Shear Stress and Turbulence Along a Streambank Due to Changes in Vegetation Canopy Density and Bank Angle

Vegetation along the toe of a streambank can alter the forces applied to the bank surface and can protect banks against erosion, though efforts are still needed to better understand the influence of vegetation on streambank hydraulics. In this study, we investigate the role of vegetation density and bank-toe angle on turbulence and shear stress on the bank-toe. Five flume experiments were used to explore how changes in vegetation density (plant frontal area per unit cross-sectional area) influence boundary shear stress on a 30° and 15° vegetated bank-toe at three discharge rates. Velocity profiles were measured to explore flow patterns around and under the canopy. Turbulent stress measures based on turbulent kinetic energy and Reynolds stress were used to evaluate boundary shear stress. Tensor fields were visualized to further our understanding of vorticity and hydraulics near the boundary. Resistance parameters Cd (drag coefficient) and Manning’s n were also estimated. Overall, we found a trend of increasing Cd with increasing vegetation density, related to low stem Reynolds number. Estimates of Cd and thus Manning’s n were high in comparison to most literature cited values, especially when vegetation density was high and leaves were present. Results demonstrate the strong influence of vegetation density on channel velocity and estimation of resistance. Calculated turbulence statistics indicate that, with increasing vegetation density, turbulence and shear stress decreased along the majority of the bank-toe and increased at the bottom of the bank-toe, thus a greater opportunity for erosion exists at sensitive locations along the bottom of the bank-toe and in the main channel. Reynolds stresses increased under the canopy, resulting in higher shearing forces along the bank-toe, especially on the 30° slope. Generally, we find that while vegetation may reduce shear stress along the bank-toe due to a decrease in velocity, turbulence induced by vegetation may increase erosion at the interface between the main channel and vegetated bank-toe.