Transport Mechanisms Of Diluted Water And Sediment During The Water-Sediment Regulation Scheme In Yellow River Delta

A 3-D hydrodynamic numerical model based on Delft3D and orthogonal curvilinear grids is established for Yellow River delta and downstream river from Lijin,in order to describe the dispersal of diluted water and sediment as precise as possible.The model has been calibrated by field investigation during water-sediment regulation scheme in 2011 as well as a storm surge in 2013.Results shows that the model is suitable to reflect not only the hydrodynamic conditions,but also salinity and sediment distribution.The influence of regulation scheme on tide-runoff interaction and the position,strength,duration of shear fronts is analyzed by several cases.And then the transport mechanisms of diluted water and sediment controlled by shear fronts is expounded.In the end,potential effects of two kind of extreme conditions,density-driven flows and extratropical storm surges are discussed.The conclusions of this dissertation are as follows.(1)Topography-induced and runoff-induced shear fronts are discovered in Yellow River Delta.The first one,which is caused by steep bed slope,is located at the right side of inlet in the whole water column,while the other only exists in the surface layer of offshore areas.River discharge affects the horizontal length scale of runoff-induced shear fronts and contributes to the strength and duration of topography-induced shear fronts.The main track of diluted water and sediment on the right side of inlet is maintained by shear fronts,which are conducive to dampen perpendicular transport.On the other side of inlet,the isolines of tidal-averaged diluted water and sediment are annular due to the disappearance of topography-induced shear fronts.(2)Two low-salt areas are distinguished by tidal-averaged salinity.The one located at the northeast of inlet is consistent with plume flow and clockwise residual circulation in surface layer.Diluted water in bottom layer limited by topography-induced shear front disperses to south and southeast along the coast in nearshore area,forming a bottom low-salt area.Advective flux related to Eulerian residual transport velocity makes the greatest contribution of fresh water transport,while tidal pumping flux reflecting the phase lag between salinity and tidal current is only noticeable in shallow regions along the coast.The phase lag is connected with tide-runoff interaction and topography-induced shear fronts.(3)There is an obvious positive correlation between the horizontal scale of sediment transport and the magnitude of river discharge,although the former decreases inevitably due to flocculation and settling velocity comparing with diluted water.Both lateral and longitudinal sediment transport during high discharge are considerable,within 14 m isoline.The length scale is 20 km and 8 km,respectively.When it comes to low discharge,suspended sediment is trapped by topography-induced shear fronts and mainly deposits in southeastern coastal areas shallower than 10 m,even though the sediment input is far larger than high discharge.The reverse direction of tidal pumping flux indicates representative settling processes in three areas,especially the one inside topography-induced shear fronts.(4)The critical suspended sediment concentration between plume flow and density-driven flow is 27±4 kg/m~3,which is corresponding with observations.Reverse salt wedge,which means diluted water dives to the bottom while salt water is restricted at the surface because of density-driven flow,provides bottom water with lower salt and higher suspended sediment concentration.The front of density-driven flow is located out of bottom topography-induced shear fronts,therefore the restriction of shear fronts can be less important.Larger bed slope on the left side of inlet contributes to higher bottom velocity of density-driven flows therefore it is possible to move further toward northwest.Salinity and sediment distribution become more similar on the two sides of inlet due to the competition between primary density-driven flow and shear front on the left and right side of inlet,respectively.Combined with higher vertical gradient of residual density and considerable vertical mixing at the final phase of density-driven flow caused by unstability of reverse salt wedge,the surface low-salt center in Gudong area can be maintained(5)Yellow River Delta suffers great damage caused by extratropical storm surge and is one of the most vulnerable areas in norther China.River discharge points to the northern Bohai Sea,which is a completely opposite direction compared to the dominated wind direction and wind-driven current during extratropical storm surge.Storm surges and surface wind-driven currents restrict the range of plume flow caused by runoff,as well as the dispersal of diluted water and sediment.Because of the concave coastline on the left side of inlet,both Eulerian residual transport and net diluted water and sediment flux are enhanced.Diluted water and suspended sediment disperse to Gudong area,especially in shallow regions along the coast.As a result,deposition rate will increase considerately during storm surges.