Effects of submerged flexible vegetation and solid structure bars on channel bed scour

Abstract

The effects of different submerged obstacle longitudinal bars with different arrangement densities on the flow profile and morphology of a scour hole were investigated under clear water conditions. Acoustic Doppler velocimetry (ADV) data were applied to plot the vertical distributions of three-dimensional velocities and turbulent contours. The experimental results indicate that arrangement density (also can represent porosity), structural material (flexible or solid), and the sidewall effect are the main factors impacting turbulent kinetic energy and the morphology of scour holes. For flexible vegetation, the maximum turbulent kinetic energy near the bed surface increased with the arrangement density. For the same structure, the depth and the magnitude of the lateral expansion of the scour hole also increased with the arrangement density. The flexible vegetation reduced the depth of the scour hole because of deflection and arrangement density. The larger volumes of scour found in the upstream and middle sections of solid structures compare well to those in flexible vegetation. The deflection of porous flexible vegetation transported the turbulent kinetic energy downstream, reduced the turbulent kinetic energy near the sediment bed, and increased the stability of the bars. Flexible vegetation bars are able to protect the bank and the bed of a river under normal conditions, making them a good alternative design in the management and restoration of rivers.