Typhoon Event Records In Coastal Deposits In Eastern Hainan Island,Southern China

Typhoons(tropical cyclones)and the resultant disasters cause huge losses of hulnan life and property.In the context of rapid population growth and accelerating urbanization,with population and social wealth being concentrated in the coastal zone,coastal protection standards need to be enhanced.With the global warning and sea level rising,the frequency and intensity of storn events will change significantly.It is the time to reappraise the extreme events.However,the time scale of data recording is too short,and the return period of extreme disastrous typhoon events is often associated with hundreds to thousands of years.Such information may be contained in Ule geological records;the purpose of this study is to analyse the sedimentary records of storm events.In 2016 and 2017,a 3-month field work was carried out in Hainan Island.Based on the results of the field investigations,the coastal dunes along the Lingshui coastlines,together with the boulder beaches at Wenchang,Wanning and the Sanya,were selected as the study sites.The overwash deposits and beach boulders were alalysed to obtain information on ancient storms.The characteristics of sedimentary sequences,particle size and magnetic susceptibility,boulder size and morphological parameters were used to identify the sedimentary characteristics and formation mechanism of the overwash and boulder deposits.Using radionuclide 14C and OSL dating,the time of occurrence of the events was determined;the empirical formula were applied to estimate the intensity of the ancient storam activity on the easterm coast of Hainan Island.(1)Based on a detailed study of the coast of the Jianling area,the coastal dune profiles(JL-1 and JL-2)containing overwash layers were identified,on the basis of the characteristics of the coarse grain sediment,particle size and magnetic susceptibility.Using AMS 14C and OSL dating analyses,in combination with historical records,the storm layers are found to be the product of multiple typhoons.The mechanism for the formation of the deposit is related to storm wave induced overwarsh patterns:the washover flow cannot return after reaching the top of the dune,resulting in the rapid deposition of the sediments carried by the storm flow,forming the overwash deposits in the back side.(2)The field investigation on the eastern coast of Hainan Island refealed boulders on the beaches at Baolnigang,Dahuajiao and the Xiaodonghai reef flat.According to boulder size and the historical documents,these boulders should have resulted from of the storms.At Baohugang,the source of the boulders is the adjacent beach rocks;because of the limited material supply,the boulders are scattered without fonning a boulder beach.In contrast,at Dahuajiao a boulder beach was formed because of relatively abundant material sourse.Sortiing of the boulders occurred because of the typhoon wave action.The boulders with a high sphericity tend to deposit in the lower part of the beach,and the disk-like boulders tend r:o pile up on the top of the beach.There are numerous large boulders in the Xiaodonghai reef flat,with a trend of landward fining.Judging their morphological characteristics and long axis direction,the largest 34 boulder in the area should have been transported in the way of moving or rolling.(3)Using the Stockdon empirical equation,the overwash height at the Jianling was calculated,indicating that the storm record at the Jianling site represents the product formed by several storms,with their return periods ranging between 100 and 200 years.The result obtained implies that the coastal dune here contain important information on the intensity and frequency of occurrence of the South China Sea typhoons.(4)According to the Nandasena empirical equation,the critical minimum wave height to initiate the transport of boulders is calculated for the Baohugang,Dahuajiao and Xiaodonghai sites.At Baohugang,the value is 10.30 m,and for Dahuajiao and Xiaodonghai they are 2.54 m and 3.22 m,respectively.However,because the model does not consider the actual distance between boulder accumulation site and breaking point,the calculation values may be underestimated.The model can.be improved by calibration with the actual transport distance of future typhoon events.