Quantifying the Erodibility of Streambanks and Hillslopes Due to Surface and Subsurface Forces

Abstract

<abstract> <bold><sc>Abstract.</sc></bold> The erosion rate of cohesive soils due to fluvial forces is usually computed using an excess shear stress model. However, no mechanistic approaches are available for incorporating additional forces, such as seepage, into the excess shear stress model parameters. Recent research incorporated subsurface (seepage) forces into a mechanistic detachment rate model for streambeds. The new detachment model, the Modified Wilson model, was based on two modified dimensional parameters (b₀ and b₁) that included seepage forces. The objective of this study was to modify the parameters (b₀ and b₁) to quantify the influence of seepage on erodibility of cohesive streambanks and to compare the results to those obtained from tests on horizontal beds. A new miniature version of a submerged jet erosion test device (“mini” JET) and a seepage column were utilized to derive the parameters of the Modified Wilson model for a silty sand soil and a clayey sand soil across a range of uniform seepage gradients. The experimental setup was intended to mimic a streambed and a streambank when the “mini” JET and seepage column were placed in vertical and horizontal orientations, respectively. The soils were packed in a standard mold at a uniform bulk density (1.5 to 1.6 Mg m^-3) near the optimum water content. Seepage forces influenced the observed erosion with a non-uniform influence on b₀ and b₁ as functions of the hydraulic gradient and density. Expected theoretical differences between Wilson model parameters for streambanks and streambeds were not consistently observed for these erodible soils, most likely due to variability in streambed and streambank samples in terms of soil preparation, packing, and seepage gradient establishment. The influence of seepage forces can be predicted by the Modified Wilson model parameters in both vertical and horizontal experimental setups using JETs on soils without seepage. Additional research is needed on the behavior of several mechanistic soil parameters under the influence of seepage.