Influence of vegetation on streambank hydraulics

Graduation date: 2010Proper use of vegetation in streambank bioengineering practices requires a comprehensive understanding of the influence of vegetation density on streambank hydraulics. A series of studies were conducted to investigate the relationship between independent variables vegetation density, bank angle, and discharge and dependent variables channel velocity, resistance, turbulence, and shear stress. Flume experiments were conducted varying vegetation stem density (number of plants/horizontal area) and frontal area (number of leaves/vertical area) on 30° and 15° vegetated bank-toes at three discharge rates. Three sets of 3D velocity measures were collected using an ADV at: 1) 0.6 x depth, 2) near-boundary, and 3) velocity profiles. Resistance parameters for drag coefficient (C[subscript d]) and Manning's n were estimated. Turbulent stress measures based on turbulent kinetic energy and Reynolds stress were used to evaluate boundary shear stress. Tensor fields were visualized to explore vorticity and near-boundary hydraulics. Results demonstrated that as vegetation density increased, water was increasingly redirected from the bank-toe to the main channel, decreasing downstream velocity along the bank-toe by 35-95% and increasing downstream velocity in the main channel by 80-240%. As vegetation density increased and water velocity decreased along the bank-toe, depths increased, surface slope flattened, and an eddy formed downstream of the vegetated patch. C[subscript d] increased with increasing vegetation density, and decreasing stem Reynolds number. Estimates of C[subscript d] and n were high relative to commonly published values, especially when vegetation density was high. Increasing vegetation density also increased turbulence and shear stress, creating greater opportunity for erosion at sensitive locations along the bottom of the bank-toe and in the main channel. Reynolds stresses also increased under the canopy, resulting in higher shearing forces along the bank-toe, especially on the 30° bank-toe. Differences in results between bank-toe angles were minimal, dominated by the influence of vegetation. Magnitude of results decreased with decreasing discharge, but patterns were similar. Findings suggest planting at higher densities may protect the bank from erosion, but may increase the potential for erosion along the interface between the bank and main channel if unprotected, though further research with natural plant communities is encouraged to confirm findings