A basis is developed for a similarity treatment of the coupled mean fields in turbulent open-channel flows over a flat sand bed in equilibrium with a suspension of sand. Hypotheses involving multiple scales, asymptotic similarity, and matching over an overlapping region are made instead of the traditional dependence on eddy diffusion and Reynolds analogies. Velocity measurements using the laser-Doppler technique in experiments with well-sorted natural sands showed that, contrary to the implications of previous models, the effect of suspended sediment on the mean velocity profile may, over a variety of laboratory conditions, be confined to a layer adjacent to the bed. This is interpreted within the similarity framework in terms of the existence of an inner lengthscale, differing from either the viscous or the grain scale, associated with the presence of sediment. The suggested lengthscales and concentration scales are examined in light of the experimental results.
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