Detachment characteristics of root-permeated soils from laboratory jet erosion tests

Abstract

The influence of vegetation on flow and sediment dynamics at various spatial and temporal scales has been well documented. Vegetation may be one of the most effective measures in streambank stabilization. Traditionally, research on the influence of vegetation roots on streambank stabilization has focused on mechanical reinforcement and reduced applied shear stress due to above ground biomass. Few studies have investigated the effect of roots on fluvial detachment of sediment. This study conducted 36 mini-jet erosion tests (mini-JETs) on bare soil samples and 29 mini-JETs on root-permeated soil samples (average root diameters of 0.5 to 1.7mm) to determine the role of roots in erosion resistance. The research also estimated parameters of the linear excess shear stress model (erodibility coefficient, kd, and critical shear stress, τc) and a nonlinear detachment model called the Wilson Model (b0 and b1) and investigated the correlations between parameters of the two models and root characteristics. Root-permeated soil samples were more erosion resistant at higher shear stress as the τc and b1 parameters were on average higher for the vegetated samples than the bare soil samples. As root diameter increased in the soil samples, erosion rates at high shear stress decreased. The erodibility coefficient parameters (b0 and kd) of both the linear and nonlinear detachment models were negatively and significantly correlated to root diameter through power functions. No significant correlation was detected between critical shear stress or b1 and root parameters which supports conclusions of previous studies. Significant correlations were observed among the parameters of the excess shear stress model and the nonlinear detachment model; especially high correlation was observed between τc and b1 for the vegetated samples. In conclusion, root-permeated soils exhibited lower erosion rates primarily through increasing the required shear stress before detachment.