Forecast Flood Inundation Mapping at Continental Scale from National Water Model River Discharges

The National Water Center (NWC), part of the National Weather Service's (NWS) Office of Water Prediction (OWP), seeks to provide forecast flood inundation maps (FIM) as services at the national scale along 2.7+ million river reaches. The National Water Model (NWM) forecasts stream discharges along these reaches at varying time horizons. Building on the methodology of previous versions, OWP FIM 3.0 provides enhanced methods to map NWM stream discharges to inundation extents with a modified version of the Height Above Nearest Drainage method for mapping stages. FIM 3.0 produces required datasets for generation of FIMs, which are relative elevations, reach-level catchments, rating curves and crosswalk tables, to convert the NWM discharge forecasts to inundation extents as a post-processing step. Computational tests estimate that producing these FIM required datasets for the continental U.S. will be around 600 total CPU hours. Only publicly available input datasets from the NWM or NHDPlusHR are required. The latest methods leveraged within FIM 3.0 include the use of consistent reach lengths and deriving FIM outputs at lower stream densities (downstream of River Forecast Center forecast points) to mitigate catchment boundary limitations. FIM 3.0 is primarily parallelized across Hydrologic Unit Codes (HUC) 4, 6, or 8 as processing units which can be selected by the user at run time. FIM capability improvements were measured by comparisons to engineering scale maps using contingency statistics. Future versions will support NWM geospatial dataset production, usage of Lidar elevations, and more advanced hydrological methods. FIM 3.0 is highly configurable, modular, and portable due to its software architecture and containerization. All software dependencies are open-source including GDAL/OGR, TauDEM, and RichDEM. The source code and Docker container are expected to be made available publicly for community review and development.