Estimation Model And Evolution Analysis Of Jiuduansha Area In The Yangtze River Estuary

The estuarine and sandbar play important roles in wetland resource.The estimation and evolution of the sandbar area has always been a concern for people.However,considering the complexity and variability in the estuary area,it is hard to obtain the chart base of Om-line.Meanwhile,measuring on the spot which is time-consuming,costly and difficult,can't be implemented too.Therefore,this status,difficult to calculating area of sandbar,make us can't carry on researching on sandbars.However,using remote-sensing(RS)data extract the waterside lines of sandbars with varying tide levels and calculate the corresponding area by relevant GIS technology.Using equilibrium profile model combined with GIS spatial analysis technology can determine the Om line of sandbar and calculate sandbar area then analyze the evolution.This study is practical and has significant theoretical meaning.Jiuduansha is the largest sand-bar wetland in the Yangtze River Estuary,important reserve land resource in Shanghai and a national nature reserve.Dramatic changes have taken place in this region under the combined effects of natural factors and human activities in recent years.We quantitatively analyze the evolution characteristics before and after large-scale water conservancy project through area estimation model and the method of determining the Om lines,then dicuss the evolution trend and their impact of Jiuduansha.We hope provide relevant scientific basis for the planning and management of Jiuduansha Nature Reserve.The main conclusion are as follows:(1)Based on equilibrium profile model,we studied the relationship between the tide level at Zhongmu station and the area of Jiuduansha,the distance between the water depth and the waterside line collecting from satellite images.We estimate the maximum sandbar area and obtain the Om line shape of sandbar through trend analysis.The effectiveness of the method has proved with existing data.Besides,applying the method to the RS data of sandbar in different periods can not only quantitatively analyze the evolution trend of the sandbar,but also further analyze the features of erosion and deposition on the sandbar profile.(2)Quantitative analysis of data from varying historical periods(self-maintaining,engineering disturbance and stabilization)can be drawn:referring to the area of sanbar,Jiuduansha was continuously rising in general from 1995 to 2013.Ranging from self-maintaining period to engineering disturbance period,the area of whole Jiuduansha increased by 4.31km2·a-1 while it increased by 5 km2·a-1 from engineering disturbance period to stabilization period.However,siltation differed from one another in each part of sandbar,Jiangya Nansha has rapidly expanded during the disturbance of the project,and it has been slowly rising and even eroding during the stabilization.Shangsha presented slowly rising and steady,but the area of zhongxiasha continued to rise,the increasing rate in stabilization period exceeded the engineering disturbance period.Due to the large proportion of Jiuduansha in the zhongxiasha,the overall zhongxiasha rise faster than other regions.From the morphological analysis of sandbars,Jiuduansha as a whole developed longitudinally along the runoff direction in the period of self-maintaining,then,under the interference of the project,the sand body moved northwestwards and began to develop horizontally along the south dike of the Deepwater Channel.Jiang Ya Nansha went up in the southwest of the northwest during the period of engineering disturbance,and then rose to the northeast.During the period of engineering disturbance,Shangsha rose to the northwest and then rose northeastwards.At the beginning of the zhongxiasha deposited to the southeast direction and then to the northeast.Judging from the ebb and flow evolution trend of sandbars,sandbars will continue to increase overall,Shangsha and Zhongxiasha will gradually connect and become a common sandbar.With the formation of tidal flat deposition from the southeast to the northeast over the entire eastern part of the zhongxiasha,the increasing rate in the zhongxiasha sand will slow,and the overall increasing rate of the Jiuduansha will slow down too.The estuarine sandbar area estimation model established in this paper and the proposed method for determining the 0m line are simple and effective,it can be applied to other estuaries and coastal sandbars.