Feedbacks of flow fields and sediment deposition to discontinuous vegetation patches: impacts of density and characteristic length of patches

Abstract:

The discontinuous instream plants, which have progressively emerged as potentially advantageous ecomorphologic engineers capable of changing hydraulics, enhancing ecosystem resilience, and influencing sediment deposition, usually grow seasonally. Thus, understanding the feedback of flow fields and sediment deposition to the density and length of vegetation patches is essential in exploring the geomorphic role of vegetation due to its seasonal evolution in natural rivers. In this study, we conducted flume experiments to clarify the interaction mechanisms between the hydraulics and suspended sediment deposition in discontinuous emergent vegetation patches. Results validated with field data demonstrate that discontinuous vegetation patches modify flow velocity and turbulence, significantly influencing sediment deposition along the streamwise direction. Within the vegetation region, represented by L_i, the enhanced turbulence and flow velocity induced by emergent vegetation were found to reduce sedimentation, while sediment deposition increased in interval regions. The sediment deposition pattern with the different lengths of patches (L_v) reveals the expansive effects of upstream patch on the downstream interval regions with different vegetation densities. When L_i/L_v > 1, the interval is sufficiently long to extend turbulent eddies that originated from upstream dense vegetation patches, which restricts sediment deposition and leads to the decreased deposition of suspended sediment compared to the scenario without vegetation. However, this effect recedes with the increase in interval length. Conversely, the sparse vegetation patch enhances the total deposition magnitude when the velocity in the interval zones is small. When L_i/L_v < 1, the decreased sediment deposition is observed in both dense and sparse vegetation scenarios.