Sedimentary Characteristic And Topographical Evolution Of Haikou Bay, Gaolong Bay And Boao Beaches In Hainan

Beach is an important sedimentary morphological unit of sandy coast. Hainan Island has long sandy coast which is its precious natural resource and tourist resource. But due to the influence of natural and human factor, half of the sandy coast in Hainan Island is at the risk of erosion and recession. Therefore, it is important for the beach protection and restoration as well as the tourist island development to further the study of beach sedimentary characteristics and topographical evolution in key areas in Hainan Island.Based on the data of beach sediment grain size and beach profile height in Haikou Bay, Gaolong Bay and Boao, which is gathered during August2010, December2010, March2011as well as before and after the typhoon "Nock-ten" in July2011, sedimentary characteristics and topographical evolution of these beaches were discussed, and the influence of the storm to these beach features were also analyzed. The result in detail is listed below:1) The main surface sediment type in Haikou beach was sand, and it also had other types like gravelly sand and sandy gravel. According to D50, the stronger wave in winter made winter beach sediment coarser and more sorting than summer. The distribution curve of sediment grain size was symmetric or negative, and it was mesokurtic or leptokuric. Because of the breakwater protection or human activity, the change of D50and σi with depth of core sediment in HKZ-1and HKZ-2was gentle, but it was dramatic in HKZ-3and HKZ-4. The underwater sediment type in Haikou Bay was focus on sand, silty sand and clayey silt. The D50was increasing from the west to the east, but decreasing from coast to sea, and it had positive correlation with σi.The main sand source of the Haikou Bay was the sand from Nandu River and the erosion from the east coast of the river estuary. The shoal in Haikou Bay was on the stage of slight erosion, which mainly happened in the west. The middle section of the shoal was in a balance state, and the east was on the stage of deposition. Most area on the beach showed the season change that was gentle in summer but steep in winter, but the beach was in the situation of relative balance during a short time.2) Sand was also the major beach sediment in Gaolong Bay, and there was a little muddy sand as well, which was more in summer. The surface sediment D50was about2.55φ that changed little between seasons. The sediment was moderately sorted. Its distribution curve of sediment grain size was symmetric or positive, and it was mesokurtic or leptokuric. The core sediment in GLZ-1had less change of D50and σi with depth than GLZ-2, because GLZ-1was in the shaded area of the arc-shaped coast, but GLZ-2was in the transition area. The D50and SKI decreased in the direction from bank to sea.The principle sand source of Gaolong Bay was the sand from tidal channel of Qinglan port and the erosion of the coral reef nearby. It showed big difference of beach width between north and south. It also stayed seasonal balance between erosion and deposition.3) The beach surface sediment in Boao had many types like sand, muddy sand gravelly sand and so on. Sand was also the main type. The sediment was coarsest in the three beaches, and one of the reason is that the coast in Boao was open so that it had stronger wave than Haikou Bay and Gaolong Bay whose coast was arc-shaped. Based on the seasonal change of the D50, the surface sediment of Boao beach was coarser in summer than that in winter. What’s more, the surface sediment north of the Wanquan River mouth was thinner man that south of the river mouth because of the flat terrain and the protection of coral reef. The change of D50and σi with depth of the core sediment in BAZ-2had a positive correlation.It had big difference of seasonal rangeability of beach height in Boao between north and south of the Wanquan River mouth. In the north side, the rangeability was small, and the variation regulation was backshore erosion but foreshore deposition in summer but the reverse situation in winter. In the south side, the variation was large, and had little seasonal regulation. 4) After the typhoon "Nock-ten", the average clay and silt content in Gaolong surface beach sediment reduced, but the sand content increased. At the same time, D50, σi, SKI and KG both decreased after the typhoon. In addition, typhoon made the middle beach section erosion and the end section deposition in Gaolong Bay. In the influence of typhoon, the clay silt and sand content of Boao beach surface sediment decreased, but the gravel content increased. What’s more, the sediment D50, σi and KG reduced, but the SKI increased a little. The change of the beach profile height in Boao during the typhoon had less regulation than that of Gaolong Bay, and the variation rangeability north of the Wanquan River mouth was smaller than that south of the river mouth in general.