Surface Sediment Grain Size And Transport Of The Beibu Gulf In South China Sea

The Beibu Gulf of South China Sea is located at the junction of China and Vietnam,surrounded by the Vietnam coastline, Guangxi coastline and Hainan Island coastline,which is a special geographical position. Numerous river bring terrigenous clast into the Beibu Gulf, Offshore materials carried by tide currents and eroded materials produced by waves also deopsited in it,the provenances are diverse.in addition,the south of the Beibu Gulf is covered by Hainan Island,ocean water flowing into the bay through the opening and the north is interlinked by the Qiongzhou Strait. Complicate hydrodynamic forces create varies seabed topography.Tidal sand ridges distributed in the estuary along the north shore, west exit of the Qiongzhou Strait and the west side of Hainan Island.Therefore,exploring the characteristics and transport trend of the bottom sediment in Beibu Gulf would contribute directly to understand the provenance, hydrodynamic force,topography of the region and make a foundation for the further research.In this paper, types of surface sediment distribution of the grain size and parameter are studied based on 96 sediment samples.Using flemming triangle graphic method to conducted a deposition dynamic partition,and Gao-Collins method to describe the transport trend of surface sediment.Moreover, bathymetric data,chart data,observed current data are applie to simulate tidal flows,wave and erosion-deposition of the research areas by MIKE21 software.The simulation results are verified by depth comparison.Combined surface sediment particle size analysis, simulation results and the characteristics of topography,we make a systematic study about the deposition-erosion environment and its impact on the formation of topography in the study areas,the main results are as follows.(1) the surface sediment types in the study areas include sandy silt, silt, sandy clay, mud, muddy sand, silty sand, gravelly muddy sand, gravelly sand, pebbly sand and sandy silt account for the majority.(2) surface sediments in the north shore are coarser, because land materials are carried into the sea by rivers and rebuilt by waves and tidal current.The west opening of Qiongzhou Strait and west side of Hainan Island the tidal sand ridge area which is constructed by the tidal current.Sediments is finer in the central of the Beibu Gulf which is the sedimentary area. in the northwest of the study area,the surface sediments do not conform modern hydrodynamic condition,which are the Palimpsests.The southern of the study area is the eroded area that are controlled by tidal current.(3) The distribution of surface sediment is affected by material source,tidal current,wave and seabed terrain in study area.the provenance is river,open waters and shoreline eroded materials.tidal currents mainly act on the Qiongzhou Strait,west side of Hainan Islang and southern of the Beibu Gulf,developing the tidal sand ridges and tidal trench.Moreover,waves mainly act on the north shoreline of the Beibu Gulf, resulting sediment differentiation.The central basin is deeper which lead to a lower energy sedimentary environment(4)Surface sediments in the study area are affected by the circulation.Tidal current that going across the Qiongzhou Strait carry sediments transporting towards north and northwest.Movement of Material consistent with the direction of circulation.Sediments in the central and south-central of study area transport to Boundaryline of China and Vietnam,it seems that changes of flow direction result in depositon,but specific reasons need to be further investigated. Sediments in the south of study area transport to the southwest and northeast,may be the tidal current flowing in and out of study areas carry sediment moving to both sides.(5)The flemming triangle,migration trend analysis and numerical simulations all show that the study areas achieve a balance between erosion and deposition.The northestem of the study area is in a state of deposition,and the amount of sediment erosion is about 0～2 cm/a.The Sorthwstern of the study area is in a state of erosion,and the amount of sediment is about 4～6 cm/a;the amout of erosion of west side of Hailan Island and the centrel of the Qiongzhou Strait are up to 8～12cm/a and 14-20cm/a Respectively;the northen shore and the Leizhou Peninsula are both eroded and deposited under the control of river,tidal current and wave.(6)The characteristics of the erosion and deposition correspond well with the seafloor in the study areas,which indicate that the erosion and deposition environment affects tremendously the formation of seafloor.