THE EFFECT OF TURBULENCE ON SEDIMENT DEPOSITION

A Lagrangian (random walk) sediment deposition model is used to invest igate the competing effects of gravity and turbulence on deposition to a fully absorbing bed. This approach permits an analysis of the proba bility 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) = w(s)/U* = 0.1, where w(s) is the fall velocity in still flu id and U is the shear velocity. Turbulence controls the transport of particles below w(c) which deposit faster than similar particles in s till water. For particles above w(c), the still water fall velocity i s a good estimate of the mean deposition velocity. A discrepancy betwe en the Lagrangian model results and experimental data (for w < w*(c)) may suggest that existing diffusion models are an incomplete represen tation of the physical system. An alternative conceptual model is sugg ested, based on coherent intermittent turbulence structures, which app ears to explain the experimental results more effectively. Many natura l and engineered sedimentation systems have w < w*(c) and are dominat ed by turbulent sediment transport. New sediment removal technologies are suggested based on turbulence enhanced deposition.