Numerical Simulation Of Residual Current In Estuary

The Yangtze River is the biggest river in China,and has the function of "golden channel".Now,the large siltation quantity with strong siltation at the specific channel position is gradually becoming one of serious problems.Therefore,the research of the hydrodynamic,salinity distribution and sediment transport plays an important role in the understanding the siltation processes of the Yangtze estuary.Residual currentsare residual parts after the current filter out the cyclical parts,the salinity and sediment particles carried in water will move along with the residual current,so the residual current plays a dominate role in the long-term sediment transport.Based on FVCOM three-dimensional hydrodynamic,salinity and sediment numerical model,five ideal numerical experiments and the Yangtze estuary experiment are set up,the residual currents of the different terrain conditions,and the relation of the residual current and sediment transport are studied.The details of this article are as follows:(1)Five cases are set up to analyze the residual flow field,salinity field and sediment deposition characteristics,and the following conclusions are obtained.The residual flowdirection are basicallythe same,and the surface residual current velocity is larger than the bottom layer,if there is no barrier of topography or structure.The dam parallel to the residual current direction will haveno obvious influence on the residual current distributions,and the dam which is perpendicular to the flow direction will make a significant change.The horn-shapeddamhasa significant impact onthe residual current.Under the influence of the reverse horn-shaped dam,the trend of residual current velocity in the channel also presents the opposite trend.The change of terrain near the North Channel has little influence on the residual flow distribution of the south channel.(2)The residual current in the north passage of Yangtze estuary is more complicated,the residual flow distributions near the north passage is similar to the residual flow structure simulated in the ideal cases.The construction of spur dikes leads to the decrease of cross sectional area and the residual flow velocity increases when the tidal current flows through spur dikes,and obvious vortices are formed in the dam field.(3)From the distribution of salinity field,the range of freshwater in the south branch is wider than the north branch,and the range of freshwater reaches the maximum between the spur dikes located in the north passage of the South channel.Compared with the distributions of the residual current and salinity fields,when the residual current velocity is large,the range of freshwater is wide.And with the smallerresidual current,the saltwater intrusion rangesbecome wider.The residual current distribution is related to the salinity distribution..(4)Comparison with the residual current and sediment siltation,it can be found that the sediment siltation thickness will increase significantly when the vertical residual flow velocity increases and the horizontal velocity decreases.The scour thickness increases significantly,when the horizontal residual current velocity increases and the vertical velocity reduces significantly.Therefore,it can be concluded that there is a certain correlation between the variation of the residual current and the sediment siltation.