Secondary flow structure and sediment deposition around flexible and rigid vegetation patches

Vegetation plays a crucial role in aquatic systems and characterizes riverine ecosystems. To predict changes in riverine environment, it is essential to understand the influences of vegetation on flow structure and sediment transport. The present study explores the effects of deflection of flexible vegetation elements on vortex structure and sediment deposition around a vegetation patch through flume experiments. First, velocity measurement by particle image velocimetry (PIV) was performed for flows around a single upright/inclined rigid strip blade and around a single/three-row patch of the inclined blades to investigate the effect of deflection of vegetation elements on flow structure. Second, the effects of vegetation flexibility on the flow structure and sediment deposition in the vegetation patch wake region were explored by vertical and horizontal PIV measurements and sediment deposition experiments. In this part, we considered three types of vegetation patch models consisting of rectangular blades arranged in a square lattice array: (1) rigid, emergent vegetation; (2) flexible, submerged vegetation; and (3) rigid, submerged vegetation. In the flexible vegetation case, a rotating secondary flow is generated by the deflection of vegetation elements and significantly contributes to momentum transport into the vegetation patch wake region, while the turbulent mixing by vertical or horizontal vortices determines the wake region behind the rigid vegetation patches. Moreover, the secondary flow around the flexible vegetation patch increases the spanwise sediment transport fluxes across the lateral patch edges and thereby enhances the sediment deposition behind the vegetation patch.