Abstract:
The Erosion experiments with saturated Bentonite (Calben), Kaolin Koclay and Kaolin Ball Clay using the Circular Couette Flow Erosion Device (CCFED) and simulated rainfall (uniform drop size) and overland flow are described. The former experiments involved erosion by bed shear stress alone while the latter were carried out under conditions in which the effects of raindrop impacts dominated. These experiments were carried out at constant temperature and in most cases, the pore and eroding fluids were the same. The experiments with the CCFED included erosion tests of the three clays with various pll-values and molar salt concentrations. The scanning electron microscopic study of the eroded clay surface indicated strongly three dimensional, pitted surfaces which suggests an erosion mechanism in which clay lumps are lifted out of the clay surface. This observation agrees with the mechanism of erosion proposed by Croad (1981). Analysis of the experimental results using the Croad Model facilitated study of the surface function of erosion rate with pH-value/molar salt concentration and bed shear stress as the parameters. The results show that resistance to erosion of clay depends on the bonding and orientation of the clay aggregates as influenced by the electrolytes present in the slurries (before consolidation). A theoretical expression for the pressure peaks due to raindrop impacts on the surface water was developed, and an erosion modal based on the Croad Model was derived . This equation involves constants which depend on soil and/or flow properties. These constants were determined using the experimental results obtained with the simulated rainfall and overland flow. The erosion equation shows, among other trends, that the erosion rate of soil increases with the square of rainfall intensity and the erosion rates of soil under two rainfalls of the same intensity but different drop sizes are different. A total erosion rate equation was formulated by assuming that the total rate of erosion by rainfall, eT, is the sum of erosion rate by raindrop impacts on overland flow and by bed shear stress. It was subsequently demonstrated that the function of the total erosion rate is a four dimensional vector, i.e. êT = f(I,S₀,x). The total soil loss rate of Kaolin Koclay along a plane of 5% slope and slope length of 30 m was estimated for two storm events. The results show that infiltration rate of soil is an important variable and if taken into account, the limited TSL estimated with the erosion model developed agrees reasonably well with the predicted soil loss rate obtained using the Universal Soil Loss Equation.