Vertical disoersion of fine and coarse sediments in turbulent open-channel flows

Through using a kinetic model for particles in turbulent solid-liquid flows, underlying mechanisms of sediment vertical dispersion as well as sediment diffusion coefficient are investigated. Four hydrodynamic mechanisms, namely gravitational settling, turbulent diffusion, effect of lift force, and that of sediment stress gradient, coexist in two-dimensional (2D) uniform and steady open-channel flows. The sediment diffusion coefficient consists of two independent components: one accounts for the advective transport of sediment probability density distribution function due to sediment velocity fluctuations, and the other results from sediment-eddy interactions. Predictions of the kinetic model are in good agreement with experimental data of 2D open-channel flows. In such flows, it is shown that: (1) the parameter gamma (i.e., the inverse of the turbulent Schmidt number) may be greater than unity and increases toward the bed, being close to unity for fine sediments and considerably large for coarse ones; (2) effects of lift force and sediment stress gradient become significant and need to be considered below the 0.1 flow depth; and (3) large errors may arise from the traditional advection-diffusion equation when it is applied to flows with coarse sediments and/or high concentrations.