Variability of fluvial erodibility parameters for streambanks on a watershed scale

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, the Wilson model, was recently developed to predict the erosion rate of cohesive soils, also based on two soil parameters: b0 and b1. The Wilson model is proposed as advantageous in terms of being a more mechanistic, fundamentally based erosion equation. 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 in Oklahoma to investigate (i) erodibility parameter variability or uniformity at a river basin scale, (ii) correlations between the derived parameters and soil texture, (iii) influence of solution technique on the estimated erodibility parameters, and (iv) the applicability of predictive relationships between kd and τc. The second objective was to demonstrate the applicability of the mechanistic Wilson model using field JET data and to investigate correlations between the excess shear stress model parameters, kd and τc, and the Wilson model parameters, b0 and b1. Erodibility parameters for streambanks of varying soil texture were measured using a miniature version of the JET device (mini JET). Data from the JETs were used to derive the erodibility parameters using a Blaisdell and scour depth approach. Soil samples were acquired at locations of the JETs to quantify particle size distribution, average particle size (d50) and bulk density. 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 = 0.62 to 0.89) and between b1 and τc (R2 = 0.31 to 0.96). Therefore, the Wilson model parameters closely resemble the empirical excess shear stress parameters.