Study On The Characteristics Of Typhoon Deposits In Jiulong River Estuary,Southern China

Typhoon is the most threatening natural disaster along the southeast coast of China.According to statistics,typhoons cause a direct economic loss of about 600 million yuan and 168 casualties every year to five provinces and cities(Shanghai,Zhejiang,Fujian,Guangdong and Hainan)in southeast China.The five provinces and cities have a total area of 444,300 square kilometers,with a total population of about 230 million,and the total GDP accounts for about a quarter of the national GDP.It is an important densely populated and economically concentrated region in China.Therefore,in order to improve the typhoon disaster prevention and resilience in coastal areas of China,it is urgent to deepen understanding of the space-time laws and driving mechanisms of strong typhoons and super typhoons affecting the southeast coast of China,and to promote the sustained and stable development of the economy in China's coastal areas.Strong typhoons and super typhoons are often "once in fifty years" or "once in hundred years",and the existing literature records or measured data focused on a relatively short time scale,for example,the hydrological station data or meteorological data is mostly since 1949,or even shorter.The literature records are mostly descriptive sentences,and the quantitative information is very vague.Therefore,the geological record has become an important carrier of studying strong typhoons in 100-year scale even in 1000-year scale.Based on this,this study choose Jiulong River Estuary in the southeast of Fujian Province as the study area.In the preliminary study of the three cores,it was found that two cores contained five layers,in which the grain size of the sediments was significantly coarser than that of the sediments in the upper and lower layers,combined with the climatic characteristics of the study area,it is preliminarily concluded that these layers may be typhoon deposits layers.Therefore,the scientific questions raised in this study are as follows:1)What are the characteristics and forming mechanism of strong typhoon deposits?2)How to use the sedimentary record to invert the period and frequency of strong typhoons?To solve the scientific questions above,this study uses grain size,organic isotope,deposition rate and major and trace elements methods to analyze 3 1 surface sediment samples and 3 sediment cores from study area,and the work mainly includes the following parts:1)Confirmation of typhoon deposits layers;On the basis of the existing sedimentary characteristics combined with ?13C?C/N data and major and trace elements data to determine the source of the sand layer and whether it is typhoon deposits layer;2)Deposition rate and dating:1By means of 210Pb and 137Cs,we can get the results of the deposition rale of cores and the age of sand layers;3)Reconstruction of the typhoon history in the study area:Corresponding with typhoon deposits layers and historical typhoon.The main results are as follows:(1)The grain size distribution of surface sediments in the Jiulong River Rstuary:The grain size of sediment in the upper estuary is coarse and the sorting is good.Some stations in the middle section of the estuary show a bad sorting because the average water depth is shallow and the hydrodynamics are complex and variable.The surface sediments in the rest of the estuary are mainly silt,and the sorting is good.(2)Combined with sedimentary characteristics and organic carbon isotope data,the two thick sand layers(S2,S4)in cores JYO and JY2 are typhoon deposits layers,S2 caused by storm surge and S4 caused by flood.The other thin sand layers(Sl,S3,S5)may also be caused by typhoons,and their source is in the estuary.Besides,the 80cm-90cm in core JYO and 60cm?70cm in core JY1 may be caused by flood and the 100cm in core JY1 may be caused by the same typhoon as the S2 in core JYO.(3)According to the dating data,the 113cm of the core JYO and the 113cm of the core JY1 are the year of 1955.The layer above 113cm is the secondary transport after 1955 and the age of the source is relatively old(older than 1955).According to the relative positions of JYO,JY1 and JY2,it can be inferred that the 113cm of the core JY2 may also be 1955.Based on other studies on deposition rates in the Jiulong River Estuary,an average of 1.53cm/year was chosen as the deposition rate for the cores in this study.(4)Summarizing the historical typhoon events affecting Fujian Province from 1659 to 1910 and from 1949 to 2018.There were 104 strong typhoons between 1659 and 1910,including 6 strong typhoons.There were 195 typhoons between 1949 and 2018,including 41 strong typhoons and 10 super typhoons.(5)Corresponding typhoon deposits layers with historical typhoon events.S1 may be caused by typhoon "Chanchu" landing in Guangdong or typhoon "Kaemi"second landing in Fujian in 2006.S2 may be caused by typhoon "Iris" landing in Fujian in 1959.The flood recorded by the layer of 85cm-90cm in core JYO may happen during typhoon "Pamela" second landing in Fujian in 1961.The flood recorded by the layer of 60cm-70cm in core JY1 may happen during typhoon "Percy"second landing in Fujian in 1980.The 100cm in core JY1 may be caused by the same typhoon "Iris" as the S2 in JY0.S3 in core JY2 may be caused by typhoon "Yancy"landing in Fujiang third times in 1990,S4 in core JY2 may be caused by typhoon"Nelson" landing in Fujian in 1985,S5 in core JY2 may be caused by typhoon "Nina"second landing in Fujian in 1 975.(6)Through the spectral analysis of the elements in core JY0,it is concluded that the possible cycle of typhoon in the study area is 22 years,13 years,7 years and 4 years,which may be affected by sunspot activity and el nino/la Nina.