Vortex Structure and Sediment Deposition in the Wake behind a Finite Patch of Model Submerged Vegetation

A submerged patch of finite-width vegetation may produce vortex structures in its wake in both the vertical and horizontal planes. This experimental study used velocity and deposition measurements and flow visualization to determine how the height ($h$) and width ($D$) of a submerged patch of model vegetation with high stem density influenced the wake vortex structures and their impact on the deposition of suspended sediment. The enhancement of fine-particle deposition can provide an important positive feedback to patch persistence and growth. The distance over which deposition was enhanced ($L_{\mathrm{Dep}}$) was strongly correlated with the distance over which velocity remained depressed within the wake ($L_w$), which in turn depended on the wake vortex structures. The aspect ratio $D/h$ determined the orientation of the vortices formed in the wake. For $h<D$, a vortex in the vertical plane occurred within a few patch heights downstream from the patch. If present, the horizontal von Karman vortex was initiated at the same position. The Karman vortex was absent for $h/D<0.7$. For these cases ($h<D$), $L_{\mathrm{Dep}}$ scaled on the patch height. Specifically, $L_{\mathrm{Dep}}=(2.3\pm 0.5)$ $h$. For $h>D$, the vortex in the horizontal plane formed first (closer to the patch) and controlled the velocity recovery within the wake. For these cases ($h>D$), $L_{\mathrm{Dep}}=(2.1\pm 0.4)$ $D$. In field studies, these relations can be used to predict the length of the region favoring deposition ($L_{\mathrm{Dep}}$) from measured patch diameter ($D$) and height ($h$).