Critical shear stress for erosion of fine and coarse-grained sediments in Georgia
Harris, Travis W.
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Erosion of a river bed has important implications with respect to scour around river structures such as bridges, transport of contaminants attached to the sediment, and disruption or destruction of aquatic habitats. Erosion occurs when the resistive strength of the sediment is overcome by the hydrodynamic forces produced by the flow of water. This resistance to erosion in a sediment originates from gravity or interparticle forces for coarse sediment (sand and gravel) and fine sediment (silt and clay), respectively. Since the erosion of fine sediment depends on the combination of many interparticle forces, and this combination fluctuates widely amongst different fine sediments, past studies have had difficulty finding a consistent method to estimate fine sediment erosion. This study analyzes sediments that fall in the transition size range between fine and coarse sediments and compares the findings with those from fine sediments (Wang 2013) and sandy coarse sediments (Navarro 2004, Hobson 2008), in order to correlate the erosion rates of both sediment types to their physical characteristics. In this study, kaolin-sand mixtures were prepared by mixing various percentages of Georgia kaolin by weight ranging from 30% to 100% with industrial fine sand and tap water. Geotechnical and other tests of sediment properties were performed to measure water content, bulk density, grain size distribution, temperature, pH, and conductivity of these mixtures. Hydraulic flume experiments measured the erosion rates of each sediment and these rates were used to estimate the critical shear stress correlating to that mixture. Relationships between the physical properties of the sediment and critical shear stress were developed by multiple regression analysis. An alternative option of estimating the critical shear stress by a weighted equation, which uses the combination of fine sediment erosion and coarse sediment erosion equations separately, was explored and found to be a viable and accurate option to estimating both coarse and fine sediment erosion from the same parameters and equation. The results from this study can be used to estimate sediment erodibility and thus river bed stability based on simple tests of physical properties of the river bed sediment and will help predict scour around bridges and other flow obstructions.