Determination of flood hydrographs for streams in South Carolina: Volume 1. Simulation of flood hydrographs for rural watersheds in South Carolina

Abstract

A typical (average) flood hydrograph corresponding to a peak discharge of specific recurrence interval can be simulated for ungaged rural basins having drainage areas less than 500 square miles in South Carolina. Three dimensionless hydrographs were developed on the basis of data collected during 188 storm events at 49 stations representing a wide range of drainage area sizes and basin conditions. The design peak discharge and a volumeadjusted average basin lagtime are required to apply the technique. The standard errors of estimate for simulated hydrograph widths at 50 and 75 percent, respectively, of observed stormflow were ±14.1 and ±18.3 percent for basins in the Blue Ridge physiographic province, ±29.2 and ±36.2 percent for basins in the Piedmont province, and ±17.8 and ±22.8 percent for basins in the Upper and Lower Coastal Plain subprovinces.Multiple-regression analyses were used to develop equations for estimating average basin lagtime. At the 95-percent confidence level, drainage area was determined to be the only significant explanatory variable needed to estimate the average lagtime for basins in each physiographic province. The standard error of estimate of regression relations developed for estimating lagtimes for the Blue Ridge and Piedmont provinces, and the Upper Coastal Plain and Lower Coastal Plain subprovinces were ±7.3, ±25.6, ±34.3, and ±25.6 percent, respectively.A regression equation that provides runoff volume in inches also was developed. The explanatory variables used in the equation for estimating runoff volume are peak discharge, average basin lagtime, and drainage area*, the standard errors of estimate of equations applicable in the Blue Ridge, Piedmont, Upper Coastal Plain, and Lower Coastal Plain were ±10.3, ±21.1, ±13.6, and ±15.1 percent respectively. The regression equations for estimating runoff volume are the basis of an adjustment to average basin lagtime, which is required to simulate flood hydrographs by use of the dimensionless hydrographs.The simulation techniques and regression equations may be useful engineering tools for estimation where time of inundation or storage of floodwater is a part of the flood prevention or structure design criteria.