Estimating Urban Time of Concentration

Abstract

Hydrologic flood‐flow design methods generally require one time parameter, such as the time of concentration, as input, and the accuracy of the estimated peak discharge or flood hydrograph is sensitive to the accuracy of the estimated time parameter. Numerous equations for estimating the time of concentration have been proposed. Eleven equations for estimating tc are compared using data collected from 48 urban watersheds located in most regions of the country. All the watersheds had areas less than 4,000 acres; the average impervious was 29.1%, and the mean time of concentration was 1.49 hr. Using accuracy and bias as the criteria for comparison, the eleven equations show wide variation in accuracy and bias. Two equations are calibrated with the data; the rainfall intensity is found to be the most important input parameter. The mean time of concentration computed by the velocity method for the 48 watersheds agrees very closely with the mean time computed from rainfall and hydrograph data. However, for individual watersheds the differences are relatively large.