Geomorphological Evolution Of Tidal Flat Around Chongming Island

Located at the intersection of land and ocean,estuary experiences the most sensitive and active land-ocean interactions.The estuarine area is characterized by developed water transport and abundant natural resource.It nurtured vast tidal flat that not only provides valuable habitats for humans and other living creatures,but also plays an important buffer role in resisting extreme weather,such as typhoon and storm surge.Therefore,estuary tends to the most developed area in culture and economy.However,the estuarine geomorphological pattern is suffering new changes and the tidal flat is facing erosion pressure due to sudden fluvial sediment decrease induced by frequent and intensive human activities.Explore the geomorphological evolution mechanism of estuarine tidal flat and put forward its change tendency,therefore,is the key to effectively maintain the long-term balanced development of estuary area.Located at the mouth of the Changjiang(Yangtze)River,Chongming Island is the third largest island in China and the largest estuarine alluvial island in the world.Being rich in tidal flat,Chongming Island is valuable reserve land resources in Shanghai City.From the first order bifurcation of the Changjiang River,the tidal flat of the Chongming Island can be divided into three sections,namely,the Northern Region(NR),Southern Shore(SS,extends from Chongtou to Bayaogang),and Eastern Shoal(ES).The tidal flat evolution of each section is extremely complicated,with the shoreline changing frequently,due to the effects of diverse dynamic environments and frequent human interferences.Therefore,investigate the evolution process of tidal flat and the change feature of shoreline of Chongming Island,and then predict the shoreline evolution trends are of vital guiding significance to the surrounding wetland ecosystem exploitation and utilization,channel regulation and embankment protection.Based on historical bathymetric charts and water depth measurements during1984-2016,this study clarified the evolution processes of Chongming Island tidal flat and the change characteristics of their isobaths,detected the influence factors that dominated the geomorphological changes of the tidal flat through ArcGIS technique and statistical analysis methods.Furtherly,the change trends of shoreline in 2030 were predicted by constructing the BP neural network model.The main conclusions are as follows:(1)The isobaths changes of each section of Chongming Island are frequently.During 1984-2016,the NR exhibited overall deposition,albeit with some local slight erosion,and thus the isobaths extended seaward constantly.Evolution of isobaths along the SS showed spatial variations.0 m isobath of the SS experienced successive phases of retrogradation and extension both on Chongtou-Miaogang Reach and BaozhenBayaogang Reach and maintained slight extending state on Miaogang-Baozhen Reach;The-2 m and-5 m isobaths of the SS both changed alternately between extension and retrogradation,with an overall extension after 2009 and 2013,respectively.0 m isobath of ES was characterized by the alternate change of ‘extension-retrogradation-extension',while its-2 m isobath showed an overall extension,being accompanied by local slight retrogradation,with the eastern tip experiencing especially apparent retreat in 2013.The-5 m isobath of ES extended to the southeast overall,displaying a variation feature of ‘sometimes retrogradation and sometimes extension,and shifts between retrogradation and extension'.(2)During 1984-2016,the tidal flat area of the three sections changed diversely.Specifically,the area of tidal flat in NR as a whole increased continually;the tidal flat area above-5 m isobath of ES exhibited sustained growth,with the high shoal showing the most significant growth,the middle shoal showing slight growth and the low shoal showing slight reduction;there is relatively less tidal flat in SS,which,however,is growing gradually,with the areas of high,middle and low shoals tripping over each other.(3)The evolution of Xinqiao Channel has potential effects on the tidal flat variation of SS.Specifically,Biandan Shoal and Xinqiao Channel trip over each other.The continuous growth and northward expandation of Biandan Shoal caused gradual shrinkage and shallow of Xinqiao Channel;the unbalance dynamic conditions of ‘stronger south channel and weaker north channel' around Baimao Shoal and the construction of Dongfengxisha Reservoir accelerated the deposition of Xinqiao Channel and its northward moving,which directly caused tidal flat extension of SS.Tidal flat evolution in NR was dominated by land reclamation,which caused the shrinkage of the North Branch,and shoreline extending and tidal flat development of Chongming Island.ES experienced frequent land reclamation,which induced seaward shifting of dike,and as a consequence,the low shoal developed to middle-high shoal and the middle shoal developed to high shoal,thus the tidal flat increased both vertically and horizontally.(4)Four data sets,including annual mean water discharge and annual sediment load at Datong station,the distances from the isobaths of-2 m and-5 m to the shoal dike were set as input parameters in the BP neural network model while the distance between the 0 m isobath and the shoal dike was output,thus the location of 0 m isobath in 2030,2050 and 2100 can be forecasted.The results showed that compared to 2016,the 0 m isobath of NR extends seaward slightly;the 0 m isobath of SS is basically in retrograde condition,which cause slight decrease in the area of high shoal;the 0 m isobath of the south and north sides of ES both show slight retrogradation,while its eastern area indicates strong seaward extension,and as a result,the high shoal increases dramatically in area.In addition,sea level rise lead to the deposition shoreline extend slowly or even retrograde(North Laoyaogang Reach and Earthen ES),and the erosion shoreline retrograde more violently(Gelonggang and Siyaogang-Bayaogang Reach).