The Crucial Role of Flood Inundation Mapping (FIM) in the July 2023 Montpelier, VT Flood Damage Assessment

The NOAA Office of Water Prediction (OWP) National Water Center (NWC), in collaboration with the National Weather Service (NWS) River Forecast Centers (RFCs), is implementing Flood Inundation Mapping (FIM) services for approximately ten percent of the United States population by 1 October 2023. As part of this effort, the NWS Northeast RFC has been evaluating the accuracy of Stage-Based Categorical FIM (CatFIM). Stage-Based CatFIM is a static FIM layer generated using the height above nearest drainage (HAND) method and river stages at minor, moderate, and major floods. The accuracy of the Stage-Based CatFIM was determined by comparing the inundation extent of FIM to the NWS Flood Impact Statements (E-19s). This process was occasionally challenging as some E-19s were vague or outdated.

The July 2023 Vermont Floods provided an opportunity to evaluate the performance of Stage-Based CatFIM as a part of a Flood Damage Assessment in partnership with the Burlington Weather Forecast Office. Catastrophic river flooding occurred along the Winooski River at Montpelier (MONV1) on July 10th. The river crested 3.85 feet above major flood stage at 21.35 feet which is provisionally the second highest crest on record at that location. The city of Montpelier, including the state office buildings, were inundated with several feet of water for nearly eighteen hours. Stage-Based CatFIM played a critical role during the flood damage assessment surveys and paved the way for deciding which portions of Montpelier to survey, i.e., locations with the greatest FIM extent at major and record flood stages. Most importantly, there were several areas within the city where the FIM extent was very accurate. This presentation will show the value of FIM for informing Impact-Based Decision Support Services (IDSS) in the future.