The dimensionless hydrograph method is a simple, easy-to-use technique for simulating an average, or typical, hydrograph that corresponds to a given peak discharge. The method has been successfully tested and applied to both rural and urban streams in the states of georgia, tennessee, alabama, south carolina, ohio, missouri, and arkansas. Most studies found that the dimensionless hydrograph developed for the georgia study, which is nearly identical to a theoretical dimensionless hydrograph developed nationwide for urban streams, will apply to streams in the other study areas. Only in the coastal plainarea of south carolina and the lowlands of west tennessee were dimensionless hydrographs found to be significantly different. An important parameter for the use of the dimensionless hydrograph method is the basin lagtime. Each of the studies developed regression equations that relate lagtime to basin characteristics. The other parameter needed to apply the method is peak discharge. The accuracy of the dimensionless hydrograph method varies from state to state, but in general the width at 50 and 75% of the peak discharge had standard errors in the range of 20 to 40%. For large floods at ungauged sites, the standard errors were between about 30 and 60% and represented the combined errors using lagtime regressions, peak discharge regressions, and the dimensionless hydrograph. The report also contains hydrograph width relations that can be used to determine the elapsed time that a specified discharge would be exceeded. Some investigators included regression equations for estimating flood volumes; however, flood volumes also can be computed by integrating the area beneath thesimulated hydrograph, or by applying an equation that yields virtually the same result as the integration method. This paper appears intransportation research record no. 1224, Rest areas, wetlands, and hydrology.
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