A physically-based study of the mechanism of sediment detachment in the soil erosion process
Owoputi, Lawrence Olusola
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This study presents a critical evaluation of the mechanism of detachment in an erosion process. Sediment detachment is defined as the dislodgment of soil particles from the soil mass during an erosion event. The dislodgment is caused by the forces applied by both overland flow and raindrops. The detachment of sediments is also dependent on a number of other factors, especially on those that are affecting the soil erodibility. The first part of this study comprises a comprehensive literature review of the general area of sediment detachment. The literature review includes the examination of the current methods of quantifying the rate of sediment detachment and the identification of the areas that require further development. The general conclusion from this review is that there is a need for the development of a more detailed theoretical and engineering-based equation for predicting the rate of sediment detachment and for assessing soil erodibility in order to provide an improved physically-based erosion model. The second part of this study consists of the development of a new equation to describe the mechanism of sediment detachment in the erosion process. The equation is based on the concept that soil particles will be detached from the parent material if there is a net upward force greater than zero acting on the particles. The forces acting on the soil particles include those from the erosive agents (overland flow and raindrops), seepage, self weight and cohesion. The primary advantage of this equation is that it helps change the basis of prediction of the rate of sediment detachment from empiricism to the utilization of physically based variables. Therefore, it is possible to separately assess the relative importance of the individual factors affecting the mechanism of sediment detachment. The third part of this study includes the laboratory and numerical tests. These tests, which involve the use of one agricultural soil and two cohesionless sands, have two purposes. Firstly, the tests were designed to examine the roles of soil density and seepage in the erosion process. Observations from these tests show that both factors (soil density and seepage) have direct impacts on both runoff and soil erosion rates. However, the results also show that the impacts of the two factors on runoff rates cannot be used to predict their impacts on the erosion rates. Specifically, both the soil density and seepage have distinctive impacts on the soil erodibility, as also indicated by the detachment equation developed in this study. The second purpose of the experimental and numerical tests was to provide quantitative testing of the detachment equation. Only the impact of seepage and density changes on soil erosion rates as obtained from the cohesionless sands were used for the testing. The analysis shows a very good correlation between the experimental results and the equation. In the fourth and final part of this study, a discussion of the practical significance and applications of the new detachment equation is presented. The discussion also includes the general limitations of this study, with particular reference to those components of the detachment equation which require further development.