Erosion Research In Donghai Island

Donghai Island, the important part of Zhanjiang Economic and Technological Development Zone, is located in the southern Guangdong Province. And the strategic position is important because of the gateway out of Zhanjiang Port. It has great significance for constructing national circular economy pilot zone to protect the Donghai Island coast and natural beach. Meanwhile, the Zhanjiang Bay entrance oceanic dynamic and sediment transporting can be analyzed via the response of beach topography and it can also provide scientific support for studying the erosion/siltation trends of Zhanjiang Port deep-water channel.Depended on the Marine public welfare industry project of “Typical island geological disaster monitoring and early warning model research in China ”, the whole coast erosion of the island is study through field surveys and high-resolution satellite image interpretation. The northeast sandy coast where has obvious erosion variation and close to the entrance of Zhanjiang Bay is focused on monitoring.Combined previous research results with means of low-altitude UAV remote sensing, beach profile monitoring, sediment grain size monitoring, the conclusions are as follows :(1) The sandy coast is mainly distributed in the southwest,southeast and Bengtang region of the island. The erosion of the southwestern coast was heavy. And it became steady after building the fending groyne. The southeastern coast deposited apparently and the speed reached up to 8.3m/yr. The coast nearby Bengtang deposited first and then became to erode and the speed of erosion was 0.38m/yr.(2) The shoreline occurred significant changes in one and half year. The entire line mainly retreate and a small part forward. The retreat of the line near the lighthouse about 200 m is most evident, with an average distance of about 10m and the most up to 17 m.(3) The whole beach is mainly siltation and part down-cuts. Wherein the south region most affected by the prawn pond sewage channel, the beach is in a perennial down-cut state. The beach near the lighthouse is also significantly affected by it and dramatic changes in a short time.(4) Sediment grain size monitoring results are consistent with the beach erosion/siltation. Because the shoreline formed where the aeolian dune retreated and the sand is finer than which from seabed, the beach down-cut, the sand finer; the beach siltate, the sand thicker.(5) Combined the shoreline and beach variation, the coast erosion situation is evaluated according to the beach catastrophic intensity scheme. The most of coast is stable or slightly deposition. The most south coast and which near the lighthouse is in the strongly erosion state.(6) The coast changes in the study area are caused by a variety of factors which general divided into natural and human factors. The sea level rise as a long time scale factor has little influence; The storm surge can cause substantial shoreline retreat in short time and it’s the major factor leading to erosion; The seabed sand translated by wave and tide is the important supply of beach sand. The impact of human factors is obvious. The prawn pond sewage channel is the most serious factor which can directly washout the beach and indirectly block the flow of sand along the coast. The structure can cause beach erosion by changing the flow field around; The reclamation activities in Zhanjiang Bay reduced the amount of tidal volume and influenced the output of sediment.