benchmark to compare with published Flood Insurance Rate Maps (FIRMs); it also predicts flood inundation for river discharge greater and less than the 100-year event. The model consists of hundreds of cross-sectional river profiles acquired using ArcGIS 10 software to interpolate a LiDAR-derived, 1-m resolution, bare earth digital elevation model (DEM) of the river and surrounding floodplain. The cross-sections are imported into the Hydrologic Engineering Center River Anaylsis System (HEC-RAS) software which models floodwater extent based on discharge, river shape, and floodplain elevations. A discharge value of 7280 ft³/sec represents a 100-year flood event on the Teton River. Coefficients of friction for Manning’s equation are selected dependent upon the ground cover of overbanks and the nature of the bed load in the river channel at each cross-sectional river profile. HEC-RAS generates a georeferenced polygon shapefile of the predicted floodwater extent based on discharge values and the coefficients of friction used in Manning’s equation. Layering the shapefile with the LiDAR data and with 1-m resolution NAIP imagery gives a clear picture of the 100-year floodplain for the Teton River and illustrates where potential flood damage to agricultural, business and residential structures may occur. An advantage of modeling riverine floods using the outlined method is that once the data is assembled, various discharge values and their resulting flood extents can be simulated quickly providing near real-time inundation maps during high water run-off.