Flow characteristics in different densities of submerged flexible vegetation from an open-channel flume study of artificial plants

摘要

The effect of submerged flexible vegetation on flow structure (e.g. flow velocity, Reynolds shear stress, turbulence intensity and Manning coefficient) was experimentally studied with a 3D Acoustic Doppler Velocimeter (ADV) in an open-channel flume. The results from flow observations over artificial plants (designed to simulate natural vegetation) showed that flow structure was affected markedly by the presence of submerged flexible vegetatioa The study provides understanding of flow patterns, variation in velocity profile and turbulence structures that are affected by plant stem density. The study also reveals how the flow patterns return to stability at the downstream end of the vegetated area which is critical in determining the length of the vegetated areas for restoration cases. Also, new mathematical expressions (equations) have been formulated to clearly express variations in velocity profile, Manning coefficient and flow discharge ratio with vegetation density. Vertically, the velocity profile could be roughly divided into three layers, including the upper non-vegetated layer, the middle canopy layer, and the lower sheath layer. In the upper non-vegetated layer, velocity profiles followed the logarithmic law, and a corresponding empirical equation was developed based on the observed data. The flow is from left to right in this study, and the velocity profile followed a left round bracket"(" with the minimum point located at the canopy area (0.7H_v, where H_v denotes vegetation height) within the middle canopy layer. However, the velocity profile followed a right round bracket")" in the lower sheath section layer with the maximum point located at the sheath section (0.2H_v). With increasing vegetation density, the velocity and corresponding flow rate increased in the upper non-vegetated layer and decreased within the middle canopy layer and the lower sheath layer. The ratio of average flow discharge in the non-vegetated and vegetated layers followed the exponential function law with increasing vegetation density. This analysis revealed the effect of vegetation on flood potential and flow bottom scour. Reynolds stresses peaked above the canopy top (z/H_v ＝ 1.0-1.2, here z denotes vertical coordinate), and the turbulence intensities reached their maximum peak at two locations including the sheath section (z/H_v ＝ 0.1 -0.4) and the canopy top (z/H_v ＝ 1.0-1.6) for all vegetation densities. Manning coefficient was highly correlated to vegetation density and inflow rate with new empirical equations being proposed.