Flood-inundation maps for the St. Marys River at Decatur, Indiana

Digital flood-inundation maps for an 8.9-mile reach of the St. Marys River at Decatur, Indiana, were developed by the U.S. Geological Survey (USGS), in cooperation with the Indiana Office of Community and Rural Affairs. The inundation maps, which can be accessed through the USGS Flood Inundation Mapping Science Web site (http://water.usgs.gov/osw/flood_inundation/), depict estimates of the areal extent and depth of flooding corresponding to selected water levels (stages) of the St. Marys River at Decatur (USGS station number 04181500). The maps are useful for estimating near-real-time areas of inundation by referencing concurrent USGS streamgage information at http://waterdata.usgs.gov/. In addition, the streamgage information was provided to the National Weather Service (NWS) for incorporation into their Advanced Hydrologic Prediction Service flood warning system (http:/water.weather.gov/ahps/). NWS-forecasted peak-stage information may be used in conjunction with the maps developed during this study to show predicted areas of flood inundation.

During this study, flood profiles were computed for the stream reach by means of a one-dimensional, step-backwater model. The model was calibrated by using the stage-discharge relation for the streamgage at St. Marys River at Decatur. The hydraulic model was used to compute 18 water-surface profiles for flood stages varied at 1-foot (ft) intervals and ranging from approximately bankfull (13 ft above gage datum) to greater than the highest recorded water level at the streamgage. To delineate the area of flood inundation for each modeled water level, maps were constructed in a geographic information system by combining the simulated water-surface profiles with a digital-elevation model derived from light detection and ranging (lidar) data. Estimated flood-inundation boundaries along each simulated profile were developed using HEC–GeoRAS software.

The availability of these maps and associated Web mapping tools, along with the current river stage from USGS streamgages and forecasted flood stages from the NWS, provides emergency managers and residents with information that may be critical for flood-emergency planning and flood response activities such as evacuations and road closures, as well as for post-flood recovery efforts.