Flood-depth frequency relations for streams in Alabama

Equations are defined for estimating the depth of water for floods having recurrence intervals of 2, 5, 10, 25, 50, 100, and 200 years on rural and urban streams in Alabama.Multiple regression analyses were made using the "maximum RSQUARE improvement" procedures.The dependent variable was the flood depth and the independent variables were 10 basin and climatic characteristics.For rural streams, drainage-area size (0.44 to 1,344 square miles) was the only statistically significant independent variable tested to estimate flood depths in six different hydrologic areas.Other variables affecting flood-depth relations are reflected in the equation constant and coefficient for each hydrologic area.These account for differences in geologic and topographic characteristics.For urban streams, drainage-area size (0.16 to 83.5 square miles) and percent impervious area (8.3 to 42.9 percent) of the basin were the most significant independent variables tested to estimate flood depths.For most streams where the drainage area is larger than 1,344 square miles, the drainage basin extends into more than one hydrologic area and the equations presented would not apply.Flood-profile data and gaging-station records are available from the U.S. Geological Survey for use in estimating flood depths for these large streams.Flood-routing and historic flood profiles are also available for many of the large streams which have flood regulation.