Spatial distribution of deposition within a patch of vegetation

Abstract

This laboratory study describes the spatial pattern of deposition observed in a patch of vegetation located at the wall of a channel. There are two sources of sediment flux to the patch: the advection of particles across the upstream leading edge and the lateral dispersion of particles from the adjacent open channel. The relative contribution of these two supplies determines the spatial pattern of net deposition in the patch. We define the advection length scale within the patch as the longitudinal distance over which advection contributes a significant sediment source. At distances from the leading edge that are within the advection length scale, net deposition in the patch is laterally uniform, reflecting the laterally uniform mean flow delivering the particles. At distances farther than the advection length scale from the leading edge, the net deposition is highest near the flow-parallel edge and decreases into the patch, which is the signature of dispersive transport from the patch edge. Two processes contribute to the lateral dispersion, both of which are associated with the shear-layer vortices formed at the flow-parallel interface between the patch and the channel. The vortices generate turbulence and enhance the turbulent diffusion of sediment across the interface. In addition, the vortices induce a wave oscillation in the flow field within the patch that appears to enhance the lateral transport inside the patch.