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[1] The present paper reports results obtained with image velocimetry to provide new insights into the two-phase nature of sediment-laden flows. The resulting two-phase flow perspective is compared with the traditional mixed-flow (or combined phase) perspective that treats sediment-laden flows essentially as flow of a single fluid. The insights are from flume experiments entailing the use of fully suspended natural sand and neutrally buoyant particles conveyed in a turbulent open channel flow of water. They confirm that suspended particles (irrespective of particle density) may affect a turbulent flow throughout its depth. Suspended particles modify flow turbulence, the main effects quantified being decreases in the bulk water velocity and in the von Kárman constant, while the flow's friction velocity remains approximately constant. Comparison of the results obtained with the two particle densities reveals differences in particle influences on water flow. In the flows conveying sand the characteristics of water and particle movement are strongly coupled, yet distinct; that is, there is a lag in the mean velocity between local water and particle movement, and intensities of water turbulence differ from intensities of particle motion turbulence. These results confirm and extend prior two-phase flow perspectives on suspended-particle transport and indicate the inaccuracies in some assumptions associated with the mixed fluid formulation of suspended-particle transport.

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