Grain size characteristics and transport trends of surface sedimentin the bays of northern Hainan Island

Characteristics of grain size distribution from 192 surface sediment samples collected in the bays of northern Hainan Island (Haikou Bay, Puqian Bay, Mulan Bay, and Dongzhai Port) were analyzed and the types of sediment were classified. The depositional dynamics were analyzed using Flemming triangle diagram, GSTA model was utilized to identify sediments transport trends. The modern sedimentary characteristics and land sea interaction in this area were revealed , and the scientific basis was provided for coastal erosion protection, channel safety assurance, and marine environment governance. Results show that the main types of surface sediments in the study area are gravel, sandy gravel, sand, silty sand, sandy silt, and silt. The distribution areas of sand were the widest, followed by silty sand. From south to north, the grain sizes of sediments changed cyclically. The sorting of surface sediments in estuary and seacoast was good, while it was poor in Dongzhai Harbor and middle of Qiongzhou Strait. The study area was dominated by high-energy environments. Sediment transport modes included river transport, coastal erosion, coastal transport, and seabed erosion. The trend of surface sediments transport in the northern part of study area was east to west, which was mainly influenced by the strong tidal residual current in Qiongzhou Strait. Baisha Shoal was a sedimentary convergence center, which was formed by the combined effects of waves and currents. The dominant sedimentary dynamics of estuary, included waves, runoff and tidal current, were alternated seasonally. The accumulated sand body that formed at estuary during flood season was eroded by northeastern waves and western coastal current, and then the eroded sediments were transported westward to Haikou Bay. Sediments of the nearshore area were transformed by wave. Sediments in the Puqian Bay and Mulan Bay were transported from sea to shore in a trend consistent with the directions of wave action and flood currents. The hydrodynamic of Dongzhai Port was weak, with no obvious trend of sediment transport. There was no obvious transport trend of sediment in the Dongzhai Port.