Discrete particle model for sheet flow sediment transport in the nearshore

Fully three‐dimensional discrete particle computer simulations of high‐concentration sheet flow transport in oscillatory flows quantify the effect of fluid acceleration on bed load transport in highly unsteady flows typical of nearshore marine environments. A simple impulse‐momentum approach explains simulation results and forms the basis for adding an acceleration‐related term to widely used energetics sediment transport formulae. Transport predicted by the acceleration term becomes increasingly significant as wave shape approaches the sawtooth profile characteristic of surf zone bores. Simulations integrate F = ma and a corresponding set of equations for the torques for each sphere. Normal and tangential forces between contacting particles are linear functions of the distance between sphere centers and the relative tangential displacement at the contact point, respectively; particle interactions are both inelastic and frictional. Pressure gradient forces generated by the passage of surface gravity waves drive fluid and particle motion in a stack of thin horizontal fluid layers that exchange momentum and exert fluid drag, added mass, and buoyancy forces on particles. Transport properties of the simulated granular‐fluid assemblage are robust to large variations in material properties of the particles. Simulated transport rates agree with available experimental data for unsteady transport of coarse sands; the mode of bed load motion, dispersion of bed load particles, and particle segregation by size and density are qualitatively consistent with available particle‐scale observations of bed load transport of natural particles.