Detail Abstract

The stabilization of “squeezed” vegetated channels

Abstract

A vegetated channel is a feature of river morphology commonly found in natural rivers and estuaries. River channels usually consist of floodplains, a transition area, and the main channel. In the right conditions, vegetation often grows in the floodplain region, creating vegetated areas. These river channels were termed vegetated floodplain or vegetated compound channels. In a vegetated channel, vegetation adds to friction and drag forces, damping the flow and boosting the magnitude of the gradient of the mean streamwise velocity at the vegetation edge. Consequently, a mixing layer develops between the vegetation and the main channel region. The parallel shear flow between the vegetation and open channel region can trigger the Kelvin-Helmholtz instability, pulling slow flow at the channel-vegetation interface toward faster flow in the main channel region. These kinds of water motions induce the flow to arise a kind of large quasi-two-dimensional turbulence structures in the mixing layer. These turbulence structures contribute up to 90% of the amount of transverse momentum exchange between the vegetated region and the open channel areas.</br>   Nevertheless, the remaining vegetation areas have been squeezed into narrow strips due to human activities. As a result, riverbanks at those locations are usually suffering from erosion. In order to obtain more insight into this issue, in this study, experimental data were presented on the instantaneous flow field inside the vegetation region in different scenarios of vegetation widths. The results suggest that the large quasi-two-dimensional turbulence structures in squeeze conditions occur more frequently but less regularly. The transverse exchange of mass and momentum induced by these large vortex structures are larger, while the space and time for sediment to be deposited are reduced. Moreover, the experimental results also reveal that if the width of the vegetation region reduces, the bed shear stress in the transition region increases significantly. In this context, the increased applied bed shear stresses on the transition slope may cause the instability of the vegetated floodplain region.