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

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Paper number  131596568,  2013 Kansas City, Missouri, July 21 - July 24, 2013. (doi: @2013
Authors:   Garey A Fox, Abdul-Sahib Al-Madhhachi, Erin R Daly
Keywords:   Erodibility; Excess Shear Stress Model; Fluvial Resistance; Streambank Erosion; Wilson Model

Abstract. Typically the erosion rate of cohesive soils is modeled using the excess shear stress equation, which includes two soil parameters: the erodibility coefficient (kd) and the critical shear stress (τc). 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 (kd and τc) 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 kd and τc. 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, kd and τc, 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 kd or τc 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 b0 and kd (R2 = 0.90) and between b1 and τc (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.

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