Estimating flood hydrographs for urban basins in North Carolina

Abstract

Flow cubic foot per second (ft3/s) 0.02832 cubic meter per second Contents ESTIMATING FLOOD HYDROGRAPHS FOR URBAN BASINS IN NORTH CAROLINA fi/Robert R. Mason, Jr. andJerad D. Bales ABSTRACT A dimensionless hydrograph for North Carolina was developed from data collected in 29 urban and urbanizing basins in the State. The dimensionless hydrograph can be used with an estimate of peak flow and basin lagtime to synthesize a design flood hydrograph for urban basins in North Carolina. Peak flows can be estimated from a number of available techniques; a procedure for estimating basin lagtime from main channel length, stream slope, and percentage of impervious area was developed from data collected at 50 sites and is presented in this report. The North Carolina dimensionless hydrograph provides satisfactory predictions of flood hydrographs in all regions of the State except for basins in or near Asheville where the method overestimated 11 of 12 measured hydrographs. A previously developed dimensionless hydrograph for urban basins in the Piedmont and upper Coastal Plain of South Carolina provides better floodhydrograph predictions for the Asheville basins and has a standard error of 21 percent as compared to 41 percent for the North Carolina dimensionless hydrograph. Purpose and Scope This report presents a technique for estimating flood hydrographs for urban basins in North Carolina and documents the development of this method. The method is based on the application of a dimensionless hydrograph described in this report. Flood hydrographs can be synthesized by using the dimensionless