Role of pressure gradients in sheet flow of coarse sediments under sawtooth waves

摘要

Discrete particle model (DPM) simulations have been performed to directly examine the role of horizontal pressure gradients acting on the wave bottom boundary layer (WBBL) during the process of sheet flow transport in the surf zone. The DPM is a two-phase WBBL model that directly couples a one-dimensional eddy-viscosity fluid flow to a three-dimensional particle model. Newton's Third Law is enforced at every simulation time step through fluid-particle interaction forces of buoyancy, drag, and added-mass. Simulations are able to reproduce bedload transport rates from a laboratory data set for coarse sediment distributions. Consequently, simulations of a monochromatic sawtooth wave were performed with three coarse grain size distributions to investigate the relative importance of bed shear stresses, horizontal pressure forces, and fluid-particle drag forces on bedload transport processes. The simulation results demonstrate that the magnitude of the horizontal pressure force acting directly on sediment embedded in the WBBL is small compared to the magnitude of the particle drag force or particle stress; however, the phasing of the onshore peak in the horizontal pressure force with respect to the particle drag force is important to sediment mobilization and enhances the observed onshore bias in bed load flux. Removing the horizontal pressure force on the sediment particles resulted on average in a 30% reduction in net bed load flux. In the surf zone, free stream fluid accelerations are typically equated to the horizontal pressure gradient. Thus, the simulations may explain why the fluid acceleration can be a successful parameter for aiding in sediment transport predictions in the surf zone.