Lagrangian Measurements, Simulation, and Theory for Incipient Motion in Oscillatory Flows

A combined incipient motion formulation was evaluated using laboratory measurements to determine the role of the fluid shear stresses (e.g., Shields parameter) and pressure gradients (e.g., Sleath parameter) at the onset of sediment motion. Observations across a range of sediment size and density suggested that incipient motion was induced by the pressure gradients in flows with large accelerations, by the shear stresses in flows with small accelerations, and by the combined effects for intermediate flows. The combined incipient motion formulation better predicted sediment mobilization under a range of oscillatory flows. Additionally, direct numerical simulations were performed for one set of experiments involving acetate beads. The resulting hydrodynamics and turbulent characteristics were compared with the laboratory experiments. Finally, here we attempt to close the problem with a theoretical investigation into the force balance on a single grain resting on the bed under prescribed monochromatic oscillatory forcing. The theoretical investigation combines formulae from a variety of sources including empirical and quasi-empirical relations to examine the incipient motion of particles across a wide range of conditions applicable in the marine environment.