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Velocity Distribution in Compound Channels with Vegetated Floodplains

Author(s): Kejun Yang

Linked Author(s): Kejun Yang

Keywords: Compound channels; Vegetated floodplain; Velocity distribution; Vegetation type

Abstract: In natural rivers with floodplains, there are many kinds of vegetation on the floodplains. The vegetation on the floodplain strongly affects the resistance of flow. Understanding the hydraulics of flow in a compound channel with vegetated floodplain is very important for determining the stage-discharge curve and for supporting the management of fluvial processes. In this paper, the author carried out 26 groups of preliminary laboratory experiments in a flume of 16 m long and 0. 3 m wide at Sichuan University, China in order that he could investigate the effect of different types of vegetation on the floodplain on the velocity distribution. For vegetation on the floodplain, the author chose plastics grass, duck feathers and plastic straws as model grass, model shrubs and model trees, respectively. To consider the effect of bed slope, S0, two slopes were used, i. e. 1. 25‰ and 0. 1785‰. Under the condition of the same bed slope, for every vegetation at least three discharges were considered, 10. 95, 14. 11 and 17. 72 l/s. But for certain cases, a discharge of 22. 21 l/s, was also used. For comparisons, the experiments for non-vegetated case were undertaken. A three-dimensional Acoustic Doppler Velocimeter (ADV) was used to measure the local flow velocities for different types of vegetation on the floodplain, and the total discharge and flume slope were measured independently. All measured streamwise velocities followed the logarithmic distribution for the case of non-vegetated floodplain, but for vegetated floodplains followed an S-shaped profile, exhibiting three-zones. The resistances to the flow offered by the different types of vegetation were different. On the whole, the grass retarded the flow the largest. After the floodplain was vegetated, the lateral gradient of streamwise velocity increased and the lateral velocity varied complexly. As a result, the apparent shear stress on the vertical interface between the main channel and the floodplain correspondingly increased.

DOI:

Year: 2005

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