Application of Excess Shear Stress and Mechanistic Detachment Rate Models for the Erodibility of Cohesive Soils

Abstract

<abstract> <bold>Abstract.</bold> Typically the erosion rate of cohesive soils is modeled using the excess shear stress equation, which includes two soil parameters: the erodibility coefficient (k_d) and the critical shear stress (τ_c). Alternatively, a mechanistic detachment rate model (“Wilson Model”) was recently developed to predict the erosion rate of cohesive soils. The general framework of the “Wilson Model” was based on two soil parameters: b₀ and b₁. The “Wilson Model” is advantageous in being a more mechanistic, fundamentally based erosion equation as compared to the more commonly utilized excess shear stress model. The objective of this research was to derive the excess shear stress model parameters (k_dand τ_c) from field jet erosion tests (JETs) on numerous streambanks across the Illinois River watershed to further investigate the erodibility parameters relative to parameter uniformity, correlations between the derived parameters and soil texture, and the applicability of predictive relationships between k_d and τ_c. The second objective was to demonstrate the applicability of the “Wilson Model” using field JET data. If a shift to the more fundamentally based “Wilson Model” is expected, similar investigations into parameter uniformity and correlations are needed. This study also investigated correlations between the excess shear stress model parameters, k_d and τ_c, and the “Wilson Model” parameters, b₀ and b₁. A new miniature version of JET device (“mini” JET) was performed on streambanks of varying soil texture within the Illinois River Watershed in northeastern Oklahoma. Soil samples were acquired at locations of the JETs to measure the particle size distribution, average particle size (d₅₀) and bulk density. When considering correlations between the derived parameters and soil texture, no significant relationships existed between k_d or τ_c and bulk density, d₅₀, percent clay, silt, or sand, or percent clay-silt. Existing empirical relationships should be used with caution considering the variability between the results observed in this research and previous relationships proposed in the literature. Strong correlations were observed between b₀and k_d(R² = 0.90) and between b₁and τ_c (R² = 0.93). Therefore, the Wilson Model parameters closely resemble the empirical excess shear stress parameters, but can be mechanistically defined to account for multiple forces acting during the erosion process.