Turbulence structure and implications in exchange processes in high-amplitude vegetated meanders: Experimental investigation

Abstract

Aquatic plants in rivers interact with flow and exert an important role in maintaining suitable habitat and ecological equilibrium. Understanding turbulence structure in the presence of vegetation is important with respect to environmental processes, such as sediment transport and mixing of transported quantities. Literature indicates that mass and momentum exchanges in the presence of vegetation are strongly influenced by the sequence of coherent structures which form between vegetated and non-vegetated zones. In the present paper we investigate turbulence structure and coherent motion in high-curvature channels with submerged vegetation. The analysis is performed with the aid of detailed experimental data collected in a meandering flume of large amplitude. Results essentially highlight that the contribution of turbulence to lateral diffusion and to streamwise and vertical transport may be significantly influenced by the relative submergence h/kv. The dominant mechanism of exchange obtained for h/kv<2 is different from that obtained for h/kv>2. Quadrant analysis has been also applied to investigate the occurrence of turbulent events and the associated exchange mechanism. In particular, at the apex section, it has been found that ejection events dominate in the outer bank region and, as literature indicates, this could cause the suspension of sediment in the flow that is transported downstream. In the central region the motion is especially characterized by high-speed fluid directed towards the bed, influencing the transport process significantly.