On temporal and spatial variations of coastal topography and sediment transport process in Sanya Bay

As a global natural disaster, coastal erosion brings huge losses to the coastal economy. Affected by the human activities and natural agents, coastal erosion is intensified, especially in sandy coast. By multi-stage beach terrain monitoring during 2016-2021, the temporal and spatial variations of the coastal topography in Sanya Bay, Hainan Island, were investigated via empirical orthogonal decomposition, numerical simulation, and coastal sediment transport calculation. Results show that the coastline of Sanya Bay was eroded obviously, and the average annual retreat distance reached nearly 2 m. The four coastline segments presents different beach erosion and siltation characteristics. In the western segment of the coast, erosion has long existed due to deficient sand supply, and the terrain is mainly controlled by storm waves. With the construction of the east sand barrier at the Xidao wharf, the erosion of the first segment in the west turned into siltation, and the amount of coastal sediment transport received by the second segment in the west also decreased, thereby erosion in this area was intensified. The eastern segment is a maintenance area. The western side of Haiyue Square was eroded, while the eastern side was silted. The topography of the eastern segment was affected by both seasonal waves and storm waves. The coastal sediment transport was gradually reduced from Xiaoqi Port to Tuanjie Road, and the net sediment transport was from west to east. Although the alongshore sediment transport type on the middle segment of the coast is accumulation type, monitoring showed that its coastline was receding due to the convex embankment effect, which decreased sand supply to the western coast and resulted in offshore sediment transport. Therefore, erosion took place in mainly the surf zone, and the outer area had largely no scouring nor silting except for near-island places. The results provide a scientific basis for coastal erosion protection and management.