Accretion-Erosion Evolution And Its Driving Mechanisms Of Yangtze Underwater Delta Over The Past Nearly 20 Years

In recent years,human activities have become the main driving forces affecting the topography of the estuary,as its influence becomes stronger,The delta underwater topography has presented a new pattern of erosion and accretion,and caused widespread concern at home and abroad.Being China’s largest estuarine delta,Yangtze Delta’s topography evolution and the driving mechanism have always become the focus of social concern.Since the 21 st century,due to the construction of several water conservancy projects in the upper reaches,the sediment volume of the Yangtze River decline sharply.In this context,the process of the underwater topography in the Yangtze River Delta and its evolution in the future are highly concerned.Although previous researchers have conducted a lot of analysis and discussion,due to the data’s inaccuracy and of the scope’s limitations,also the research methods are different,the understanding differs.Therefore,on the basis of drawing on previous researching,it is very necessary to make good use of high resolution and large-scale research methods to study this issue in depth.In this paper we measured and collected the depth data of the Yangtze River estuary and the historical topographical change in characteristics data and we used ArcGIS,Surfer and Origin software for processing and analysis,to obtain the historical topographical characteristics of the entire Yangtze River estuary,key areas and key monitoring sections over the past 20 years.The analysis of the driving mechanism of incoming sand and sea level changes in the river basin identifies the main driving factors controlling the erosion and accretion of the Yangtze River estuary in the past 20 years,providing scientific predictions for the evolution of tidal flats in the Yangtze River Estuary,maintenance of major projects,and maintenance of waterways.in accordance with.The results are shows: The main results are: 1.Features of underwater topography evolution in the study area:The region generally presents the characteristics of "near bank accretion and far bank erosion".In the space,the degree of scouring and accretion in the vicinity of the mouth is stronger than that of the underwater delta.In the vicinity of 6.4 m,accretion-erosion interface The accretion trend is basically maintained in plain shoals,but the accretion amplitude gradually decreases with increasing water depth.The area between-6.4m and 9m is dominated by erosion,the degree of erosion gradually increases as the water depth increases.And for the area between-9m to-19 m the erosion reduced.topography changes are smaller and more stable from-19 m to-50 m.The accretion and erosion volume in the entire research area showed a decreasing trend,and the erosion extent was larger than the accretion rate.Since 1997,The study area has undergone such changes: net accretion(1997-2002)-weak accretion with deposit tendency decreased(2002-2007)-weak accretion with deposit tendency increased(2007-2009)-Weak accretion with deposit tendency decreased,erosion tendency increased(2009-2011)-Scoured sharp increase in erosion(2011-2013)-Strong accretion with greatly increased accretion(2013-2015),The overall situation is erosion 2.the driving mechanism of topography evolution:From the view of time,The underwater topography in this area changed from accretion to erosion around 2007.Taking this time as the boundary,this area has drastically reduced the amount of accretion and the rate of erosion has increased dramatically,which is consistent with the dramatic decline of the sand due to THREE GORGES DAM storage in about 2003.This indicates that the evolution of Yangtze River estuary estuary is still controlled by the sediment yield.The annual runoff of the Datong Station mainly affects plain shoal with a shallow depth of 6.4m in the Yangtze River Delta,and has a weak influence on the river channel.There has been almost no correlation between the changes in shallow areas and deep trough areas in the Yangtze River Delta over the past two decades and changes in Shanghai’s sea level.Therefore,the Yangtze River sediment flux into the sea are the main factors affecting the study area control erosion and deposition of underwater topography,runoff was a secondary factor affecting shoal terrain.3,The topography change tendency of the Yangtze River delta in the recent years:Considering that the amount of sediment discharge coming from the Yangtze River basin continues to fall.It is very likely that the front of the Yangtze River Delta will be eroded in the future.This deserves the attention of relevant departments and further attention.Due to the construction of the reclamation engineering of the Yangtze River estuary,Chongming Dongtan、Hengsha Dongtan、Jiuduansha and Nanhui Dongtan still a state of net accretion at present.However,the accretion rate has slowed down.It is suggested to strengthen the environmental monitoring and early warning measures of the coastal zone of the Yangtze river estuary to reduce or even avoid the possibility of erosion of the beach in the future and to achieve sustainable development of the coastal resources.