Study of overland flow phenomenon using experimental and modelling approaches

Abstract

Rainfall-runoff analysis of a watershed is needed for the purposes of water resources planning, design flood estimation, flood forecasting, study of movement of pollutants, and many other applications. For a mathematical simulation of this phenomenon, controlled rainfall-runoff experimental runs were conducted on the V-catchment system placed over the advanced hydrologic system to obtain runoff hydrograph data. The approximate convection–diffusion (ACD) equation–based overland flow model was used for simulation of these observed hydrographs. Considering various combinations of runoff simulation scenarios, a total of 111 laboratory experiments were conducted on two types of overland flow and channel flow roughness conditions generated (1) on a V-catchment model made up of acrylic sheet surface and (2) with artificial roughness surface generated with the help of sand paper pasted on the acrylic V-catchment model. These 111 experimental runs were made corresponding to six rainfall intensity inputs, that is, varying from 54 to 84 mm/hour, and for four overland plane slope conditions varying from 0.22% to 1.54%, for each of the three channel slopes used. Overall, it is demonstrated that the ACD equation–based overland flow model is able to reproduce the observed hydrographs closely, in many cases with Nash–Sutcliffe efficiency greater than 90%. But in all the experiments, the recession limb of the observed hydrographs could not be reproduced very closely because of prolonged flow of water. The reason behind this behaviour could be attributed to the retention of runoff of the recession limb because of surface water tension prevailing over the overland flow bed, channel bed and even over the weir model used for flow measurement. Except for this problem, the experimental set-up established in the laboratory reasonably generates overland flow hydrographs which could be reproduced by the overland flow model developed based on the ACD equation, including those cases of diffusion behaviour of the V-catchment experimental hydrographs.