A 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 localized groundwater seepage forces 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 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 (b0 and b1) that included seepage forces due to localized groundwater flow gradients. The proposed model provided a general framework for studying the impact of soil properties, fluid characteristics, and seepage forces on cohesive soil erodibility. The proposed model will be described and methods of analysis will be presented for deriving the material parameters from flume tests and jet erosion tests (JETs). To investigate the influence of seepage on erodibility, innovative submerged JETs and larger-scale flume experiments were conducted including cases with and without seepage. Seepage forces influenced the erodibility parameters (b0 and b1) and the corresponding predicted erosion rates. As expected, increased seepage gradients or forces decreased b1 and increased b0 for both flume tests and JETs. The influence of seepage on erosion can be predicted using the "Modified Wilson Model" parameters with a priori flume and/or JET experiments without seepage. Erodibility parameters with or without seepage from flume experiments were statistically equivalent to those derived using JETs. The "Modified Wilson Model" is advantageous in being a more mechanistic, fundamentally based erosion equation that can replace the more commonly used empirical detachment models such as the excess shear stress model.