Turbulent flows over smooth erodible sand beds in flumes

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Abstract
The characteristic flow parameters of an open channel flow with an erodible hydraulically smooth sand bed were investigated for sand grains at rest, and for the flow regime in which an intermittent transport occurs. The experimental results show the following: (1) The grains are only transported by sweeps. (2) The sediment transport is responsible for up to 30% of the wall shear velocity. (3) No noticeable drag reducing effect occurs. (4) At high intermittent sediment transport the sweep frequency was found to be constant over the water depth. (5) With increasing wall shear stress the intensity distribution of the sweeps becomes more homogeneous and the maxima smaller. All these results are discussed based on a quadrant analysis of the main features of the so-called coherent structures of turbulent flows.