Dynamics of bedload transport under run-up wave by gravel resolved scheme based on 3D DEM-MPS coupling

Accurate predictions of morphological changes in swash zones require a detailed understanding of sediment transport mechanisms, which are strongly related to bore-induced vortices and turbulence, surface-subsurface interactions, namely, infiltrate/exfiltrate flow, and swash-swash interactions. However, obtaining experimental or field measurements of instantaneous velocity and sediment flux is challenging owing to the suspended sediment, turbulence, and shallow depth characteristics of these regions. The present study simulates the gravel bedload transport under a dam-break bore at a grain-resolved spatial scale. The simulation uses a 3D Lagrangian–Lagrangian solid–fluid coupled model comprising the moving particle semi-implicit (MPS) method for a violent swash flow and the discrete element method (DEM) for gravels. The simulated water depth, velocity, and sediment flux agree with existing experimental results during a run-up. The gravel transport mechanisms for the lower, mid, and upper swash zones were discussed. Discussions on bedload mechanisms reveal that bore-generated horizontal vortices can reduce the onshore velocity near the beach surface, reducing sediment flux in the lower swash zone. Modified Shields numbers investigate the seepage effects: the frequently used standard Shields number value is insufficient to estimate bedload flux under the intense infiltration in the mid-swash zone. The simulation result also elucidates the turbulence characteristics in the upper swash zone.