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.