Evaluation and prediction of cohesive soil erosion parameters among varying soil properties and testing procedures: Implications for river bed scour at bridge piers

Sediment entrainment in cohesive soils is influenced by interactions among the soil physical, geochemical and biological properties, which eventually define the inter-particle forces. Research was conducted for better understanding cohesive soil erosion using an in-situ mini-jet device for estimating erodibility parameters: critical shear stress (τ_c) and erodibility coefficient (k_d), and develop a modified field operational and data analysis procedure to more consistently predict these parameters. This newly developed procedure accounted for sediment property changes in surface and subsurface layers on stream beds/banks. Field data were collected from 21 streams using this method among geologically diverse formations across Tennessee, USA and identified that geological origin affected the erodibility parameters estimation. Predictive models were also developed among four physiographic provinces based on controlling soil physicochemical properties, in which a comprehensive suite of soil properties were assessed.Because existing studies report a wide variability in estimating erodibility parameters per device, research compared results between two fundamentally different devices: conduit flume and mini-jet device. Estimated results showed a reasonable correlation between the two devices for τ_c (R² = 0.58) but deviated for k_d. This fundamental research on measurements of the erodibility parameters with relationships to soil properties were applied to a civil infrastructure issue of scour at highway road bridges. Through an extensive hydrological modeling effort, the influence of cumulative effective stream power on scour depth around piers in cohesive sediments was explored. It was identified that shear duration, number of flow events above critical, and age of foundation were significantly affecting the observed scour depths around bridge piers.In addition, a laboratory flume experiment was also conducted using natural cohesive sediments around cylinder. The effect of multiple flow events on scour depth propagation and the stress history (memory effect) were studied. Results from these experiments showed significant influence on final scour depths based on the flow sequence. Finally the available scour depth predictive equations were also used to compare the results from this study. In general, findings from this research expand the body of knowledge on improving our understanding of erosion behavior and associated processes in cohesive soils in stream banks/beds.