Evaluation of National Weather Service Flood Inundation Mapping for Operational Use

The public release of National Weather Service (NWS) forecast-based flood inundation mapping (FIM) offers exciting new possibilities for enhancing communication about river flood hazards and forecasts. By visually representing flood threats on maps, forecasters and users gain real-time and projected flood insights to support life and property-saving decisions.

By leveraging NWS river forecasts and guidance from the National Water Model, FIM data now covers thousands of river miles that have historically lacked dynamic flood mapping resources. While these new dynamic flood maps rely on current forecasts, most of the flood inundation output has never been evaluated at the local-scale for accuracy.

Further, the NWS FIM workflows employ the Height Above Nearest Drainage (HAND) technique and digital elevation models (DEMs) for nationwide flood mapping capability, however limitations exist, including deriving synthetic rating curves to relate stage and discharge, and deficiencies in the DEMs, such as missing bathymetry information and spatial resolution issues in some important areas. These limitations can affect the accuracy of flood inundation output, particularly in watersheds featuring complex flood control infrastructure as well as streams with extensive flow regulation for navigation and other purposes.

Therefore, to help build confidence in these new FIM resources, ongoing evaluations of FIM are identifying strengths and areas for improvement by comparing historical flood observations, FEMA floodplain maps, and other hydraulic engineering flood models.

This presentation demonstrates how NWS River Forecast Centers assess local-scale accuracy of the new FIM resources, informing operational FIM use, partner collaboration, and decision support services. The presentation features FIM evaluation procedures, shares examples of FIM output, and details methods for improving FIM output using ground truth information.