Erosion analysis of coarse and fine grained sediments native to the state of Georgia
Krehbiel, Paul Richard
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Sediment erosion in aquatic environments plays an important role in the design of bridges and other hydraulic structures with regard to scour, contaminant transport, and preservation of ecological systems. Erosion is the action of hydrodynamic forces overcoming the resistance by a sediment particle to being entrained and transported such that significant local erosion occurs. Sediments can be characterized as either non-cohesive or cohesive, as a classification determined by certain geotechnical properties. Non-cohesive sediments, consisting of sand and silt, primarily resist erosion due to the submerged weight of the particle and packing density of the sediment. Cohesive sediments, consisting of silt and clay, resist erosion via interparticle interactions, as determined by clay size fraction, water content or bulk density, and fines content, as well as other properties such as pH, organic matter, and mineralogy. Erosion of non-cohesive sediments that are primarily coarse-grained has been studied and documented by many researchers. While cohesive sediments have been investigated extensively, they are inherently more difficult to study because of the physicochemical properties that determine interparticle binding forces. This study focuses on a few geotechnical parameters to predict the erodibility of sediment mixtures on the coarse-fine transition boundary and mimic sediments native to Georgia. Previous researchers have investigated the erosion properties of coarse-sediment field samples in Georgia (Navarro 2004 and Hobson 2008) and predominantly fine, laboratory-prepared samples (Wang 2013 and Harris 2015). In order to span this collection of data, a series of samples was prepared and tested in an erosion flume in the Georgia Tech Hydraulics Laboratory using the same methodology as previous investigators to measure critical shear stress. The silt to sand ratio was held constant at 0.75, which is consistent with prior investigations of native Georgia sediments. Sand, silt, and Georgia Kaolinite were added to the samples, increasing the quantity of Kaolinite by weight in each subsequent sample from 10-30%. Sediment properties measured included water content, grain size distribution, clay size fraction, pH, temperature, and conductivity. Erosion rates for the mixtures were measured using a hydraulic flume. From these experiments, a critical shear stress for each mixture was determined based on water content. The critical shear stress data were analyzed as a function of measured geotechnical parameters using multiple regression analysis which provided a series of estimation equations. The relationships for critical shear stress derived in this research include a three-variable equation depending on water content, clay size fraction, and an interaction term; a fourth term that adds a fines content variable to the previous relationship; and a pair of equations that are implemented on separate coarse vs. fine data sets based on water content and particle size. While a weighted equation, which uses a combination of cohesive and non-cohesive equations, or two separate equations for coarse vs. fine sediment have merit, the optimal solution found in this research is the three-variable equation based on water content, clay size fraction, and an interaction term applied to all available data. However, more research should be conducted investigating the idea of two equations that are implemented on two separate data sets and on the criteria that best separate the data sets relative to cohesive vs. noncohesive erosion behavior. The results of this research can be used to find better predictions of sediment critical shear stress for Georgia sediments as a function of easily measured geotechnical parameters thereby providing better estimates of bridge scour and sediment stability for other structures in aquatic settings.