EVALUATION OF FLOOD INUNDATION MAPS FROM HURRICANE IDA IN THE MIDDLE ATLANTIC REGION

Flood-related impacts are associated with most federal disaster declarations with Hurricane Ida being no exception. In September 2021, the remnants of Hurricane Ida arrived in Pennsylvania and New Jersey causing millions of dollars of destruction and claiming many lives. Most of these damages involved inland flooding, affecting public buildings and entire neighborhoods. To minimize these impacts, public agencies use Flood Inundation Maps (FIMS) to identify vulnerable structures for flood warning and long-term planning purposes. This project presents the results of a geospatial analysis made for the Middle Atlantic, specifically, the river basins of Delaware, Lehigh, Raritan, Schuylkill and nearby headwaters, Juniata and Upper Potomac, and Lower Susquehanna. We evaluated FIMs and depth grids provided by the National Water Center (NWC). The maps are driven by a combination of observed streamflows from USGS gages and the National Water Model routing. Evaluating maps driven primarily by observed flows helps us to focus more on mapping and hydraulic modeling errors as opposed to errors associated with the hydrologic forecasting. We compare the Ida FIM depth grids to the United States Geological Survey (USGS) observed highwater marks (HWMs) collected post-Ida. Our analysis concluded that more locations within the domain were under-estimated in comparison to the USGS HWMs than over-estimated. While 41% of these differences fell within the -2 to 2 (ft) error range, we infer hydraulic reasons why larger differences exist in some locations. Also, a comparison between NWM flood extent and the Advanced Hydrologic Prediction Service (AHPS) inundation maps was made. FIM performance was best in the Juniata and Upper Potomac regions whereas the Schuylkill and Nearby Headwaters were the poorest performing, likely due to urbanization. Furthermore, the findings of this research will be useful for informing future versions of the NWC FIM development within the National Weather Service. Ultimately, these new FIM services will support emergency managers and enhance flood awareness and community response to aid in the nation’s protection of life and property.