Cross-shore grain-size distribution and sediment transport on different beaches of a coral island in the South China Sea

Sediment transport processes play a key role in the geomorphic stability of coral island beaches, however, these processes remain poorly understood. Using sediments collected from the southern and northern beaches of the North Island in the Xisha Islands of the South China Sea, we constructed a unique grain-size descriptor of irregular coral sands by evaluating the fitting performance between particle dimension parameters and approximated particle volume, and applied this descriptor to characterize cross-shore variations in grain-size distributions on both beaches. Finally, the driving forces and transport processes of different coral sand grain-size components were jointly analyzed using End-Member Modelling Analysis (EMMA), integrated with regional hydrodynamic background information and established beach transport mechanisms. The results indicate that cross-shore sediment transport in the intertidal zones of both beaches is characterized by gravel deposition under combined wave and tidal action, with limited transport of coarse sand and sustained transport of medium-coarse sand. The two beaches display distinct transport patterns in the supratidal zones. The southern (windward) beach is jointly influenced by aeolian transport and wave-splash events, resulting in the alternating depletion and replenishment of medium-coarse sands. In contrast, the northern (leeward) beach is dominated by aeolian transport, leading to the continuous depletion of medium-coarse sands. These findings provide process-based insights into sediment transport on coral island beaches, although an island-wide assessment is needed in the future.