Numerical Simulation Of Effects On Hydrodynamic And Water Quality Of The North Branch Middling Narrowing In Yangtze Estuary

With the river channel scouring and silting in the North Branch,the North Branch is shrinking in the Yangtze River estuary,and the phenomenon of sediment and salt water intrusion into the South Branch appears from the North Branch since the 1950 s.Especially since the 1980 s,the intersection point of the tidal current and runoff water moves down in the North Branch and the South Branch,leading to this problem is more prominent,which has seriously affected the urban development and the production and life of residents.To avoid large amounts of sediment and saltwater intrusion into the South Branch from the North branch and threats to water security in Shanghai,Shanghai city is ready to implement the reclamat io n project named the North Branch middling narrowing to solve the "saltwater intrusion" problem.The North Branch middling narrowing uses the reclamation program,which reclaims land from water,so that the northeastern part of Chongming Island expands outward to reduce the diameter of the North branch mouth.In this paper,firstly,the research progress of numerical model at home and abroad is introduced in detail,including hydrodynamic,salinity,water quality and other aspects.This paper introduces the development status of the domestic and international reclamation project and the research status of the Yangtze River estuary.This paper introduces the location of the Yangtze River Estuary and its adjacent areas,as well as the hydrological conditio ns,meteorological conditions and social environment.The basic governing equations of MIKE mathematical model and numerical method are described systematically.The main purpose of this paper is that based on MIKE three-dimensional numerical model considering water baroclinic which couples the hydrodynamic model(HD)with the temperature and salinity module(TS)and the water quality model(Ecolab)together,a lar ge scale calculation model is established to simulate the Yangtze River Estuary.Tidal elevatio n,tidal current,temperature and salinity are tested and analyzed.Based on the large scale model,a small scale model is constructed and repeatedly verified.The simulation results is verified with measured distributions of inorganic nitrogen,phosphate and chlorophyll-a.The distribution characteristics of nutrients are analyzed.Based on the model validated,considering the impact of the reclamation project,the influence of the North Branch middling narrowing in the Yangtze River Estuary in terms of hydrodynamic and salinity and water quality on the surrounding waters.The North Branch of the flood slack tide decreases significantly and the ebb slack tide is complex affected after the project is completed.After fil ing,the velocity of the North Branch is reduced to some extent.The flood velocity in the South-North-Branch bifurcation has a tendency to increase,which is affected by the tides in the South Branch.The North Branch middling narrowing makes the salinity of the North Branch decrease obviously,and plays a controlling role on saltwater intrusion from the North Branch into the South Branch.After the project is completed,in winter,the content of phosphate,inorganic nitrogen and chlorophyll-a increases obviously in the lower segment of the North Branch;the average concentration of phosphate and inorganic nitrogen decreases slightly,while the content of chlorophyll-a increases in the middle and upper segment of the North Branch.After the implementation of the project,in summer,the average concentration of phosphate,inorganic nitrogen and chlorophyll-a decreases slightly in the middle and upper segment of the North Branch;the average concentration of phosphate and inorganic nitrogen increases obviously in the lower segment of the North Branch,while the content of chlorophyll-a increases because of the high nitrogen content.The North Branch middling narrowing has little effect on the hydrodynamic characteristics and nutrient content in the South Branch and offshore area.Finally,the numerical simulation is summarized,and the problems are analyzed and constructive suggestions for further research are proposed.