Hydraulic Erosion of Cohesive Riverbanks in Response to Urban Runoff

Urbanized rivers experience increased peak discharges due to increases in non-permeable surface area within the watershed, most likely leading to higher and more frequent excess shear stresses (τ - τc), where τ is applied shear stress by channel flow and τc is critical shear stress for entrainment of boundary material. Typically, higher excess shear stress leads to increased bank erosion, but due to the complexities of cohesive soil detachment, the relation between excess shear stress and cohesive bank erosion has not been established. Cohesive bank material is detached by two processes: (1) hydraulic erosion - the lift and drag imposed by channel flow, and (2) subaerial erosion - the weakening and weathering of bank material imposed by dynamic soil moisture conditions. In this study, we analyzed hydraulic erosion of cohesive riverbanks by measuring discharge and bank erosion rates at three transects along a 600 m segment of Sand River, an urban ephemeral stream located in Aiken, SC. We evaluated magnitude, duration, event peak, and variability (number of peaks) of calculated excess shear stress distributions, and correlated the values of these four independent variables to bank erosion measurements at the three transects. Stepwise regression, in conjunction with rank correlation, found that the explanatory variable for amount of cohesive bank erosion was variability of excess shear stress at the transect with the lowest critical shear stress and event peak of excess shear stress at the two transects with higher critical shear stresses. Based on these observations we propose that (1) the event peak of excess shear stress dictates the amount of hydraulic erosion of cohesive riverbanks with moderate critical shear stresses, and (2) the variability of excess shear stress dictates the amount of hydraulic erosion of cohesive riverbanks with low critical shear stresses.