Magnitude and frequency of urban floods in Alabama and Mississippi using data through the 2024 water year

To improve flood-frequency estimates for urban streams in Alabama and Mississippi, the U.S Geological Survey, in cooperation with the Alabama and Mississippi Departments of Transportation, estimated streamflows corresponding to selected annual exceedance probabilities (AEPs) for 64 streamgages on urban streams in the two States—48 in Alabama and 16 in Mississippi. Using estimates from 42 of the 64 streamgages, regression equations were developed that can be used to predict streamflows corresponding to the selected AEPs at ungaged locations on urban streams. Annual peak-flow data through September 2024 were used in the at-site flood-frequency analyses. Drainage area and the percentage of developed land use in the drainage basins of the streamgages (from the 2011 National Land Cover Database) were identified as statistically significant explanatory variables. Pseudo coefficient of determination (pseudo-R2) values ranged from 81 percent for the 0.2-percent AEP (500-year flood) to 91 percent for the 4- and 2-percent AEPs (25- and 50-year floods, respectively). Standard errors of prediction ranged from 23.1 percent for the 4-percent AEP (25-year flood) to 36.1 percent for the 2-percent AEP (500-year flood).