Overland flow time of concentration on flat terrains

Abstract

Time of concentration parameter is defined very loosely in literature and it is calculated rather subjectively in practice (Akan 1986). The situation becomes adverse as the terrain slope approaches zero; because the slope generally appears in the denominator of any formula for time of concentration, this time goes to infinity as the slope goes to zero. The variables affecting this time parameter on flat terrains have been studied through plot scale field experiments. It has been found that the antecedent moisture and rainfall rate control this parameter. Some of the existing time of concentration methods have been compared, and it is found that all the empirical models compared under predict this time parameter. This under prediction can be attributed first to the differing concepts of time of concentration previous researchers have modeled, secondly to the absence of any accounting for the initial moisture content in their respective equations and thirdly to the watersheds where these models have been calibrated. At lower time of concentrations, Izzard-based model predictions show some results close to the observed values. A methodology to determine the plot scale surface undulations has been developed to estimate the depression storage. Regression equations have been derived based upon the experiments to determine the overland flow times on a flat plot of 30 feet length with uniform rainfall intensity. The application of these equations on other lengths cannot be ascertained. Equations for the hydrograph slope on flat terrains have been determined for bare clay and grass plots.