Improved Time of Concentration Estimation on Overland Flow Surfaces Including Low-Sloped Planes

Abstract

Time of concentration (Tc) is one of the most used time parameters in hydrologic analyses. As topographic slope (So) approaches zero, traditional Tc estimation formulas predict large Tc. Based on numerical modeling and a review of relevant literature, a lower bound for slope (Slb) of 0.1% was identified as a threshold below which traditional Tc estimation formulas become unreliable and alternate methods should be considered. In this study, slopes less than Slb are defined as low slopes. Slopes equal to or exceeding Slb are defined as standard slopes where traditional Tc estimation formulas are appropriate. A field study was conducted on a concrete plot with a topographic slope of 0.25% to collect rainfall and runoff data between April 2009 and March 2010 to support numerical modeling of overland flows on low-sloped planes. A quasi-two-dimensional dynamic wave model (Q2DWM) was developed for overland flow simulation and validated using published and observed data. The validated Q2DWM was used in a parametric study to generate Tc data for a range of slopes that were used to develop Tc regression formulas for standard slopes (So≥0.1 %) and low slopes (So<0.1%).