Turbulent Flow Structure in a Symmetric Compound Channel With Flexible Emergent Vegetation Over Floodplains

Abstract

An experimental study was conducted to investigate the influence of emergent flexible floodplain (FP) vegetation on the turbulent flow structure in a symmetric compound channel. The foliage patterns of the artificial flexible vegetation used in our experimental sets were as follows: no vegetation, homogeneous grass and shrubs and a heterogeneous mixture of grass and shrubs arranged in a staggered pattern. An analysis of the velocity data revealed an observable decrease in velocity across the FP owing to the presence of vegetation and a significant increase in the mean streamwise velocity in the middle of the main channel (MC). The effect was higher in the case of heterogeneous vegetation. This indicates that a heterogeneous mixture of grass and shrubs provides a higher resistance than homogeneous grass or shrubs separately. It enhances the lateral momentum transfer owing to the fast-moving MC flow and low-velocity FP flow. The turbulent kinetic energy (TKE) and Reynolds shear stress (RSS) were higher in the vegetative MC than in the non-vegetative channel. In contrast, these were lower in the vegetative FP than in the non-vegetative FP. An octant analysis showed a decrease in ejection events in both FP and MC and an increase in sweep events in the MC owing to FP vegetation. The reduction in ejection events contributed to the breakdown of vortices and reduced turbulence. Meanwhile, the increase in sweep events enhanced the sediment entrainment. These results indicate that heterogeneous FP vegetation significantly affects the flow dynamics and turbulent parameters in a compound channel.