Modelling Typhoon-induced Sedimentary Processes In Coastal Lagoons,Southeastern Hainan Island

Under the background of rising sea level,intensified climate change and the concentration of social wealth to the coastal area,typhoon,one of the severest natural disasters,not only has a major impact on the coastal topography and ecosystem,but also poses a serious threat to people's lives and property.Therefore,the study of coastal typhoon activities is of great significance.Due to the difficulty of in situ observations during typhoon events,numerical experiments as a new research method play an important role in typhoon research.Hainan Island is one of the areas with the severest typhoon disasters in China.A number of coastal lagoons and dunes have developed in the coastal areas,which are suitable for preserving storm deposits and ideal for typhoon sedimentary process research.In this dissertation,Delft3D model is used to simulate the typhoon-hydrodynamic-sediment transport-geomorphology evolution of the lagoons in the southeastern Hainan Island,and the reliability of the model is verified by the information on wind field,astronomical tide,storm surge and wave height.Based on the simulation of the actual typhoon "Doksuri",the hydrodynamic process and sediment transport process of the lagoons during the typhoon are investigated.By designing different typhoon numerical experiments,the influence of typhoon track and intensity is analyzed for the hydrodynamics and sediment transport during the typhoon event.The major findings are as follows:(1)According to the simulation result of the 201719 typhoon "Doksuri",which had a great influence on the Xincun and Li'an lagoons,the typhoon caused a maximum enhancement of 0.3 m storm water level in the study area.The significant wave height outside the lagoons reached around 7 m,and it reached 1.2 m and 0.8 m,respectively,inside the lagoons.The wave height in the two lagoons decreased from the west towards the east.(2)Under the influence of "Doksuri",sediment re-suspension and re-accumulation occurred in lagoons.The accretion areas in Xincun are mainly located at the entrance and the southeastern part,with a maximum accumulation thickness of more than 10cm at the entrance.The thickness reached up to 5?6 cm at the southern part of the Xincun dock,and 4?5cm at the lagoon center.Accretion also occurred in other places,with a thickness of around 1cm.In the Li'an lagoon,the southern part and shallow water area near the entrance are subjected to erosion,with a thickness of around 4cm,and the accretion thickness in the south is 5?6cm in maximum.Sedimentation occurred mainly in the north,and it was larger in the west than in the east,with a maximum thickness of 4?5cm.(3)A comparison between the simulation results and the characteristics of sediment cores shows that the near-bed sheer stress during the typhoon can exceed the threshold for sediment motion in the lagoon.The simulated storm layer thickness is similar to that actually identified in the cores,indicating the reliability of the simulation of the sedimentary dynamic process during the typhoon event.(4)The hydrodynamic and sediment transport processes are related to typhoon intensity and track.With the same typhoon intensity,the resultant induced bed changes varies with the typhoon track:the spatial range and storm layer thickness is largest if the typhoon center passes through the study area,and they were reduced if the track passes to the north or south of the study area.With the same typhoon track,the typhoon intensity controls the intensity of erosion or accretion,but the overall spatial distributions are similar.This study aimed at simulating the sedimentary process caused by a single typhoon event.If additional simulations for a series of typhoons can be carried out,then long-term storm deposition sequences at different locations in the lagoon can be predicted.On such a basis,the formation and preservation potential of storm deposits can be evaluated,providing a guidance for locations of sediment core collection where long-term record of typhoon events may be found.In addition to the typhoon intensity and track,other factors such as the speed of typhoon center movement,duration,and the radius of maximum wind speed should be considered in the modelling work.An understanding of the responses of the study area to different typhoon events will be useful in disaster mitigation.