EFFECTS OF BED PERMEABILITY ON FINE SEDIMENT TRANSPORT

The transport behavior of silt and fine sand in suspension is important to understanding contaminant fates, water quality, and sediment accumulation in rivers and the sea. We report the results of experiments conducted to examine the effects of, among other things, bed permeability on fine sediment transport. These results are compared with existing observations of silt transport over smooth and granular-rough, impermeable beds, and are analyzed in terms of a simple model that accommodates simultaneous arrival to, and pick-up of, grains from the bed. Over permeable and impermeable beds alike, we find that the rate of fine-particle arrival to a bed diminishes linearly as the value of the dimensionless parameter S = u_*³/g'ν approaches unity, where u_* is the conventionally-defined friction velocity of the slow, g' is the density adjusted acceleration due to gravity, and ν is the kinematic viscosity of the transporting fluid. Entrainment of fine particles from impermeable beds increases exponentially with S but is all but eliminated from a flat, permeable bed comprising an open framework of screen mesh. This transport behavior suggests that noncohesive, silt-sized grains can neither gain nor remain on an impermeable bed under flow conditions for which intense lift in the viscous sublayer equals or exceeds submerged weight of individual particles. In this instance, the lift force on a spherical particle in a linear shear flow is appropriately predicted by an expression first advanced by Saffman (1964; J. Fluid Mechanics). In contrast, an open-framework bed with pore spaces offers refugia for particles slowly depositing from suspension, and thus significantly affects fine-particle entrainment and transport behavior.