Vertical sorting process in oscillating water tank using DEM-MPS coupling model

In natural beaches, sea-beds consist of graded sediments. In swash and surf zones, wave actions lead to selective transportation and segregation of sediments. Thus, size segregation needs to be considered while determining the sediment transport rate and morphological dynamics on beaches. However, because of the strong non-linearity and the large deformed free surface, previous experimental or observational investigations failed to obtain enough knowledge about the sorting process in the swash zone. In this study, as the first investigation step of the size sorting process in the swash zone, an experiment was conducted with an oscillatory water tank, while the vertical sorting process was observed. Moreover, to further investigate the structure of the flow and movable bed, a 3D numerical simulation was performed. A coupling scheme of the discrete element method (DEM) and the moving particles semi-implicit (MPS) method was applied to reproduce the vertical sorting progress. The result revealed that the development of the vertical sorting brought about bed surface armoring and a horizontal velocity transition. Additionally, an acting force investigation disclosed that the inertia of the water surface gradient induced oscillations of the sediments, which consisted of elements, in the water tank while leading to frequent element collisions and contacts, which caused the elements to be pushed upward.

Automated summary

This study investigated the size sorting process in the swash zone through experiments and numerical simulations, revealing that vertical sorting in the zone leads to bed surface armoring and horizontal velocity transition. The inertia of the water surface gradient induced oscillations of sediment elements, causing frequent collisions and contacts among the elements.