Velocity and Turbulence Evolution in a Flexible Vegetation Canopy in Open Channel Flows

Dams intercept the river flow and reduce the water input of downstream lakes, which leads to the decreased water level and deteriorated water environment. Environmental water releases are becoming regular ways to maintain ecological water demand of lakes. This study was carried out to describe how natural submerged flexible vegetation altered velocity and generated turbulence under different water releases discharge scenarios. Natural Ceratophyllum demersum L. was collected from a large shallow lake, Baiyangdian Lake, China. Two vegetation densities were set in this flume experiments. The flow velocity was measured using particle image velocimetry (PIV) techniques and used to study the mean flow features (velocity and Reynolds stress) and distributions of vegetative drag and development length. The observed velocity profiles presented vertical stratification. The greatest gradient of velocity and shear stress appeared around the top of the canopy. The canopy drag of submerged flexible vegetation presented a parabolic vertical distribution, and smaller drag was generated by larger flows under the same density. The initial adjustment length of water flow was linearly correlated with the deflected height of flexible vegetation and was little affected by the vegetation density in the two studied densities. The most intense turbulence in the vegetation canopy appeared at its top, where the largest shear stress was also observed. Strong canopy-scale vortices on the top of the canopy were produced and penetrated closer to the bed in the cases of denser vegetation under small discharges (Cases 3.1–3.5); penetration was also restricted by canopy drag and was unable to reach the bed.