Evaluation of Flood Inundation Mapping Activities at the NOAA National Water Center

Abstract:

The Office of Water Prediction Flood Inundation Mapping (FIM) project uses the Height Above Nearest Drainage (HAND) method to create flood inundation libraries throughout the continental U.S. To document performance and guide development efforts, evaluation of FIM maps is performed against publicly available, model-backed benchmark datasets using a contingency table approach and calculating metrics such as Critical Success Index, False Alarm Ratio, and Probability of Detection to assess performance. One benchmark dataset is managed by the multi-agency FEMA-sponsored Interagency Flood Risk Management (InFRM) team. The estimated Base Flood Elevation viewer provides watershed level (HUC-8) HEC-RAS modeling studies known as Base Level Engineering (BLE), found in the south-central United States. BLE datasets contain flows and inundation extents for the 1% and 0.2% annual chance flood events. Comparative analysis of BLE data is used to evaluate the performance of NWS HAND-derived FIM on a watershed level as well as guide development of HAND. Another benchmark dataset, supplied by the NWS, are calibrated stream-scale HEC-RAS flood maps co-located at NWS Advanced Hydrologic Prediction Service (AHPS) forecast sites and USGS gage sites. These model-backed output datasets extend across a wide range of flows ranging from action stage to major flood stage. With flows obtained from USGS rating curves, FIM mapping is evaluated for nearly 120 AHPS sites. Comparison of FIM rating curves with BLE HEC-RAS rating curves to evaluate conveyance properties for a wide range of flows (10%-0.2% annual chance flood) are also demonstrated. Finally, evaluation of flood depth is demonstrated using structure data to provide insight on the potential human risk during flooding events. Performing evaluations of FIM across a range of benchmark datasets guide development and document the performance of FIM maps which support the National Weather Service in fulfilling its mission to protect life and property.