Abstract:The low-velocity circulation pattern in groyne fields could promote the sediment deposition and lengthen the mean residence time of nutrients, thus creating a favorable growth environment for aquatic plants. Although flow hydrodynamics in cavity zones had been well studied, vegetation effects on the recirculating flow were rarely considered. Laboratory experiments were adopted to investigate the flow structure and sediment deposition in groyne fields with emerging vegetation, where the PIV (particle image velocimetry) was employed to measure the velocity vector in clean water and the combined method of photo, weighing and particle size analysis was used to investigate the sediment phenomenon in muddy water. The results revealed that the presence of vegetation reduced the circulation velocity and rearranged the circulation systems inside the cavity, i.e. from the double gyres to a sing one. In addition, the plants suppressed the evolution of coherent eddies in the mixing layer, with a distinct decrease in the eddy scale. Due to the deflecting effect of consecutive groyne on the main-stream, sand in the main channel was transported into the groyne field in a form of fine suspended load through two ways, i.e., by the circulation structure and by the turbulent vortex at the interface. As the sediment transport was subject to hydraulic characteristics, different flow structures between the non-vegetated and the vegetated cases could lead to the difference of sediment deposition. Under the same diachronic condition, the deposition with the coarse particle size and the relative large weight was mostly concentrated in the primary circulation area in the non-vegetated case; but for the vegetated condition, as the restriction effect of vegetation community could reduce the oscillation intensity of Kelvin-Helmholtz shear layer and the water exchange coefficient between the cavity and the main channel, the deposition with the fine size and the small weight was almost evenly distributed in the whole groyne filed. Such differences were particularly evident in the upstream groyne fields (i.e., the transitional stage of of hydraulic characteristics).