Modified Excess Shear Stress Model Parameters based on Mechanistic Predictions from a Detachment Rate Model

Abstract

<abstract> <bold>Abstract.</bold> The excess shear stress approach is commonly used to quantify the erosion rate of cohesive soils due to fluvial forces, dependent on two major soil parameters: the critical shear stress (τ_c) and the erodibility coefficient (k_d). The excess shear stress equation is commonly used in streambank, hillslope, and runoff models. However, mechanistic approaches are currently unavailable for incorporating multiple forces, such as localized groundwater seepage, into these parameters. A more mechanistically-based detachment model, the “Modified Wilson Model,” was recently developed based on two soil parameters (b₀ and b₁) for modeling the erosion rate of soils using the hydraulic analysis of an open channel or a jet erosion test (JET). The objective of this study was to mathematically derive relationships for modifying the excess shear stress parameters (τ_c and k_d) based on parameter predictions by the mechanistic detachment rate model (b₀ and b₁). Therefore, a more mechanistic determination of erodibility parameters can be achieved without altering the structure of many sediment transport models constructed on the basis of the excess shear stress approach. Expressions for the modified τ_c and k_d were based on equating two detachment rate models and assuming that the power term in the excess shear stress equation was 0.5. Data from previous laboratory and in-situ JET devices and flume tests performed on two cohesive soils (silty sand and clayey sand) for a case with a single force (fluvial only) and a case with multiple forces (fluvial and seepage) were utilized to derive the original excess stress model parameters along with the modified τ_c and k_d parameters. The modified parameters in the excess shear stress equation allowed improved predictions of the observed data for both the flume tests and JETs.