Effect of Vertically-Layered Vegetation on the Velocity of Open Channel Flow

Widely-exiting aquatic vegetation in rivers and canals affects the velocity structure and hydrodynamics characteristics of flow, due to the additional drag and the interaction of vegetation with the flow. The change in velocity will affect flow and ecological processes, drawing close attention to flood risk management, river rehabilitation and aquatic biodiversity. Previous studies have mainly focused on the effect of single-layer vegetation, which is far less common in natural rivers, where vegetation of various heights often co-exists. The effect of such multilayered vegetation on the flow structure remains very limited understanding. In this paper, we performed a novel experiment in a flume covered with the vegetation of three heights (10, 15 and 20 cm) in a linear pattern and under submerged conditions. Velocities at different positions behind the vegetation were measured using a micro propeller velocimeter, showing that the streamwise velocity was strongly influenced by the height and position of vegetation. The measured results also showed significant differences in velocity in the regions of directly behind vegetation and behind the vegetation gap. Generally, velocity profiles deviate significantly from the logarithmic law of flow velocity without vegetation. More specifically, the velocity is relatively small and almost constant from the bed until 0.4 h (h: short vegetation height) and then increases continuously until 1.5 h (middle vegetation height), and the velocity increases rapidly to the water surface. There are two distinct reflections occurring at about 0.4 times the short vegetation and at the middle vegetation height. This finding would help practitioners understand the role of complex riparian vegetation in ecological and riverine environment management.