The Effects of Bed Configuration on Incipient Motion in Oscillatory Flows

Abstract:

Small-scale incipient motion experiments were conducted in an oscillating flow tunnel with acetate beads and coarse gravel-sized electronic sediment grains. The effects of bed configuration on incipient motion were investigated by comparing the hydrodynamics associated with one exposed grain, two exposed grains, and a packed bed at the onset of sediment motion under similar free-stream forcing conditions. Direct numerical simulations were also performed for the acetate bead experimental trials. As more grains were exposed to the flow, the critical Shields parameter for sediment motion increased by as much as a factor of three but decreased with separation distance as sheltering effects decreased. A combined incipient motion criterion, accounting for the effects of the fluid shear stresses, pressure gradients and bed configuration was also evaluated. The critical threshold of the combined parameter was dependent on the static coefficient of friction and the packing concentration of the mobile bed layer. The turbulence field was intensified when more grains were added and turbulent vortices shed by nearby sediments may have induced sediment motion for some trials.