Modelling sheet–flow sediment transport in wave–bottom boundary layers using discrete–element modelling

Sediment transport in oscillatory boundary layers is a process that drives coastal geomorphological change. Most formulae for bed–load transport in nearshore regions subsume the smallest–scale physics of the phenomena by parametrizing interactions amongst particles. In contrast, we directly simulate granular physics in the wave–bottom boundary layer using a discrete–element model comprised of a three–dimensional particle phase coupled to a one–dimensional fluid phase via Newton's third law through forces of buoyancy, drag and added mass. The particulate sediment phase is modelled using discrete particles formed to approximate natural grains by overlapping two spheres. Both the size of each sphere and the degree of overlap can be varied for these composite particles to generate a range of non–spherical grains. Simulations of particles having a range of shapes showed that the critical angle—the angle at which a grain pile will fail when tilted slowly from rest—increases from approximately 26° for spherical particles to nearly 39° for highly non–spherical composite particles having a dumbbell shape. Simulations of oscillatory sheet flow were conducted using composite particles with an angle of repose of approximately 33° and a Corey shape factor greater than about 0.8, similar to the properties of beach sand. The results from the sheet–flow simulations with composite particles agreed more closely with laboratory measurements than similar simulations conducted using spherical particles. The findings suggest that particle shape may be an important factor for determining bed–load flux, particularly for larger bed slopes.