Observations on flow and local scour around submerged flexible vegetation

Abstract

This study experimentally investigated the effects of submerged vegetation on the characteristics of flow and the formation of a scour hole. The submerged vegetations were modeled by bundled plastic fibers. Experiments were performed for various spacings of the plastic fibers, resulting in vegetation density ranging from 0.21 to 0.65. The vegetation models were aligned with the approaching flow in a rectangular channel. Vertical distributions of time-averaged velocity and turbulence intensity at various streamwise distances were evaluated using an acoustic Doppler velocimeter (ADV). The characteristic lengths of the scour hole, including scour depth, dune height, the length of the scour hole, the horizontal distance of the maximal scour depth, and the horizontal distance of the dune crest, were determined from the bed profiles. Velocity and turbulence intensity plots indicated that the deflected flow over the vegetation was promoted with the decrease in spanwise spacing. Downstream of the vegetation zone, two shear layers developed from the vegetation zone. As the spanwise plant spacing increased, the turbulence intensity of the upper shear layer decreased because of the strong flow through the vegetation. Vegetation densities were used to examine the effects of vegetation on the characteristic lengths of the scour hole. Scour depth, dune height, length of the scour hole, horizontal distance of the maximal scour depth, and horizontal distance of the dune crest increased linearly with a decrease in vegetation density.