Flood of July 2023 in Vermont

A major storm caused catastrophic flooding in many parts of Vermont on July 9–12, 2023, resulting in millions of dollars in damages. The high amount of rainfall caused several rivers to peak at record levels, in some cases exceeding records set during Tropical Storm Irene in 2011. The U.S. Geological Survey, in cooperation with the Federal Emergency Management Agency, collected and analyzed data that characterized the flood in Vermont. The data collected included peak water-surface elevations, taken from high-water marks at bridges, dams, and roads, and peak streamflow and annual exceedance probabilities (AEPs) at streamgages, lake gages, and selected ungaged locations. At 11 of the 80 streamgages with 12 to 94 years of record, the July 2023 peak streamflow was the peak of record. Ten streamgages recorded a peak streamflow with an AEP of less than or equal to 1 percent (greater than or equal to a 100-year recurrence interval).

The July 2023 flood affected many of the same communities as the historical flood caused by Tropical Storm Irene in 2011. Twenty of the 45 continuous-recording streamgages running during both events recorded greater peak streamflows during the July 2023 flood than during Tropical Storm Irene in 2011. Four of the 11 U.S. Geological Survey streamgages with period-of-record maximum peak streamflows observed during the July 2023 flood had previously recorded their maximum period-of-record peak streamflows during Tropical Storm Irene. There were 17 rivers in Vermont that were surveyed for high-water marks during both Tropical Storm Irene and the July 2023 flood. On those 17 rivers, a total of 103 sites contained surveyed high-water marks for both events. Thirty-two of these sites had higher surveyed elevations for the July 2023 flood than Tropical Storm Irene, including Black River in Newport, Black River in Springfield, Jewell Brook, Middlebury River, Missisquoi River, Ottauquechee River, Otter Creek, Wells River, Whetstone Brook, and Winooski River. Peak water-surface elevations were not collected on the Lamoille River in 2011.

Federal Emergency Management Agency flood insurance studies were evaluated in the context of the July 2023 flood. Peak streamflows at streamgages and nearby locations were assessed to determine the influence of the July 2023 flood on the AEPs used in past studies. Overall, 21 of 26 streamflow-computation locations in the flood insurance studies had more than a 10 percent difference in the 1-percent AEP streamflow. A hydraulic evaluation of surveyed water-surface elevations following the July 2023 flood was compared with the AEP profiles from past studies. Four of the 10 streamgages analyzed had poor alignment between the AEPs of the observed streamflows and the AEPs of the observed peak water-surface elevations as computed from flood insurance studies.