| Global warming increases the risk of storms,flooding and seawater intrusion in coastal zones.The coastal area of Hangzhou Bay is a typical area suffering from storm disasters in China,with a dense population and intensive economy.Therefore,it is an urgent need to know about the occurrence characteristics of extreme storm events in this area.As one of the cradles of Neolithic culture and rice agriculture in China,the civilization development on the coast of Hangzhou Bay was influenced by environmental factors,such as the change of sea level and climatic condition.For example,the world-famous Liangzhu civilization suddenly decayed at about 4400 cal yr BP.Previous studies reported several transgressive layers frequently found in Neolithic sites on the coast of Hangzhou Bay.These natural deposits may reflect the threat to the development of human civilization due to sea-level fluctuations and extreme events like typhoons during the middle-late Holocene.The transgressive layers could be the results of extreme events or relative sea-level rise induced by the enlargement of the local tidal range or global/regional sea-level rise.Therefore,this study aims to clarify the formation mechanism of the transgressive layers,analyze the intensity and frequency of extreme storm events in eastern China at the end of Liangzhu Culture and explore the relationship between seawater intrusion,extreme events and the development of ancient human civilization.This study helps to understand possible links between future climate change and increased storm intensity,which is of great significance to deal with the risks to coastal zone under the background of global warming.This study combining geological records and numerical simulation methods:on the one hand,we collected and drilled sediment cores and profiles from Yangtze Estuary,Hangzhou Bay and its coastal plain,to identify the geological records of transgression and storm events during middle-late Holocene and to reconstruct the paleotopography of Yangtze Estuary and Hangzhou Bay;On the other hand,on the basis of paleotopography,the astronomical tide and storm surge inundation scenarios of Yangtze Estuary and Hangzhou Bay at different stages during middle-late Holocene were simulated by using the ocean numerical model(FVCOM),to evaluate coastal flooding risk induced by the paleo-tide,paleo-sea level change and storm events.The simulated results were compared with the stratigraphic records to verify the accuracy of the model.In addition,this study combined the inverse modeling using sedimentary records with the forward modeling,to predict the path and intensity of extreme storms at the end of Liangzhu Culture.The main conclusions were as follows:(1)During 7600–4500 cal yr BP,sediments were filled in the Yangtze Estuary and Hangzhou Bay,and the funnel shape of Hangzhou Bay had formed at the end of Liangzhu Culture;most of the paleo-Ningbo Bay and Yaojiang Valley were infilled.Due to the changes of the shoreline and water depth,there was prominent tidal amplification in Hangzhou Bay,where the mean high water level from 1.2–1.4 m increased sharply to 2.0–2.6 m.By contrast,tidal amplification did not appear in paleo-Ningbo Bay located in the southern bank of Hangzhou Bay,where the astronomical tidal range decreased.(2)The simulated results of storms showed that the intensity of typhoon and the distance between the typhoon center and Hangzhou Bay were the main controlling factors of the inundation range of coastal zone,especially the inundation range of lowland on the south bank of Hangzhou Bay.Super typhoons would bring serious flooding risk to the Neolithic sites along Hangzhou Bay at the end of Liangzhu Culture;but typhoon precipitation had little influence on the south bank of Hangzhou Bay,which was close to the coastline;in addition,sea level rise will significantly increase the frequency and intensity of coastal flooding by storms along Hangzhou Bay.In the case of sea level rise of 1 m,a super typhoon event at the end of Liangzhu Culture will have a serious impact on the north and south coast of Hangzhou Bay,including Liangzhu ancient city.(3)In the late period of Liangzhu Culture,the Yushan site was located in the middle of the semi-open piedmont plain in the northeastern margin of the Cinan Mountain,and the nearby freshwater marsh extended about 3 km to the east and 10km to the south.Under such conditions,the formation of overwash storm deposits with a D95of 1.3 mm at Yushan site required a maximum water level of storm surge and wave run-up more than 20 m.After a sea level rise,the instantaneous water level caused by a super typhoon was sufficient to form the Yushan storm deposits.(4)This study again confirmed that a sudden sea-level rise of about 1 m may occur along Hangzhou Bay during 4.5–4.4 cal kyr BP.The transgressive layers which were generally found in the Neolithic sites along Hangzhou Bay were inferred to be caused by the rise of relative sea level rather than the enlargement of tidal range due to the change of paleo-topography at the end of Liangzhu culture.Meanwhile,extreme storm events were an important mechanism of sea water intrusion in the coastal lowlands along the Hangzhou Bay in short-time scale.Frequent typhoons accompanied by rising sea level posed a serious threat to the rice-farming activities on which Liangzhu ancestors depended for their livelihood,which may be closely related to the decay of Liangzhu culture.(5)Yangtze Estuary-Hangzhou Bay in the eastern part of China,is located in the transition zone of the landfall location of two main track typhoon in the Northwest Pacific Basin,westbound landfall(South China)and northbound landfall(Japan and Korea).Influenced by the warm climate background of the middle Holocene and El Ni(?)o-Southern Oscillation,the frequency of extreme storm events landing along the Hangzhou Bay at the end of Liangzhu Culture may increase. |
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