Sedimentary And Geomorphological Processes Of The Nearshore Tidal Flat Of Southern Lingang Special Area In The Northern Bank Of Hangzhou Bay

Located at the sensitive region with intensive land?ocean interactions,tidal flats not only provide potential land resources and coastal disaster buffer zone for coastal cities,but also have the functions of maintain bank safety,degrade environmental pollutants and provide biological habitats.So tidal flats have remarkable economic and social worth.However,due to the influences of a series of natural and human causes,tidal flats worldwide have undergone new changes,and are facing the risk of erosion and retreat.Understanding and mastering the geomorphological changing processes and evolution pattern of tidal flats are of great guiding significance for the development and sustainable utilization of tidal flat resources.The southern tidal flat of Lingang Special Area(STFLSA),China(Shanghai)Pilot Free Trade Zone locates at the eastern end of the north bank of Hangzhou Bay.It adjacents to the Changjiang Estuary and faces the Hangzhou Bay,thus,experiences intensive flood-ebb currents interactions.STFLSA is an important passage for water and sediment transport and exchange between the Changjiang Estuary and Hangzhou Bay,with complex and changeable sea floor relief.Lingang Special Area of the China(Shanghai)Pilot Free Trade Zone(LSA)was officially established in 2019,as a natural base for high-tech industries development in Shanghai.Explore the sedimentary and geomorphological changes of the STFLSA in the context of fluvial sediment decline and human activities strengthening is of vital significance to tidal flat safety and resources conservation.This paper divides the STFLSA into two sub-sections,with Luchaogang to Zhonggang as Area I and Zhonggang to Jinhuigang as Area II.On the basis of sediment samples and topographic data of STFLSA in the past 30 years,combined with water and sediment records at Datong Station since 1980,this paper clarifies the sediment variations and geomorphological evolutions of Area I and Area II following the variations of fluvial water and sediment and the construction of siltation promotion and reclamation projects.Meanwhile,this paper explores the response of sediment dynamics of STFLSA to high-energy typhoon event using sediment samples and hydrological data pre-and post-Typhoon Matsa(TM).The main conclusions include:(1)Silt and sandy silt are the main contents in the surface sediment of STFLSA,with the grain size becoming finer from Luchaogang to Jinhuigang.Overall,the sediments have poor sorting and positive skewness.During the period from 1982 to2020,the surface sediments of STFLSA present relatively complex changes.Specifically,the average median diameter of sediment became smaller wholly from1982 to 2006,indicating sediment coarsening.The average median diameter of sediment ranges from 6.12? in 2006 to 6.29? in 2011 and then to 6.16? in 2020,suggesting sediment finer first and coarsening later.The sediments suggest relatively large spatial differences in Area I and Area II.Compared with 2006,Area I exhibited sediment coarsening in 2020 while Area II suggested sediment finer.The changes of sediment component content in STFLSA are reflected in the decrease of silt content,the increase of sand component content,while the clay content changed little.Besides,the sediment indicated the trends of worse sorting,more positive skewness and widen kurtosis.(2)STFLSA is characterized by frequent erosion and siltation changes,with erosion and siltation overlapping in time and space,but as a whole faces the risk of erosion.The geomorphological evolution of STFLSA could be classified into 3 stages.During the first stage(1989-2006),Area I increased continually,but the STFLSA wholly experienced erosion.During the second stage(2006-2014),STFLSA presented a cyclic process of “erosion-deposition-erosion”,but still showed a trend of erosion in general.During the third stage(2014-2018),STFLSA indicated erosion in most area.(3)Typhoon significantly changed the sediment dynamics of the STFLSA.The sediments were mainly composed of coarse silt and fine sand before TM.The content of coarse silt decreased while fine sand increased after TM,which decreased the average median diameter of sediment and resulted in sediment coarsening.STFLA sedimentation indicated spatial differences in response to TM: In high tidal flat,the content of fine sand and medium sand decreased while fine silt increased after TM,consequently,the average median diameter of sediment increased,the sediment became finer with worse sorting.In middle and low tidal flats,however,the content of fine sand and medium sand increased while the content of fine silt and coarse silt decreased after TM,thus the sediment became coarsen with better sorting.(4)In the past 30 years,STFLSA exhibits two sedimentation modes.The main mode: tidal dominated composite delta sedimentation mode formed by the combined effects of fluvial discharge from the Changjiang River and tidal current in the bay.The secondary mode: the shoal-channel tidal current sedimentation mode formed by the deep channel and west shore coarse-grained sediments transport with ebb current.In addition,the strengthen of wave effect caused by high-energy typhoon is the main cause for the sediment spatial changes.Storm surge elevation induced wave influence range changing also contributes to the changes in STFLSA sedimentary process.