Overland (Surface) Flow and Time of Concentration for Highly Permeable Sandy Soil Slope: An Experimental Study Using Rainfall Simulator

Time of concentration is defined in many ways in literature and it is calculated rather subjectively in practice. The situation becomes adverse as the terrain slope reaches to zero; because the slope generally appears in the denominator of any formula for time of concentration, this time goes to infinity as the slope becomes zero. Estimation of it seems very paradox in nature (Grimaldi, 2012). The variables affecting this time parameter on highly permeable slope plot have been studied through plot scale laboratory experiments using rainfall simulator on artificial catchment prepared. It has been found that the initial moisture content and rainfall rate control this parameter. Some of the existing times of concentration methods have been compared, and it is found that 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 been modeled, secondly to the absence of any accounting for the initial moisture content in the irrespective 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 as well as logical to the observed values. Regression equations have been derived based upon the experiments to determine the overland (surface) flow times on a plot of 2.02 meter length, 1 meter wide and 0.15 m depth soil plots of highly permeable natural sand (at the slope of 5 to 150 to the horizontal plane) with uniform rainfall intensity. The application of these equations on other lengths cannot be ascertained. Equations for the hydrograph have been determined for highly permeable sand plot. From the matrix solver for multiple regression analysis, regression model assumed and the exponents of the parameters are obtained as: 0.5 for length, -0.14 of rainfall intensity, -0.11 of surface slope, and -0.05 for initial moisture content for highly