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.

Velocity and turbulence characteristics in an open channel with multi-row staggered vegetation patches

A staggered distribution of vegetation is very common in nature,which might significantly affect the flow structure.To investigate its impact,three rows of staggered vegetation groups are designed under seven different working conditions.The horizontal and vertical distances between the vegetation groups are constant,while the vegetation density is variable.The flow field is measured by the acoustic Doppler velocimeter(ADV).It is indicated that both the patch exit velocity and the steady wake velocity decrease with the increase of the density,which is also related to the vegetation rows.Except for the first patch,the upstream adjustment regions of all patches are diminished to some extent,due to the effect of the contralateral patch.The steady wake region also shows a decreasing trend,with a reduction of 0.5D each time when the water flows through the patch.This means that the flow structure is also affected by the number of vegetation rows.The intensity of the turbulence increases along the channel and has a non-axisymmetric distribution similar to that of the velocity.The Karman vortex street generated by the patches on the same side would merge into a larger vortex street.Depending on its intensity,the vegetation at the back has different effects on the vortex street.

The velocity patterns in rigid and mobile channels with vegetation patches

The hydraulic characteristics in an open channel with vegetation are very important in controlling the environment pollution and restoring the river ecology. This paper studies the influence of the bed form and the vegetation patch density on the spatial velocity pattern. Rigid fiberglass circular cylinders are used to simulate the vegetation and a 3-D acoustic Doppler velocimeter (ADV) is used to measure the local flow velocities. Two types of bed forms and a series of vegetation patch densities are considered. The experimental results show that the bed form significantly influences the vertical distribution of the streamwise velocity. Besides, the velocity is affected by the bed form while the lateral distribution shape of the streamwise velocity as well as the lateral and longitudinal distribution shapes of the depth-averaged velocity is less affected. When all test conditions but the bed condition remain the same, as a result of the water energy consumption by the sediment movement, the velocity in the mobile bed case is smaller than that in the rigid bed case. The vegetation patch density has a significant effect on the flow velocity. The distributions of the flow velocity at different locations show different trends with the increase of the vegetation patch density. The upstream adjustment length is not affected by the vegetation patch density, but the steady wake length is very much affected.