Modern Typhoon-induced Sediment Record In The Lagoon Of Southeast Hainan Coast

Typhoon or storm events not only seriously affect the geomorphic,ecological and sediment dynamic environment of coastal regions,causing the abnormal accumulation or erosion of coastal sediment,but also threaten the life and property security of the coastal zone.To mitigate the typhoon effects,we need to understand the sediment dynamic processes during the storm,at different special and temporal scales.Modern instrumental data are limited to the recent 100 years,so we need to develop new methods to analyse sediment records to obtain longer-term storm information.Furthermore,it is important to use the modelling technique on the basis of modern sediment dynamic processes to understanding the mechanism of storm deposit formation.Previous research in storm deposits focused mainly on the overwash deposit in the lagoons or lakes which are separated from the sea by a low-lying sand dune,particularly for thoese on the coast of Atlantic and Mediterranean.However,if the sand dune is too high,it is difficult to form overwash-type stom deposit in the coastal lagoon.The present study is concerned with the following scientific questions:what is the sediment dynaimic process of such high sand due-lagoon systems during the storm?And how high is the preservation potential for the sedimentary record on large temporal scales?Hainan Island is seriously affected by typoon in China which is not only influenced by northwestern Pacific cyclones but also affected by local typhoon of the South China Sea.The coast of Hainan Island(especially in the southeastern part of Hainan)is characterized by sand dune-lagoon systems,and the height of sand dunes ranges from 10 to 20 m.Hence,southeastern Hainan is highly suitable for the study on the stom.In this research,the photograph,grain-size characteristic,elements variety and 210Pb dating data are used to analyse the storm records from a lagoon from southeastern Hainan Island.Together with the historical documets,the corresponding typhoon events which may have produced the storm sediment records were determined.At the same time,the typhoon activity patterns of the sudy area identified from the instrumental datasets,in combination with numerical experiment,were used to model the reworking process of lagoon sediment under the different conditions of wind fields and to discuss the preservation potential of the event deposits.During December 2015 to Jannuary 2016,we collected two cores XCL-01 and LAL-01 from two lagoons,i.e.,the Xincun and Lian lagoons,in southeastern Hainan.Further,we use the cores collected in 2013 to collate additional data for this research.Based on the analyses of core characteristics,grain size,210Pb dating and XRF core scanning,the storm record for the recent 100 years,were identified,including 5 modern typhoon events between 1922-2992 in core XCL-01,4 between 1919-1985 in core LAL-01 and only one modern typhoon in core XC-03 and LA-05.To reveal the sediment's response to typhoon activity under diferent conditions,a series of numerical experiments were carried out.The input file representing realistic condition includes the bathmetry of the Xincun lagoon.The sediment resuspension-accumulition/erosion processes were modeled by setting different wind speeds and directions.The results suggest that:(1)with the same wind speeds,the northerly and southwesterly winds have the greatest contribution to sediment transport;(2)the average concerntration of noncohensive suspened sediment is spread to a wider area in the case of southerly and southwesterly winds;(3)in the case of different wind speeds and directions,accumulation tends to occur in the northern part of the Xincun lagoon;and(4)For the case of the typhoon passing southern Hainan with a wind speed of 30 m/s,a maximum thickness of accumulation reaches 10 cm in terms of the order of magnitude.Meanwhile,the numerical experiments can provide the information on sutiable sampling sites for future studies on the storm deposits.The results,as outlined above,indicate that the sand dune-lagoon environment of southeastern Hainan has a relatively high preservation potential,especially in the center of the lagoon;but as the rise of fish breed since 1990s,the potentiaol was subjected to limitations;base on the typhoons' detail information that be recorded in the these cores,we suggest that the potential varying with the typhoon paths.The high preservation locations appear to be distributed over the southern part of the Xincun lagoon.