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Near-bed velocity and shear stress of open-channel flow over surface roughness

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Abstract

Near-bed flows in rough-bed open-channels are of relevance to river engineering, freshwater ecology and hydraulic design. Detailed measurements of near-bed flow velocity and turbulence distributions are rarely available. This paper aims to improve our understanding of near-bed flow characteristics. We performed experiments of turbulent open-channel flow over roughness elements, and made fine-scale measurements of three-dimensional velocity and turbulence quantities, using an acoustic Doppler velocimeter. The experiments allowed for the d-type, the intermediate-type and the k-type of surface roughness. The fine scale measurements covered the entire flow region in the bed vicinity, including cavities below the crests of roughness elements. Below the crests, the longitudinal velocity field displays one weak eddy in the cavity between adjacent elements for the d-type of roughness, two eddies of different strengths for the intermediate-type of roughness, and boundary-layer velocity structures for the k-type of roughness. Above the crests, the main (or longitudinal) flow velocity shows a variety of vertical structures for the three types of surface roughness. At the elevation equal to one half the roughness height above the crests, there is a lack of longitudinal variations in the main flow velocity over the length of element—cavity sequence. In other words, main-flow velocity fluctuations are small; the velocity distributions have a standard deviation equal to 5%, 6% and 7% of their mean values for the d-type, the intermediate-type, and the k-type of roughness, respectively. Practically the influence of roughness surface may be considered as capped at that elevation. The secondary flow has the highest intensity nearest the bed. Turbulence shear has the highest average value at the crest level of roughness elements.

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Acknowledgements

This study received financial support from the Natural Sciences and Engineering Research Council of Canada through the Discovery Grant held by S. Li.

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  1. Department of Building, Civil and Environmental Engineering, Concordia University, 1455 de Maisonneuve Boulevard West, Montreal, QC, H3G 1M8, Canada

    Jing Li & S. Samuel Li

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  1. Jing Li
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Correspondence to S. Samuel Li.

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Li, J., Li, S.S. Near-bed velocity and shear stress of open-channel flow over surface roughness. Environ Fluid Mech 20, 293–320 (2020). https://doi.org/10.1007/s10652-019-09728-3

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  • DOI: https://doi.org/10.1007/s10652-019-09728-3

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