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Effect of Vegetation Configuration on Turbulent Flows in a Rectangular Open Channel

Author(s): Satoshi Yokojima; Yoshihisa Kawahara; Takuya Yamamoto

Linked Author(s): Satoshi Yokojima, Yoshihisa Kawahara

Keywords: Vegetated open-channel flow; Large-eddy simulation; Large-scale vortices; Drag reduction

Abstract: Riparian vegetation exerts significant effects on the flow structure and the geomorphology of rivers. In one sense vegetation provides a rich wildlife habitat as well as a protection from bank erosion. On the other hand it can increase the risk of flood inundation by reducing the discharge capacity. In the present study, the effect of vegetation configuration on the discharge capacity and the flow resistance in a rectangular open-channel flow has been investigated both experimentally and numerically. The laboratory experiments were conducted in a 24-m long by 80-cm wide flume with a bed slope of 1/555. A regular array of emergent circular cylinders of diameter 3mm with the distance of 3cm in the horizontal directions were used as laboratory models for vegetation. Velocity profiles were taken with an electro-magnetic velocimetry while the water-level fluctuations were measured using water-level gauges. Flow visualization using dye was also carried out to reveal the basic characteristics of large horizontal vortices developing along the edge of the vegetated zones. The present large-eddy simulation (LES) employs the standard Smagorinsky model with the van Driest damping function as a subgrid-stress model. The effect of vegetation on flow is represented by the classical drag-force model where the vegetation density and the drag coefficient are the model parameters. A free-slip assumption is used at the free surface. A continuous vegetation belt with the lateral width of 27cm was installed along the centerline of the channel in the reference case, where the applicability of the present LES was critically examined against the corresponding experimental data. Then, flows through patched vegetation zones along the centerline or the bank side were investigated with a systematic change of the size or the spacing of the vegetation patch in the streamwise direction. The present LES with a drag coefficient of 1. 0 was found to reasonably reproduce not only the mean flow statistics but the size and the frequency of the large horizontal vortices characterizing the flow. It is shown that the extent of a recovery of the flood carrying capacity (an enhancement of the drag reduction) obtained by various arrangements of the vegetation patches is, in most cases, solely dependent on the total volume of the vegetation zones, irrespective of details of the configuration such as the size or the spacing of the patches. An exception is found in the case where the vegetation belt or patch is installed off the centerline of the channel, i. e., asymmetric configuration of the vegetation zone in the lateral direction brings further reduction in the flow resistance.


Year: 2013

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