Mechanistic Detachment Rate Model to Predict Soil Erodibility Due to Fluvial and Seepage Forces

The erosion rate of cohesive soils is typically computed using an excess shear stress model based on the applied fluvial shear stress. However, no mechanistic approaches are available for incorporating additional forces, such as groundwater seepage into the excess shear stress model parameters. Seepage forces are known to be significant contributors to streambank erosion and failure. The objective of this research was to incorporate seepage forces into a mechanistic fundamental detachment rate model to improve the predictions of the erosion rate of cohesive soils. The new detachment model, which is referred to as the modified Wilson model, was based on two modified dimensional soil parameters ($b_0$ and $b_1$) that included seepage forces due to localized groundwater flow. The proposed model provided a general framework for studying the impact of soil properties, fluid characteristics, and seepage forces on cohesive soil erodibility. Equations were presented for deriving the material parameters from both flume experiments and jet erosion tests (JETs). In order to investigate the influence of seepage on erodibility, the erodibility of two cohesive soils (silty sand and clayey sand) was measured in flume tests and with a new miniature version of the JET device (mini JET). The soils were packed in three equal lifts in a standard mold (for JETs) and in a soil box (for flume tests) at a uniform bulk density (1.5 and $1.6\text{Mg}/{\mathrm{m}}^3$) near the soil’s optimum water content. A seepage column was utilized to induce a constant hydraulic gradient on the soils tested in the flume and with the mini JET. The modified Wilson model parameters, $b_0$ and $b_1$, were derived from the erosion rate data both with and without the influence of seepage from the flume and JETs. Seepage forces had a significant but nonuniform influence on the derived $b_0$ and $b_1$ as functions of the hydraulic gradient and soil density. The more fundamental detachment model can be used in place of the excess shear stress model with parameters that can be derived using JETs, transforming the modeling of cohesive soils for hillslopes, embankments, gullies, streambeds, and streambanks.