Influence of particle size and density, and channel velocity on the deposition patterns around a circular patch of model emergent vegetation

Abstract

This laboratory study examined the influence of particle size and density, and channel velocity on the spatial deposition pattern around an emergent (extending through the entire water depth), circular patch of model vegetation located at the center of a channel. Three flow conditions and three particles of different size and density were considered. Across all particle and velocity conditions, three basic deposition patterns were observed: (1) high deposition in the patch wake and low deposition in the zones adjacent to the patch; (2) high deposition in both the wake and adjacent zones; and (3) low deposition in both the wake and adjacent zones. The observed deposition pattern correlated with the ratio of channel shear velocity (u∗) to critical shear velocity (u∗c). Specifically, for u∗/u∗c<0.7 or u∗/u∗c>3, the deposition was high (or low, respectively) over the entire channel with little difference between the wake and adjacent regions. In contrast, for 0.7 <3 divergence in net deposition between the wake and the adjacent zones occurred, with higher net deposition in the wake and lower net deposition in the adjacent zones. The peak divergence was observed at u∗/u∗c=1.6. The deposition patterns were more closely correlated with the ratio u∗/u∗c than with ws/u∗ (with ws the particle settling velocity), suggesting that the spatial variation in net deposition was driven by resuspension (associated with u∗c) and not settling (associated with ws).