Reconstruction Of Storm-and Flood-event Series Within The Inner Shelf Of The East China Sea Over The Last 100 Years

Scenarios of future climate change and accelerated sea-level rise will lead to an increase in the intensity and frequency of extreme events,which brings an issue in updating protection standards for coastal disasters.Due to the short length of the observed typhoon and flood records,it is necessary to find alternative proxies to extend the time span of typhoon and flood records.Historical documents and sedimentary records in the muddy area of continental shelf provide two options to expand the available typhoon and flood records well beyond the instrumental capacities.Here,two questions were raised:?1?what is the relationship between the meteorological records of typhoon/flood events and their corresponding sedimentary records of the same period??2?whether the typhoon/flood events identified in the sedimentary records are consistent with the typhoon/flood events recorded in the historical documents.To solve these problems,this study attempts to reconstruct the storm and flood events on the continental shelf of the East China Sea over the last 100 years by coupling the observations and sedimentary records of typhoons and floods.In combination with the historical typhoon and flood events,this study assesses the preservation potential of typhoons and floods in the muddy deposits of the continental shelf,as well as other possible typhoon information?such as pathes and intensities?.Two cores were collected from the Zhengjiang coast?J1,2 m long?and the Changjiang estuary?A7-3,2.13m long?in 2018 and 2019,respectively.Core scanning,grain size analysis,²¹⁰Pb and ¹³⁷Cs specific activity analysis were performed in the laboratory;TOC and TN of A7-3 was also tested.The radioisotope ²¹⁰Pb dating method was used to determine the age-depth model of the cores.At the same time,meterological data of typhoons?1949²019?from the Central Meteorological Observatory Typhoon Network,the annual maximum runoff of the Yangtze River at the Datong Hydrological Station?1949²019?and the historical documentats of the Changjiang River floods from 1840 to 1949 were collected.Based on multiple indicators,the typhoon deposition judgment procedure of J1 is established:when the average grain size and Zr/Fe of the sediment increase,it is judged as typhoon deposit with zircon composition;when the average grain size,Sr/Al and Ca of the sediment increase,it is judged as typhoon deposit with non-zircon composition.Based on multiple indicators,the typhoon/flood deposition judgment procedure of A7-3 is established:when the average particle size and Zr/Rb of sediment increase,if TOC/TN also increases,it is judged as typhoon deposit or flood deposit,which can not be effectively distinguished;When the average grain size and Zr/Rb of the sediment increase,if TOC/TN becomes smaller,it is judged as typhoon deposit with zircon composition;when the average grain size,Sr/Al and Ca of sediment increase,it is judged as typhoon deposit with non-zircon composition.The typhoon and flood events identified in the sedimentary records were therefore compared with the observational typhoon and flood records through an age-depth model,and the following conclusions were drawn.The average sedimentation rate of core J1 was about 1.10 cm/a,and the time span of this core was about 1836 to 2018.Twenty-five typhoon events were identified in core J1,including 6 recent typhoon events during the observation period?1950²018?.The wind speed of the six typhoons are close to that of all typhoons affecting J1 with an average value of 28 m/s.In terms of typhoon pathways,most of them can be categorized as type III and type IV,both of which are characterized by landing in Zhejiang and Fujian Provinces and causing severe hazards.Through multivariate comprehensive analysis,it is found that the preservation potential of the typhoon events recorded in core J1 is about 20%³0%.The average sedimentation rate of core A7-3 was about 2.32 cm/a,and the sediments of this core were dated between 1926 and 2019.Seven typhoon or flood events were identified in core A7-3,of which two typhoon events and three flood events were recorded during the observation periods?1950²018?.The preservation potential of typhoon and flood events in core A7-3 are about 6%¹5%and 10%²0%,respectively.High-energy flooding events are more likely to leave traces in sediments;however,the typhoon event recorded in sediments is independent of the intensity of typhoons.Although the muddy area off the Changjiang Estuary has a high sedimeantation rate,the muddy area off the Zhejiang-Fujian coast has greater potential to preserve typhoon events.These findings will help to make better use of sedimentary records to expand the time span of typhoon records,and will also help guide future sampling sites to obtain much complete typhoon records.