Numerical Simulation On Hydrodynamic Environment Effects Of The Reclamation Project Of Nanhui Tidal Flat In Yangtze Estuary

Yangtze Estuary is the end segment of Yangtze River which locates at the entrance of the East Sea.Yangtze Estuary starts from Jiangyin of Jiangsu Province,ends at Jigujiao which locates at the outside of the mouth,and is totally about 232 km long.Shanghai is the economic center of China with the rapid development and the huge population,and the shortages of land resources have been one of the most significant constraints on the economic sustainable development of Shanghai.The reclamation project of Nanhui tidal flat is the largest reclamation project in Yangtze Estuary in recent years,and it will effectively alleviate the current situation of the shortage of land resources in Shanghai,which will significantly benefit the society and economy after the project's completion for farming.However,a series of reclamation projects have profound impacts on the hydrodynamic environment variables such as tide,sediment transport,salinity distribution and water quality et al.Therefore the study of the offshore hydrodynamic environment effects of the reclamation projects is of great value.Based on MIKE3,this paper adopts the unstructured triangle mesh to establish the three-dimensional baroclinic hydrodynamic model of Yangtze Estuary and Hangzhou Bay which is well verified with the field data,and establish the three-dimensional baroclinic hydrodynamic model coupled with water quality of Yangtze Estuary to study the hydrodynamic environment effects of the reclamation project of Nanhui tidal flat.The main contents of this paper are as follows:(1)The research progresses of the reclamation projects and the mathematical models of hydrodynamics,salinity and water quality are stated,as well as the social conditions and natural conditions such as topography,hydrology et al.in Yangtze Estuary.(2)The relevant basic equations and the numerical computation methods in MIKE3 are introduced in detail.(3)A three-dimensional baroclinic hydrodynamic model of Yangtze Estuary and Hangzhou Bay is established with the adoption of the unstructured triangle mesh.The simulated tide level,current,temperature and salinity of the model match well with the field data.The characteristics of the simulated temperature and salinity distributions in Yangtze Estuary and Hangzhou Bay in summer and winter are similar with those in the marine atlas.The temperature is low near the coast and in estuary,high in the outer sea in winter,while high in estuary and the outer sea,low around Zhoushan Islands in summer.The salinity is low in estuary and at the top of Hangzhou Bay,high in the outer sea in both winter and summer,and the salinity in winter is obviously higher than that in summer.(4)A three-dimensional baroclinic hydrodynamic model coupled with water quality of Yangtze Estuary is established with the good verification of the three-dimensional baroclinic hydrodynamic model of Yangtze Estuary and Hangzhou Bay.The simulated distributions of DIN,PO43-and Chlorophyll-a match well with the measured distributions.At last,based on the coupled model,the study of the hydrodynamic environment effect of the reclamation project of Nanhui tidal flat is conducted in terms of velocity,tide level,salinity and water quality.The results indicate that,during spring tide in dry season after reclamation,the averaged peak velocity during ebb tide will be reduced by 13% in the North Channel,while increased by 21% in the South Channel.The mean low tide level will be increased by 19%,while the mean high tide level will be reduced by 5% in the South Channel,and there will be a general decline of the tidal range in all outlets of the South Branch.The saltwater intrusion in the North Channel will be aggravated,and the contour of 2PSU will reach the middle of Changxing Island.The salinity will be reduced in the South Passage and around the reclamation project area.The saltwater intrusion of the North Branch will be aggravated.The concentration of DIN and PO43-will be reduced in the North Channel and the South Channel,increased around the reclamation project area.The value of N/P will be averagely increased by 4% in all outlets of the South Branch except that in the North Channel will be reduced,resulting in a slight aggravation of the phosphorus restriction effect in the South Branch.The concentration of Chlorophyll-a will spatially change in the opposite way of N/P.