In recent years, increasing attention has been paid to the ecological role of groyne fields as habitats for aquatic vegetation; however, knowledge on interactions between vegetation and recirculating flow is still lacking, especially vegetation effects on large-scale coherent structures in the mixing layer, which control the mass exchange between the side-cavity and the main channel. In this paper, the hydrodynamics of the mixing layer in straight open channels without sediments in the flow, with consecutive groyne fields, of different vegetation densities, is investigated both experimentally through particle image velocimetry and numerically through large eddy simulation. The results show that the presence of plants rearranges the circulation systems in the groyne field, namely, from double gyres to a single gyre. With an increase in the vegetation density, the exchange coefficient between the cavity and the main channel gradually decreases. Note that the exchange rate is calculated from a newly proposed exchange layer, which is located away from the groyne tip. Based on the analysis of the Kelvin−Helmholtz eddies along the shear layer, a phenomenological model is proposed for the evolution of coherent structures and the variations in flow hydrodynamics associated with these eddies. Compared to the non-vegetation case, the presence of vegetation could suppress the evolution of coherent eddies in the mixing layer, with a consequent effect on the flow hydrodynamics around the interface.

The traditional profile model of the sediment diffusion coefficient (), which is parabolic along the water depth, cannot precisely predict the vertical distribution of 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 () monotonically decreases with increasing relative water depth, while the depth‐averaged momentum exchange coefficient () 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 () of to the momentum exchange coefficient in nonvegetated flow, 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 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.

Vegetation in natural river interacts with river flow and sediment transport. This paper proposes a two-layer theoretical model based on diffusion theory for predicting the vertical distribution of suspended sediment concentration in a flow with submerged vegetation. The suspended sediment concentration distribution formula is derived based on the sediment and momentum diffusion coefficients through the inverse of turbulent Schmidt number ( $S{c}_t$ ) or the parameter $\eta$ which is defined by the ratio of sediment diffusion coefficient to momentum diffusion coefficient. The predicted profile of suspended sediment concentration moderately agrees with the experimental data. Sensitivity analyses are performed to elucidate how the vertical distribution profile responds to different canopy densities, hydraulic conditions and turbulent Schmidt number. Dense vegetation renders the vertical distribution profile uneven and captures sediment particles into the vegetation layer. For a given canopy density, the vertical distribution profile is affected by the Rouse number, which determines the uniformity of the sediment on the vertical line. A high Rouse number corresponds to an uneven vertical distribution profile.

The hydrodynamics in a straight rectangular open channel containing novel lateral cavities constructed by an array of square emergent vegetation patches discontinuously distributed along the bank were explored numerically using three-dimensional large eddy simulations (LES). Five vegetation densities (Φ), ranging from 0.02 to 0.25, as well as the traditional lateral cavities created by impermeable solid media, were tested. The effects of the cavity aspect ratio (AR) were also examined. The LES results showed that the mean recirculation pattern inside the vegetation cavities and coherent structures in the horizontal shear layer were closely dependent on Φ and AR. When Φ ≥ 0.06, a main recirculation vortex that formed inside the vegetation cavities resembled that within solid media cavities, whereas the extent of the former increased upstream as Φ increased. Compared with the solid cases, the vegetation cavities exhibited a higher turbulent intensity within the shear layer and wider regions of enhanced turbulent kinetic energy, which decreased with increasing Φ. The penetration depth of the elevated turbulent kinetic energy into the cavities also decreased with increasing Φ, whereas a deeper penetration was expected at larger AR values. The interfacial turbulence was dominated by “cavities field”-scale coherent vortices at Φ ≤ 0.06, whereas “cavity element”-scale at Φ ≥ 0.15. When Φ = 0.1, the shear vortices of both scales contributed to the enhancement of the interfacial turbulence. The mean mass exchange showed a non-monotonic relationship with Φ and reached maximum values at Φ = 1. The total momentum transport efficiency decreased monotonically with increasing Φ. Despite the AR and Φ values, the turbulent motions dominated the momentum transport over most of the cavity length.