Sedimentary Feature And Morphodynamic Evolution In The Sigengsha Spit And Its Littoral Zone, Hainan

The littoral zone operates as a constantly changing system, with complicated morphodynamic processes. Our understanding of its hydrodynamic, sedimentation and morphological evolution is still far from definitive. Sigengsha and its littoral zone is located at the west of Hainan Island. The littoral zone of Sigengsha consists of many typical landforms, such as beaches, sand spits, bays and deltas, this zone shows diverse sediment sources, complicated hydrodynamics and variable morphology, where special coastal evolution process is closely connected with its sedimentary environment. Based on the surface sediment samples collected from Sigengsha and its littoral zone, We acquired the sediment classification, grain-size parameters and sedimentary environment partition. And the controlled mechanism of sediment distribution was investigated combined with hydrodynamic factors and morphology. Additionally, this study aimed to analyze the shoreline evolution along the Changhua delta, Sigengsha sand spits and Beili bay using 8 Landsat images from 1972. A simple human-computer interaction remote sensing assessment was applied to evaluate the change of shoreline. At last, the sedimentary characteristics and regular morphological evolution of Sigengsha beaches was investigated combined with the sedimentation at Sigengsha beaches and the elevation of beach profiles measured with RTK.①The results suggest that the entire study area comprise 12 sediment types, mainly including sandy silt, silt, silty sand and gravel-bearing sand. Overall, the mean grain size of study area ranges widely (-0.70 Φ to 7.13 Φ) and presents a fining trend from north to south.②Based on Fleming ternary diagram, combined with sediment sources, hydrodynamic and topographic condition, the study area was divided into three sedimentary districts:Sigengsha nearshore zone, middle Beili Bay zone and Basuo port nearshore zone. Sedimentary distribution in all 3 regions are under the influences of material source, hydrodynamic conditions and bathymetry.③ In the sediment distribution pattern, the western study area was mainly influenced by strong tidal currents. The nearshore was mostly impacted by wave winnowing and transport and was characterized by fine sediment distributed in the middle and coarse sediment in the two sides. Laterally, the Sigengsha nearshore zone is a high-energy coarse-grained sand area, mainly affected by sediments from Changhua River. The Beili Bay zone is a medium-energy transitional area where the grain sizes range widely. The Basuo port nearshore zone is a low-energy area where the sediment is the finest and most likely comes from open sea.④The shoreline of Changhua River delta has changed intensively, especially its three sand spits. Coasts along the Changhua Estuary and Sigengsha spit are almost under erosion, only those under the effects of human activities can keep deposition. The area of Sigengsha spit keeps increasing. Its middle part grows more quickly than other parts. Head part of the spit is changing frequently and the spit root is under erosion. Due to the control of human, the coast along Beili Bay is stable.⑤Grain size change from coarse to fine alternatively twice from south to north. Particle size is scattered because both waves and currents affect sediment transportation.⑥Seasonal change in Sigengsha beach is obvious. Many southern profiles suffer erosion in winter and get deposited in summer. In the long period, Sigengsha beaches is erosional coast.All of these results help us better understand the morphological processes in the littoral zone. We also hope to aid the Sigengsha beach management.