Hydrologic and Hydraulic Models from Effective and Pending Flood Insurance Studies compared to the July 2023 Flood in Vermont

Flood Insurance Studies (FISs) published by the Federal Emergency Management Agency (FEMA) provide 10-, 2-, 1-, and 0.2-percent annual exceedance probability (AEP) streamflows and water-surface elevations for many stream reaches that were affected by the July 2023 flood in Vermont. After a flood, it is useful to compare observed streamflows and water-surface elevations to flood flow-frequency and water-surface elevations published in a FIS report (Federal Emergency Management Agency, 1980–2014). Theoretically, the AEP for the peak water-surface elevation and the AEP for the peak streamflow from the same event at the same location should be the same. Reaches for which the AEPs do not align show where existing FISs could be updated with new hydrologic data and hydraulic models. Flood streamflows for 10-, 2-, 1- and 0.2-percent frequencies using annual peak-flow data through water year 2023 were compared to peak streamflows in FEMA FIS reports and pending (2024) FIS studies. The proposed peak streamflows from pending FIS studies are based on statistical analyses of annual peak flow data prior to water year 2023. Although flood-frequency estimates were updated for 80 streamgages after the July 2023 flood, only a fraction of the streamgages have detailed hydrologic analysis in FIS reports (or pending FIS reports) and can be used in the streamflow comparison. Peak-water surface elevations from the July 2023 flood were also documented at surveyed high-water marks (HWMs) along selected river reaches impacted by the July 2023 flood in Vermont. Reaches that had HWMs, a streamgage on the same river with a drainage area within fifty percent of the reach drainage area, and an effective FIS were selected for evaluation. Streamflows and HWMs from the July flood event are presented alongside statistical streamflows and elevations for the 10- to 0.2-percent AEPs from the applicable FIS.

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