The effect of turbulence on sediment deposition

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Abstract
A Lagrangian (random walk) sediment deposition model is used to investigate the competing effects of gravity and turbulence on deposition to a fully absorbing bed. This approach permits an analysis of the probability density function of deposition velocity as well as its mean. A distinct change in transport behavior is observed at a critical value of w* c = ws/U * = 0.1, where ws is the fall velocity in still fluid and U * is the shear velocity. Turbulence controls the transport of particles below w* c which deposit faster than similar particles in still water. For particles above w* c , the still water fall velocity is a good estimate of the mean deposition velocity. A discrepancy between the Lagrangian model results and experimental data (for w * < w *) may suggest that existing diffusion models are an incomplete representation of the physical system. An alternative conceptual model is suggested, based on coherent intermittent turbulence structures, which appears to explain the experimental results more effectively. Many natural and engineered sedimentation systems have w * < w * and are dominated by turbulent sediment transport. New sediment removal technologies are suggested based on turbulence enhanced deposition.