Research Of Factors Leading To Variation Of Ebb-tide Sediment Diversion Ratios In The South And North Passage In The Changjiang Estuary

Changjiang Estuary is the largest estuary in China.As a typical bifurcated estuary,it has three bifurcations and four outlets.The diversion ratio of discharge(DRD)and sediment(DRS),which influences the development of bifurcated waterways,is an important parameter.The variation of DRS and DRD can be used in the assessment of the development pattern of waterways.The Deepwater Navigation Channel Project started since 1998.The project significantly changes the hydrodynamic and sediment-transportation situation in the North Passage.DRS of the North Passage decreases from 53%in 1998 to about 35%in 2015,DRS of the South Passage and the North passage has reversed.Research on reasons of this variation is of great scientific significance.This paper analyses and summarizes reasons of variation of DRS,mainly through three perspectives.Through the research several conclusions are obtained:1.Field observation in 2015 demonstrated that averaged ebb DRS of the North Passage in spring and neap tide was 35.1%and 39%respectively,the difference was3.9%.The result indicated that the ebb DRS of the North Passage in spring and neap tide was dependent on current velocity,suspended sediment concentration and section area of the North and South Passage.Current velocity and section area ratio of the North Passage to the South Passage was almost the same within neap-spring tidal cycles,as well as DRD.Section-averaged suspended sediment concentration(SSC)ratio of the North passage to the South Passage induced the decrease of 3.9%from the spring tide to the neap tide.2.From 1998 to 2012,trend of yearly variation of the ebb DRS of the North Passage was near to that of DRD,which generally kept decreasing.According to the order of magnitude,this period can be divided into three parts,the first part was from 1998 to 2000,DRS went down from 53%to 44%,the second part was from 2001 to 2006,it fluctuated between 45%and 50%,the third part was from 2006 to 2012,it gradually decreased from 48%to 35%.A positive correlation existed between DRD and DRS of the North Passage.Influenced by the Deepwater Navigation Channel Project and projects of dikes and groins,DRD decreased,which leaded to the variation of SSC of the entry-section of the North and South Passage,further induced the decrease of DRS of the North Passage.3.Results of numerical model denoted that the scour of Ruifeng Sandbar from 2000 to 2015 made the ebb DRS in the up-section of the North Passage increased by 1.4%,in the down-section it increased by 0.9%.Thus,the influence of geomorphic change of the down-area of the South Channel to DRS was little.The reason of yearly decrease of the ebb DRS is the construction of Deepwater Navigation Channel Project.