Comparison of Velocity and Reynolds Stress Distributions in a Straight Rectangular Channel with Submerged and Emergent Vegetation

Abstract

Vegetation in rivers and streams plays an important role in preventing erosion and improving bank stability. Comparison between emergent vegetation (bank vegetation) and submerged vegetation, in terms of velocity and Reynolds stress distributions, for the same aspect ratio and flow discharge, has received limited attention in the literature. This study investigates the velocity and Reynolds stress, as well as the log law for submerged and emergent vegetation in a laboratory flume and compares the results for a different set up with different sediment size and aspect ratio but the same discharge. The results indicate that the influence of submerged vegetation on the secondary currents generation is less than emergent vegetation. In addition, the log law application is valid for both submerged and emergent vegetation cases, however, it is valid up to y/h = 0.75 for emergent vegetation (in which the vegetation cover in banks is partly out of the water) but up to y/h = 0.25 for vegetation bank. For both submerged and emergent vegetation, Reynolds stress distribution presents a convex form but with a different turning point. Comparison of the results with those in an artificial pool over submerged vegetation and low aspect ratio (<5) keeps almost the same form for velocity and Reynolds stress distributions but decreases the turning point in Reynolds stress distribution. For the submerged vegetation cover, the location of zero shear stress superposes that of maximum velocity, but for the emergent vegetation approaching the bank vegetation and shifting the maximum velocity towards the bed, the location of zero shear stress approaches the bed.

Effect of Rigid Aquatic Bank Weeds on Flow Velocities and Bed Morphology

“…The results showed that the vegetation pushed the flow toward the opposite bank and caused localized bank erosion. Lin et al [42] numerically and experimentally deduced the effect of emerged bank weeds on the velocity profiles. They concluded that the emerged bank weeds reduced the velocity near the bank and that the level of velocity reduction was proportional to the weed density due to the high drag of the weeds.…”

“…They concluded that the emerged bank weeds reduced the velocity near the bank and that the level of velocity reduction was proportional to the weed density due to the high drag of the weeds. Mofrad et al [42] presented an experimental comparison between the vegetation on the channel bed and the channel banks and their effects on the velocity distribution. The study proved that, in the case of the submerged vegetation (i.e., the bed vegetation), the location of maximum velocity was observed near the water surface.…”

Incipient Motion of Bed Material in a Channel with Varying Width and Vegetated Channel Walls

“…The presence of vegetation in rivers affects various aspects of the environment and aquatic systems, such as banks stability, sediment transport, water quality, and aquatic habitats. To address issues regarding river restoration and water conservation projects, the impacts of vegetation in rivers have attracted a lot of attention from researchers [17,18]. Tang et al [19] claimed that due to generation of secondary currents resulted from vegetation in the channel bed, the velocity for the incipient motion of bed material declined.…”