A semi-empirical model to predict erosion rate of cohesive soil, determination of critical shear stress of non-cohesive soils, and field prediction of watershed erosion rate under drought and low data conditions.
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Sang, Joseph Kipyegon.
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Prediction of critical shear stress and determination of erosion rate due to concentrated flow plays a significant role in sustainability of engineering structures and watersheds management. Critical shear stress of a soil marks the onset of erosion and unless the hydraulic shear stress exceeds it erosion won't occur. Determination of critical shear stress finds application in construction of engineering structures and use of various hydrological models in evaluation of watershed management strategies. Given this importance, it is significant to improve further on the existing methods. This dissertation developed a semi-empirical model to predict erosion rate of cohesive soil using Turbulent Kinetic Energy (TKE), and applied Soil and Water Assessment Tool (SWAT) in assessment watershed erosion rates. The main questions addressed in this study were (1) is semi-empirical model a better predictor of erosion rates of cohesive soil? (2) is TKE a better predictor of critical shear stress?, and (3) can multi-year drought sediment yield dynamics be simulated under low data conditions? The adopted methodology involved laboratory experimentation using a direct tensile apparatus, Jet Erosion Test (JET) apparatus, MicroADV, and simulation studies. After the introductory chapter, the results of developing the semi-empirical model are given in Chapter Two. The results showed that the model output were comparable to those predicted by an existing model. In addition, the results showed that moisture content significantly influenced the soil erodibility characteristics of cohesive soils. In Chapter Three results of determining soil erodibility using TKE and JET apparatus for non-cohesive soil is presented. The result of the study showed that TKE is a better predictor of critical shear stress of non-cohesive than published methods. Furthermore, the use of the JET apparatus provides a fast and easier approach to determining critical shear stress of complex flows. In Chapter Four, the results of applying SWAT to simulate the impact of multi-year drought under low data conditions in two basins in the Edwards Plateaus, Texas, are presented. Scenario analysis showed that in a multiyear drought maintenance of cover is an important part of strategy to reduce erosion rates. Finally, Chapter Five presents a summary and conclusion of the research.