The Impact of Vegetation Morphology on Turbulence and Bedload Transport

Abstract

By promoting sediment deposition and retention, aquatic vegetation can contribute to river bank stabilization, biodiversity, as well as carbon sequestration. The morphology and distribution of aquatic plants influence the velocity field, turbulence intensity, and sediment transport in wetlands, which impacts the erosion and deposition processes. By combining physical and numerical experiments, this thesis quantified how vegetation geometry impacts turbulence and sediment transport near the bed.

In aquatic canopies, turbulence generated at the stem scale, and for submerged canopies, also in the canopy shear layer, could contribute to the near-bed turbulence. Results of flume experiments using a constant channel average velocity revealed that bedload transport was predominantly correlated with near-bed turbulence, but was also weakly correlated with near-bed velocity. First, in emergent canopies, if vegetation was not clustered, turbulent kinetic energy (TKE) and bedload transport did not depend on the arrangement and stem diameter(s) and can be predicted from plant biomass and velocity. If vegetation was clustered in patches, TKE and bedload transport decreased with increased clustering and can be predicted from plant biomass, patch geometry, and velocity. Second, in submerged canopies, for constant channel velocity, submerged canopies could enhance or reduce bedload transport, depending on their degree of submergence. With increasing submergence, H/h (defined as the ratio of flow depth H to canopy height h), the near-bed velocity and TKE decreased, and the source of near-bed turbulence shifted from stem wake to the shear layer at the canopy top. A model to predict near-bed TKE in submerged canopies was developed and used to explore bedload transport under more realistic conditions with constant energy slope and flexible vegetation. For a constant energy slope, the denser the canopy, and/or the larger fraction of flow depth occupied by the canopy (decreasing H/h), the greater the sediment transport was reduced compared to unvegetated beds. This thesis provides essential parameterizations of vegetation to hydrodynamic and morphodynamic models, which can be used to predict the vegetation conditions that promote or diminish erosion, offering a useful guide for river and coastal restoration.