Bed Shear Stress in Channels with Emergent Vegetation

Wednesday, 17 December 2014
Qingjun Yang1, Francois Kerger1 and Heidi Margaret Nepf2, (1)Massachusetts Institute of Technology, Cambridge, MA, United States, (2)MIT, Cambridge, MA, United States
The shear stress at the bed of a channel influences important benthic processes such as sediment transport. Several methods exist to estimate the bed shear stress in open channel flow, but most of these are not appropriate for vegetated channels due to the impact of vegetation on the velocity profile and turbulence production. This study proposes a new model to estimate the bed shear stress in both vegetated and bare channels. The model is based on the observation that, for both bare and vegetated channels, within a viscous sub-layer at the bed, the viscous stress decreases linearly with increasing distance from the bed, resulting in a parabolic velocity profile at the bed. For emergent canopies of sufficient density, the thickness of this linear-stress layer is set by the stem diameter, leading to a simple estimate for bed shear stress. For bare channels, the model describes the velocity profile in the overlap region of the Law of the Wall. The model is supported by high-resolution experiments. Furthermore, the changes in turbulence isotropy and integral length across a range of vegetation density, from bare bed to dense canopy, have been explored.