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Impact of Secondary Flow on Depth-Averaged Flow Velocity in Vegetated Open Channels

Author(s): Jungsun Oh; Taejoon Kim; Sung-Uk Choi

Linked Author(s): Sung-Uk Choi

Keywords: Lateral distribution method; Secondary flows; Bed shear stress; Transverse shear stress; Vegetated open channels

Abstract: In a natural open channel, flow velocity is determined by a combination of hydraulic, geomorphologic and vegetational factors in that each factor can cause momentum transfer in fluids. In particular, secondary flows superimposed on the primary flow frequently appear due to uneven water level by centrifugal force in a meandering channel. Secondary flows occur in a distinctive structure and direction according to the shape of cross sections in a channel. This study begins with a question about how secondary flows contribute to the lateral momentum exchange in channels (how secondary flows affect streamwise flow velocity). To solve the question, this study presents a quasi two-dimensional model using the lateral distribution method to calculate the lateral distributions of depth-averaged flow velocity. The purpose of the calculation of depth-averaged flow velocity both without and with consideration for secondary flows is to investigate the influence of secondary flows on estimation of the depth-averaged velocity. As a result, the existence of secondary flows has influenced depth-averaged flow velocity depending on the (clockwise/counter-clockwise) direction and magnitude of secondary flows. A greater magnitude of secondary flows leads to a smaller flow velocity in the main channel and a larger flow velocity in the floodplain. Furthermore, we examine the three resulting terms of the lateral distribution methods, i. e., depth-averaged velocity, bed shear stress and transverse shear stress. When the drag force due to vegetation is considered in the proposed model, the magnitudes of the three terms are affected accordingly. The comparison of results between a plain condition and a vegetated condition has been performed by employing the experimental data of open channels in the existing studies. It is concluded that the floodplain vegetation increases the difference between flow velocity in the main channel and flow velocity in the floodplain, and thus increases the transverse shear stress.


Year: 2013

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