Application of a boundary shear stress model along vegetated streambanks

Vegetation plays a key role in channel morphology. This study analyzed the effects of differing vegetation types on boundary shear stress (BSS), a parameter used to estimate channel erosion rates. A flume study was conducted analyzing BSS on a sloping streambank for four streambank vegetation conditions: no vegetation, tree, shrub, and grass. Boundary shear stress was measured using a flush mount hot-film anemometer and was estimated using the ray-isovel method with measured velocity fields. Velocity fields were measured using an acoustic Doppler velocimeter (ADV) and a miniature propeller (MP). Boundary shear stress distribution along the streambank boundary was also predicted using an existing BSS model (Kean and Smith, 2004). The Kean and Smith model predicted velocity fields in similar distribution to that measured by the ADV and MP. Compared to the measurement and estimates of BSS, average boundary shear stress calculated with the flow depth and slope over estimated BSS along the streambank by at least an order of magnitude. In general, the predicted BSS distribution of the boundary increased with water depth and decreased with increasing vegetation density. The predicted BSS at the shrub toe indicated a spike in shear stress consistent with turbulent kinetic energy estimates. This area is susceptible to fluvial erosion and the presence of dense, semi-rigid vegetation may encourage the formation of a wider channel with a vertical streambank.