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Knowledge of the magnitude and frequency of floods is needed for the design of highway drainage structures, for establishing flood insurance rates, and for many other uses by urban planners and engineers. Urban flood hydrographs also are needed for the design of many highway drainage structures and embankments and floodwater storage structures. This report describes methods that can be used to estimate peak-discharge-frequency relations, flood hydrographs, and flood volumes for ungaged urban streams in the Piedmont and Coastal Plain provinces of South Carolina. Data from stream-gaging stations on 34 urban watersheds in South Carolina, Georgia, and North Carolina, ranging in size from 0.18 to 41.0 square miles, were used in the analyses. A rainfall-runoff model was calibrated for 23 urban drainage basins in South Carolina. The model, long-term rainfall data, and observed and synthetic evaporation data were used to synthesize a series of annual peak discharges for each site. The logarithms of the annual peaks were fitted to a Pearson Type III distribution to determine the frequency of peak discharge. Multiple regression equations were developed for estimating peak discharges having recurrence intervals of 2, 5, 10, 25, 50, 100, and 500 years using data from 34 gaging stations in South Carolina, North Carolina, and Georgia. The explanatory variables affecting peak discharge were drainage area, total impervious area, and rural discharge of equivalent recurrence interval. Average standard errors of prediction for the relations range from +25.6 to +34.3 percent. A method is presented for estimating flood hydrographs by applying a specific peak discharge and adjusted basin lag time to one of two dimensionless hydrographs that were developed by using data from 30 stations in South Carolina and Georgia. The standard errors of estimate for the simulated hydrograph widths at 50 and 75 percent, respectively, of observed peak discharge were +27.0 and +29.8 percent for basins in the Piedmont and upper Coastal Plain and +19.8 and +24.5 percent for basins in the lower Coastal Plain. An equation for estimating average basin lag time for use in applying the dimensionless hydrograph technique also was developed. Significant explanatory variables for estimating lag time were total impervious area; the 2-year, 2-hour rainfall amount; and a variable combining main channel length and slope (length/slope0 * 5 ). The standard error of prediction for the lag time relation was +23.8 percent. Two regression equations that provide average runoff volume, in inches, for a flood hydrograph with a specific peak discharge also are provided. The explanatory variables used in the volume equations were peak discharge, average basin lag time, and drainage area. The standard error of prediction for the volume equations was +18.7 percent. The regression equations for estimating runoff volume are used to adjust average basin lag time before application of the dimensionless hydrograph method. This adjustment provides la more accurate estimate of the volume associated with the simulated hydrograph than would be obtained by using the dimensionless hydrograph method with the unadjusted average basin lag time.

water-resources investigations report

Determination of flood hydrographs for streams in South Carolina: Volume 2. Estimation of peak-discharge frequency, runoff volumes, and flood hydrographs for urban watersheds