Study on suspended sediment particle transport in turbulent flow with fully submerged vegetation using CFD–DEM simulations

In coastal and riverine environments, the presence of submerged vegetation fundamentally alters turbulent mixing, consequently altering the transport dynamics of suspended sediment particles. Such vegetation–flow–sediment interactions are central to understanding tidal flat morphology, salt marsh accretion, and nearshore sediment budgets. Traditional continuum-based formulations usually describe sediment distribution as a scalar of sediment concentration, which avoids tracking discrete particle motions. The follow-on assumption that the quasi-steady state is a balance between upward turbulent diffusion and downward settling in uniform tidal channels leads to the well-known steady concentration profile – the Rouse formula. This classical framework is challenged when vegetation appears on the riverbed, which affects the hydrodynamic structures and the force balancing of suspended sediment particles. To re-examine more realistic sediment transport processes in complex vegetated flows, we employ the coupled Computational Fluid Dynamics–Discrete Element Method (CFD–DEM) to resolve individual sediment trajectories under a discrete framework, and model the flow as a continuous medium using Large Eddy Simulation (LES). A series of simulations are carried out for cases with different vegetation densities (λ=0.1∼0.9) and submergence ratios (H/h=1.5∼5). Results show that Kelvin–Helmholtz (K–H) vortices at the canopy top reshape the vertical structure of turbulence and dominate vertical sediment exchange. The vegetation canopy layer demonstrates vertically stratified flow velocities and more homogenized sediment concentration. To quantify how vegetation modifies sediment residence and replenishment rates, a predictive ‘sediment renewal time’ model is established. A two-layer suspended sediment concentration formulation is derived that describes vegetation effects using newly proposed suspension indices. The CFD–DEM method and predictive models advance understanding of sediment transport, bed evolution, and nutrient exchange in vegetated coastal and estuarine environments.