Experimental invesitigation and modeling of rainfall runoff process

Abstract

Rainfall-runoff process of a watershed is important for water resources planning, flood forecasting, design flood estimation, movement of pollutants transport through rainfall runoff process, and many other applications. For a mathematical simulation of this phenomenon, controlled set of experiments were conducted to investigate this process through experimentation on the catchment system placed over a rainfall simulator to obtain runoff hydrograph data. The experiments were carried out over a non-cohesive sediment layer having sediment size of 0.5 mm to 1 mm placed over an impermeable plane surface (smooth metal sheet), with a uniform rectangular cross section of dimension one meter wide and two meter long. Total 12 laboratory experiments were conducted to know behavior of this phenomenon using rainfall simulator. The generated experimental data were simulated using a one-dimensional finite difference numerical model of kinematic wave equation for overland flow to investigate the effects of variation in rainfall intensity and surface slope on the overland hydrograph. The data was collected for catchment slope between 1% to 4 % and rainfall intensity varies from 30 to 90 mm/hr. The comparison of observed and simulated runoff hydrograph reveals that the kinematic wave model simulates the rising, equilibrium discharge and upper part of recession limb reasonably well in observed hydrograph with Nash-Sutcliffe efficiency greater than 95%. However, in all the experiments, the lower portion of recession limb of the observed hydrographs remained under predicted because of prolonged flow of water. The reason behind this variation may 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. The study further reveals that the resistance due to flow decrease linearly with increase in catchment slope for a given rainfall intensity. It was also found that for a given rainfall intensity, an increase in the catchment slope reduces the time to peak.