Sediment suspension affected by submerged rigid vegetation under waves, currents and combined wave–current flows

Abstract

Laboratory experiments and mathematical analyses were carried out to investigate and quantify the impacts of submerged rigid vegetation on sediment suspension under waves, currents and combined wave–current flows. Mimic canopies constructed from wooden cylinders were used in the experiments with three configurations (sparse, dense and vertically varying density). It was found that more sediment was suspended within the vegetation canopies, where smaller velocities and higher turbulence were observed, indicating that vegetation-induced turbulence rather than mean flow was the main driver of sediment suspension over vegetated beds. Meanwhile, higher sediment concentration near the bed was observed within a denser canopy, especially in the case of vertically varying vegetation density. The near-bed suspended sediment concentration could be described by an exponential formulation using an effective bed shear velocity considering different turbulence components (i.e. vegetation wake turbulence, bed shear turbulence and near-bed coherent structures). The formula was applicable in both vegetated and bare beds with different hydrodynamic conditions. To predict the vertical distributions of sediment suspension, two theoretical models of turbulent diffusion were improved by incorporating the vertically varying turbulent intensity in the water column.