The influence of vegetation on flow and transport mechanisms

Abstract

Vegetation on river banks and bars contribute to the stability of these morphological features; however, the exact processes that contribute to this stabilization are still subject to research. This study has looked into the flow phenomena at the streamwise interface between a vegetated and non-vegetated river bed. Flow velocities in vegetated flows are lower than in non-vegetated flows under otherwise similar conditions due to the increased drag by the plants. Therefore, a shear layer develops across the interface. Within this shear layer, Kelvin-Helmholz like vortices develop, inducing exchange of mass and momentum through the interface. These vortices are present in the water column, but their strength near the bed governs sediment transport mechanisms, which are the basis of morphological activity. The objective of this study was therefore: to obtain a better understanding of the coherent flow structures in the interface between vegetated and non-vegetated river beds and the effect of these structures on sediment transport mechanisms. This has been done in a series of flume experiments with a compound channel.