Flood-inundation area for the Withlacoochee River in Lowndes County, Georgia from Skipper Bridge Road to St. Augustine Road

Digital flood-inundation maps for a 12.6-mile reach of the Withlacoochee River from Skipper Bridge Road to St. Augustine Road, Lowndes County, Georgia, were created by the U.S. Geologicay Survey (USGS) in cooperation with the city of Valdosta and Lowndes County, Georgia. The inundation maps, which can be accessed through the USGS Flood Inundation Mapping Science Web site at http://water.usgs.gov/osw/flood_inundation/, depict estimates of the areal extent and depth of flooding corresponding to selected water levels (stages) at the USGS streamgage Withlacoochee River at Skipper Bridge Road, near Bemiss, Georgia (023177483). Real-time stage information from this streamgage can be obtained at the National Water Information System Web Interface (NWISWeb; https://waterdata.usgs.gov/ga/nwis/rt) and can be used with these maps to estimate near real-time areas of inundation. In addition, the information has been provided to the National Weather Service (NWS) for incorporation into their Advanced Hydrologic Prediction Service (AHPS; http:/water.weather.gov/ahps/) flood warning system (http:/water.weather.gov/ahps/). The NWS forecasts flood hydrographs at many places that are often collocated at USGS streamgages. The forecasted peak-stage information, also available on the Internet, may be used in conjunction with the maps developed in this study to show predicted areas of flood inundation. In this study, flood profiles were computed for the stream reach by means of a one-dimensional step-backwater model. The model was calibrated using the most current stage-discharge relations at the streamgage Withlacoochee River at Skipper Bridge Road, near Bemiss (023177483) and documented high-water marks from recent floods. The hydraulic model was then used to determine 23 water-surface profiles for flood elevations at 1.0-foot intervals referenced to the water surface elevation and ranging from NWS action stage of 131.0 feet (10.7 feet streamgage stage) to 153.0 feet (32.7 feet streamgage stage) which is near the highest recorded water level at the streamgage. The flood-peak inundation area was modeled in a GIS by combining steady-state hydraulic modeling and available lidar digital elevation model (DEM) data. Information about the study, floods, and methods used can be found in the USGS Scientific Investigations Report 2018-5011 online at https://doi.org/10.3133/sir20185011.