Experimental study on velocity distributions, secondary currents, and coherent structures in open channel flow with submerged riparian vegetation

Riparian vegetation, which is commonly found in natural rivers and open channels, has a strong influence on flow structures. This paper describes a laboratory experiment on velocity distributions, secondary currents, and coherent structures in narrow open-channel flow under the influence of submerged riparian vegetation. The channel is divided into a near-bank vegetated zone (VZ) and a main channel (MC) between which horizontal shear-induced coherent structures take place. A Prandtl mixing length model is applied to estimate the lateral distributions of horizontal Reynolds stress. Based on the experimental data, patterns of secondary currents induced by riparian vegetation in a narrow open channel are reported for the first time, which are found to be the cause of an S-shaped vertical profile of longitudinal velocity at the VZ-MC interface. This S-shaped velocity profile further results in two vertical mixing layers where vertical shear-induced coherent structures develop leading to correlations between longitudinal and vertical velocity fluctuations. Secondary transversal velocity also influences the characteristics of horizontal coherent structures close to the VZ-MC interface, that different inclination directions (clockwise or anti-clockwise) of these coherent structures are observed at different depths corresponding to the local transversal velocity.