Effects of vegetation patch density on flow velocity characteristics in an open channel

The aquatic plants are often found in natural rivers, and they affect the channel flow structure significantly. To study the effects of the vegetation density a (the frontal area per volume) on the flow velocity characteristics, rigid bamboo circular cylinders are chosen as the model emergent vegetation. In the experiments, the density of the vegetation takes various values while all other flow parameters are kept constant. A 3-D acoustic Doppler velocimeter (ADV) is used to measure the local flow velocities for different vegetation densities. The results show that the existence of the vegetation patch leads to an increase of the depth-averaged velocity on the right and left sides behind the vegetation patch, and it increases monotonically with the vegetation density. For different vegetation densities, the lateral distribution of the stream-wise velocity behind the vegetation patch follows approximately an S-shaped profile when a ≤ 60 m-1, and a logarithmic profile if a > 60 m-1. The vertical distribution of the stream-wise velocity along the channel varies with the vegetation density and also follows an S-shaped distribution in a certain range, with the locations and the ranges being affected by the vegetation density.