The effects of stream bank vegetation on the flow characteristics and turbulent flow field of the main channel.

Abstract:

Over the last two decades, the role of vegetation in the environmental and ecological restoration of surface water bodies has received much attention. Numerous studies have investigated the effects of vegetation on flow velocity at various locations ranging from the floodplain, river-bank and main channel. There is a general agreement amongst such studies, that the presence of vegetation may cause a decrease in flow velocity locally, followed by increased flows laterally, in non-vegetated regions. This experimental study attempts to present the findings of an elaborate set of flow measurements that aim at quantifying changes to the flow field at the main channel, at the bank inside the vegetated region and at their interface, for increasing vegetation density. To this goal an inclined section is constructed with acrylic panels sloping at, in a 1.8m wide re-circulating flume, to simulate the stream-bank. The main-channel bed comprises of coarse sand of 1.5mm nominal diameter, while 6mm diameter acrylic rods are used to simulate the vegetated river-bank. Ten velocity and turbulent intensity profiles are developed across the test cross-section, for each vegetation density, via acoustic Doppler velocimetry along a fine measurement grid. The experiments are run under uniform flow and stable bed conditions, for a range of six different vegetation densities changed by adding more vegetation elements in a staggered arrangement along the stream-bank. These detailed observations are further analyzed, with emphasis on the effects on the main channel. Measurements near the bed with implications for sediment transport are further discussed.