Flow and deposition in and around a finite patch of vegetation

This laboratory study describes the flow and deposition observed in and around a finite patch of vegetation located at the wall of a channel. Two patch densities are considered with 2% and 10% solid volume fraction. The velocity field, measured in and around the patch by acoustic Doppler velocimetry, revealed three distinct zones. First, there is a diverging flow region at the leading edge of the patch, where the flow in line with the patch decelerates, and the bulk of the flow is diverted toward the open channel. Second, there is a fully developed region within the vegetation, where the velocity is uniform across the patch width and along the length of the patch. Third, a shear layer forms at the interface between the patch and adjacent open channel. The pattern of deposition in and around the vegetation was characterized by releasing 12-micron spherical, glass particles and recording net deposition on a set of glass slides. In the diverging region, net deposition increases in the stream-wise direction, as the local velocity decreases. In the fully developed region of the patch, deposition decreases with longitudinal position, as the concentration in the water column is depleted. The deposition pattern is nearly uniform across the patch width, consistent with the velocity field and suggesting that turbulent diffusive flux across the lateral edge of the patch is not a significant source of particles to the patch under the conditions studied here.