Feedbacks of flow and bed morphology from a submerged dense vegetation patch without upstream sediment supply

摘要

Laboratory experiments were performed to investigate how flow patterns and bed morphology are affected around a submerged vegetation patch in the condition without an upstream sediment supply. Five patches with different submergences, including four submerged patches and one emergent patch, were considered. In all cases, a wake region with depressed velocity and turbulent kinetic energy (TKE) occurred directly behind the vegetation patches. Beyond the end of the wake region, the presence of Karman vortices contributed to the recovery of the velocity and the peak of the TKE. However, a distinct von Karman vortex street was present at the ratio h/D>1 with patch height h and diameter D, and the ratio was not influenced by the change in the channel bed. Because of the presence of stem-scale turbulence, net erosion was observed inside the patch in all cases. The motion of the colored sediment (placed inside the patch) indicated that the sediment inside the patch moved downstream and deposited in the patch wake. For a deeply submerged patch, a net deposition region was observed in the patch wake. For slightly submerged and emergent patches, a net erosion region was observed in both the patch wake and the adjacent region. Although the sand inside the patch was transported in the wake, the degree of net deposition was smaller than the degree of net erosion, which caused significant bed erosion. The greatest net erosion in the adjacent region was related to both the submergence (h/H) and the solid volume fraction (phi) of the patch. A greater h/H and phi resulted in a higher maximum net erosion value.