Coral gravel beaches, composed of biogenic reef debris, provide important ecological functions and coastal protection. Their large grain size, low density, and irregular shape produce swash processes distinct from those on sandy beaches. Understanding their morphodynamic response is important under sea-level rise, which threatens the stability of coral reef islands. Yet coral gravel beach morphodynamics remain poorly understood, particularly the effects of permeability on swash hydrodynamics and sediment transport. This study investigated coral gravel transport and beach profile evolution in a two-dimensional wave flume under solitary and regular waves, focusing on particle motion during swash events and its relation to swash hydrodynamics, and examining profile evolution toward equilibrium. High-speed cameras and image analysis estimated water depth, flow velocity, seepage rate, particle trajectories, and bed-level change, thereby characterizing the swash hydrodynamics and morphodynamics. A particle force balance evaluated the contributions of drag, inertia, gravity, friction, and seepage.
Beach permeability had limited influence in the subaqueous zone but substantially altered the swash hydrodynamics. Infiltration dominated surface-subsurface exchange and reorganized the surface-groundwater budget, reducing the water volume and momentum available for return backwash flow, which weakened and shortened the backwash, enhancing uprush-backwash asymmetry. The cross-shore distribution of infiltration reflected a balance among infiltration capacity, hydraulic forcing, and surface water supply, with the strongest infiltration in the middle swash zone. Under these asymmetric swash conditions, landward transport during uprush was only weakly offset by seaward remobilization during backwash, producing net onshore transport and deposition in the upper beach. Force balance analysis showed that the inertia force associated with fluid acceleration was as important as drag force owing to the large particle size. Although the direct seepage force was minor, seepage strongly influenced sediment transport by reshaping the swash flow rather than acting directly on individual particles.
Under regular waves, a berm and beach step formed as the profile approached equilibrium. Onshore-dominated transport steepened the upper beach, while the growing berm slope enhanced offshore transport and progressively weakened the permeability-induced transport asymmetry, providing negative feedback on berm growth. The step developed between the closure boundary and backwash limit, where sediment mobility was close to its threshold conditions. Infiltration effects also depended on wave period: a longer period allowed more time for surface-subsurface exchange, reducing uprush intensity and limiting runup height and the equilibrium beach slope.