Effects of vegetation density on shear layer in partly vegetated channels

Vegetation occurring along river margins deeply alters the hydrodynamic flow structure, inducing a characteristic shear layer profile and triggering the onset of large scale vortices, which, in turn, govern the lateral exchanges across the vegetated interface. In such systems, vegetative drag plays a key role, regulating the velocity difference between the vegetated area and the adjacent main channel. In this study, the link between vegetation density, differential velocity ratio, and presence of large scale coherent structures is experimentally explored. Tests were performed considering different vegetation density and focusing on the lateral distributions of velocity statistics and presence of coherent turbulent structures. Results showed that, with decreasing vegetation density, the favorable conditions for the onset of large scale structures can disappear with direct effects on velocity distributions and efficiency of lateral momentum transport. Based on the test results, a critical value of differential velocity ratio is suggested.