The Effects of Plant Stem Inclination on Water Flow in Open Channel

Abstract

【Objective】 Most open channels are planted with vegetation to avoid erosion of channel beds and slopes while in the meantime facilitating decontamination of organic and inorganic pollutants in the water. However, the vegetation could detrimentally affect water flow. How the depth and growing direction of the vegetation affect water flow across the channel is an issue not well understood. The objective of this paper is to plug this gap. 【Method】 A physical model was established to simulate water flow in open channel with the vegetation taken into account. Water in the channel was divided into two layers: A vegetation layer and an overlaid free layer. The impact of the growing direction of the vegetation, characterized by their stem inclination angle, was studied experimentally. 【Result】 ①When the inclination angle of the stems was the same, the water velocity in the vegetation layer, Vv, tended to level off as the water depth in the channel, h, increased. In the free layer, the average velocity, Vs, the Reynolds number Res, Froude number Frs increased asymptotically as h increased. ②We found that Vv, Rev and Frv in the vegetation layer were spatially anisotropic varying with the stem inclination angle, while in the free layer Vs, Res and Frs decreased as the stem inclination angle increased. There was a delay in the critical water depth above which the vegetation was submerged. 【Conclusion】 Water velocity Vs, Res and Frs in the free layer were higher than that in the vegetation layer. Water flow in the vegetation layer was slow and turbulent, while in the free layer the water flow was rapid, in a transition from fast turbulence to slow turbulence. Our results can help open channel design on how to reduce erosion of its bed and slope by growing vegetation without risking flooding.

Keywords

• pitching angle
• vegetation layer
• free water flowing layer
• water flow regime