Impact of flexible emergent vegetation on the flow turbulence and kinetic energy characteristics in a flume experiment

• Flexible emergent plant had a great resistance on flow to reduce the velocity.
• In vegetation zone, vertical velocity profiles were roughly divided into two layers.
• Flexible emergent plant could change flow structures and turbulence in the channel.
• Investigate the relationship between Manning's coefficient and vegetation density.
• Explore the flow structure with vegetation along the streamwise in the flume.

Flexible emergent vegetation has a remarkable impact on flow structure, flood control and ecological restoration. In this study, the variation of flow turbulence and kinetic energy characteristics caused by artificial flexible emergent vegetation were studied by measuring the flow velocity with a 3D acoustic Doppler velocimeter (ADV) in an open flume. Experiments were carried out in five vegetation densities at two flow discharges, which commonly occur in rivers. The findings revealed that flexible emergent vegetation had a great resistance on flow to quickly reduce the average velocity, especially at the foliage part. In vegetation zone, vertical velocity profiles were roughly divided into two layers: the upper layer (z/z0 > 0.3) and the bottom layer (z/z0 < 0.3). The demarcation line of foliage and sheath stem (z/z0 = 0.3) were observed to be a key point to impact the Reynolds stress, turbulence intensity and turbulence kinetic energy. This area was the momentum exchange area, turbulence and Reynolds stress increased gradually along with the streamwise distance. At the same time, the larger vegetation density, the greater turbulence momentum exchanged. The experiment also measured Manning's coefficient n and obtained that vegetation density was a more important factor to influence roughness than flow discharge. A linear relationship was obtained between vegetation density and Manning's n. The findings in this paper will be useful for understanding the impact of emergent vegetation on the flow pattern, flood control and designing aquatic vegetation restoration.