Recent Evolution And Simulation Of Mouth Sandbars In The Yangtze River Estuary Under The Influence Of Human Activities

The Yangtze River estuary is one of the most important estuaries in China,which is very important to the economic development of the Yangtze River Delta.The mouth sandbars of the Yangtze River Estuary has a significant impact on the flood discharge and navigation of the Yangtze River Estuary,and has a certain restriction on the comprehensive economic development of the Yangtze River Delta.Under the influence of multiple factors such as human engineering and sediment reduction into the sea,great changes have taken place in the plane shape of the Yangtze River Estuary in recent years.It is necessary to analyze the evolution of erosion and deposition in the Yangtze River Estuary mouth sandbars and grasp its evolution trend,so as to provide scientific basis for the comprehensive management of the Yangtze River Estuary.Firstly,based on the measured water depth data and chart data of the Yangtze River Estuary,this paper analyzes the changes of scouring and silting distribution,vertical and horizontal sections and characteristic Isobaths of the mouth sandbars from 1991 to 2016,and statistically analyzes the characteristic values of the mouth sandbars of the Yangtze River Estuary.At the same time,it also discusses the influencing factors of the scouring and silting evolution of the mouth sandbars of the Yangtze River Estuary.The research results show that the recent scouring and silting evolution of the mouth sandbars of the Yangtze River Estuary Influenced by human activities,the mouth sandbars in the North Channel,North Passage and South Passage are scoured.Secondly,based on the topographic data and hydrological observation data of the Yangtze River Estuary in 2016,a numerical model is established to simulate the impact of the sharp drop of sediment into the sea on the suspended sediment in the Yangtze River Estuary.At the same time,the model is used to predict the distribution of erosion and deposition in the North Passage and South Passage and the evolution trend of the mouth sandbar in the South Passage.The main conclusions are as follows:(1)The mouth sandbar in the Nouth Branch is less affected by the sharp decrease of upstream sediment inflow,and there is no trend of scouring to the sea.It is mainly affected by the rising tide,which has moved into the mouth under the action of the rising tide hydrodynamic force,forming a sandbar in the mouth.During the study period,the average water depth at the front of the mouth sandbar in the Nouth Branch is about 1.42m,and that at the back is about 2.86m.On the whole,the depth and position of the beach top of the mouth sandbar in the Nouth Branch are relatively stable,but its longitudinal profile is eroded to the mouth by the tidal current,and the length of 3 m and 6m mouth sandbars are shortened to some extent.From the amount of scour and deposition,the mouth sandbar section of the North Branch shows the trend of first flushing and then silting,and the erosion and silting are alternately.(2)During the study period,the beach top water depth of the mouth sandbar in the Nouth Channel increased slightly,about 5.42m.The shallow section of 8m mouth sandbar is shortened by about 9 km,and the volume of mouth sandbar is reduced by about 4.83 million m3.In addition,the amount of erosion in the lower part of Nouth Channel is about 6 times of the amount of deposition.(3)The North Passage is most affected by the construction of the Yangtze Estuary deep-water channel project.Recently,the form of mouth sandbar in the North Passage has gradually disappeared,and the depth in the North Passage is about 12m.However,the water depth in the lower part of the North Passage is slightly shallower than that in the middle and upper parts of the North Passage,and there is still a tendency of siltation in the middle part of the North Passage.(4)During the study period,the water depth at the top of the mouth sandbar in the South Passage changed little,and the position of the top of the beach moved down nearly 3km.The shallow section of 7m mouth sandbar shows a trend of shortening at both ends.The beach length is shortened by about 8.4km,and the volume of mouth sandbar in 2016 is about 60%of that in 1991.From the recent amount of scouring and silting in the South Passage,the amount of scouring is about 21 times of the amount of silting.(5)Suspended sediment content and suspended sediment flux in the Yangtze River Estuary decrease with the decrease of sediment inflow,and the decrease range decreases from inside to outside of the estuary,and the decrease range in the flood season is significantly greater than that in the dry season.The simulation results using 2016 as the initial topography show that with the sharp decrease in the amount of upstream sand,the mouth sandbar in the South Passage will be scoured as a whole ten years later,and the water depth at the top of the beach will deepen to about 6m.In addition,the mouth sandbar at 7m The length of the shallow section will be shortened to about 18km,while the basic shape of the mouth sandbar in the South Passage has not undergone major changes,and it still assumes a "single peak" shape.The study of the temporal and spatial variation of the mouth sandbars in the Yangtze River Estuary and the evolution law of erosion and deposition,and the prediction of the future evolution trend of the mouth sandbars in the Yangtze River Estuary under the condition of the sharp decrease of the sediment inflow in the Yangtze River Estuary can provide reliable theoretical data support for the flood control planning,channel regulation,beach development and utilization,and the study of river channel evolution in the Yangtze River Estuary.