Modelling Sheet-Flow Sediment Transport in Wave-Bottom Boundary Layers Using Discrete-Element Modelling

摘要

Sediment transport in oscillatory boundary layers is a driving mechanism of coastal geomorphologic change. Most formulae for bed-load transport in near-shore regions subsume the smallest-scale physics of the phenomena by parameterizing interactions between particles. In contrast, the authors directly simulate granular physics in the wave-bottom boundary layer using a discrete- element model consisting 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 modeled using discrete, non- spherical 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 degrees for spherical particles to nearly 39 degrees 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 degrees 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. (5 figures, 18 refs.).