SEDIMENT DEPOSITION IN A SIMULATED RILL UNDER SHALLOW FLOW CONDITIONS

Abstract

Eroded soil from hillslopes will deposit downslope or be transferred to waterways and deposited downstream. Understanding the interactions between shallow flows in a rill and factors such as slope, rainfall intensity, infiltration, and incoming sediment concentration are important in determining where and when sediment will be deposited. The main objectives of this study were to determine how these factors affect the deposition of non-cohesive sediment under shallow water flow in a rill and to test whether the turbulence parameter in the WEPP model deposition equation was adequately represented for those conditions. An experimental laboratory hydraulic flume under rainfall simulators was used to study sediment deposition in a 25 cm wide by 3.6 m long rill. A laser scanner was used to quantify deposition after each experiment, and sediment samples were taken from the flume outlet to quantify sediment transport. The experiments were conducted using silica sand, glass beads, and artificial plastic/glass aggregates. Combinations of different flow rates, rainfall intensities, and sediment feed rates were studied for each sediment type at slopes varying from 1% to 5%. The interaction of rainfall intensity and flow depths had a more significant effect on deposition of particles of low specific gravity and under greater interrill sediment contributions; however, this was not true for denser sand particles. Sediment deposition in the rill was less under no rainfall and high-intensity rainfall than under medium-intensity rainfall. The effect of infiltration on sediment deposition under high-intensity rainfall was related to the slope steepness. At slopes greater than 3%, less deposition was observed under saturated conditions than under unsaturated conditions. The opposite was true for slopes less than 3%. Modeling deposition based on measured deposition rates of the non-cohesive sediment showed that the turbulence factor for the particle and flow conditions in these experiments could be 10 or more times less than the 0.5 value currently used in the WEPP deposition equation.