Study of Suspended Sediment Diffusion Coefficients in Submerged Vegetation Flow

The traditional profile model of the sediment diffusion coefficient (epsilon(s)), which is parabolic along the water depth, cannot precisely predict the vertical distribution of epsilon(s) in vegetated flow. Therefore, in this study, a series of flume experiments were conducted to clarify the influence of submerged vegetation on the diffusion characteristics of suspended load particles. First, the submerged canopy increases the efficiency of momentum exchange but restrains the vertical diffusion of suspended sediment, indicating that the presence of submerged plants promotes the siltation of solid particles. Second, the depth-averaged sediment diffusion coefficient (epsilon(s)) over bar monotonically decreases with increasing relative water depth, while the depth-averaged momentum exchange coefficient (epsilon(m)) over bar presents two opposite trends, likely owing to the dominant turbulence event changing from ejections to sweeps and the different variation tendencies of the velocity gradient and Reynolds shear stress with increasing vegetation density. Compared with the ratio (beta) of epsilon(s) to the momentum exchange coefficient in nonvegetated flow, beta in submerged canopy flow is significantly less than 1, which means that the solid particles diffuse less readily than liquid particles because of the stem-scale vortices generated by the vegetation. In addition, vertical profile models of epsilon(s) and suspended sediment concentration were proposed and validated by experimental data. The models exhibit a high accuracy, correlation and applicability and can provide critical information to promote research on riverbed deformation and nutrient dynamics in vegetated rivers, wetlands and estuaries.