Effect of rigid vegetation on the velocity, turbulence, and wave structure in open channel flows
Dorcheh, Saied Ahmad Mortazavi
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Vegetation plays an important role in changing the flow characteristics and flooding of natural channels. In this study a series of laboratory programmes were undertaken to investigate the effects of emergent and submerged vegetation on the flow structure in compound, simple and wide channels. The vegetation consisted of rigid rods replicating tree vegetation. In a laboratory flume, wooden rods were used to replicate this with three density states and one non-vegetated state being used to compare their effects on the flow characteristics in the flume. Velocities were measured using 3-D Acoustic Doppler Velocimeters (ADV), with one downward facing and one upward facing probe. The results showed that the velocities decreased within and near the vegetation zones, and the flow accelerated and transferred towards the upper part of the rods, or to the non-vegetated zone, along the flume. The vegetation density was shown to control the magnitude of these effects. Also, in the transition zones between the floodplain and the main channel in the compound channel, or in transition zones between the vegetated and non-vegetation zones, or near the top of the vegetation in the simple and wide channels, the velocity fluctuations and Reynolds stresses were observed to have relatively high values. The magnitude of the velocity fluctuations and the Reynolds stresses were shown to be highly depended upon the vegetation density. Vegetation reduces the energy and increases the flow depth, with a reduction in the velocities. It also protects the channel bed against erosion due to resistance of the flow and enhances settlement of sediment. Behind the rods the longitudinal velocity was observed to be very low, but other velocities (transverse and vertical) were high. Also velocity fluctuations and Reynolds stress components were higher behind the rods. However, as the distance from the rods increased, then the longitudinal velocity increased and the transverse and vertical velocities and Reynolds stresses all decreased. For many conditions in vegetated open channel flows, rods may produce transverse waves due to vortex shedding. Some experiments in the wide channel, of 1200 mm width, 10 m length, and 24 mm rods diameter were undertaken using different rod arrangements and densities to investigate the frequency and amplitude of these waves. From these experiments, and a subsequent analysis of the results, a new equation was developed for calculating the Strouhal number for water and two new equations were established for calculating the wave amplitude for different rod arrangements and densities. These equations were experimentally proved to better match the data variations than the conventional equation.