Gravitational Circulation, Tidal Straining, Mixing And Stratification In The Changjiang River Estuary

To gain further insight into the physics of circulation,mixing and stratification in the Changjiang River estuary,based on the idea of tidal straining and one-dimensional/and three-dimensional potential anomaly equations,this thesis has used both field observational data analysis and mathematical modeling.Firstly,simultaneous field observations were made of time series of water level,current velocity,salinity and suspended sediment concentration at ten hydrological gauging stations along the North Passage in the Changjiang River estuary on 17 to 24 February(dry season)and 12 to 18 August 2012(wet season),respectively.Quantitative analyses of those data are attempted to understand circulation and mixing along the Deepwater Navigational Channel in this estuary.Landward subtidal flow only appears during the neap tide.The maximum bottom landward velocity is in the order of 0.05?0.1 m·s-1 in the dry season and 0.15?0.2 m·s-1 in the wet season,respectively.In both dry and wet seasons,calculated mixing parameter(M),which is the ratio of the tidal timescale to the vertical mixing timescale,is below the critical value(1.0)on a neap tide but above it on a spring tide within the middle and lower reaches of the Channel.This suggests that tidal variation of mixing may be able to generate the tidal straining circulation during the periodic stratified spring tide.The interaction between significant river shear flow and longitudinal density gradient,a process termed "river effect" in this study,is revealed as the maj or reason for seasonal variation of stratification.The competition between tidal stirring,gravitational circulation and river effect could produce an increase in tidal mean value of the potential energy anomaly(?)from spring to neap tides.Tidal mean value of the Simpson number(Si)over a neap tide surpasses the critical value(8.4×10')within the middle reach of the Channel,suggesting the occurrence of persistent stratification there.Tidal mean Si over a spring tide is in the order of 8.4×10-1?18.8×10-2 within the middle reach,suggesting the occurrence of strain induced periodic stratification(SIPS).Shear prevails within the pycnocline in the middle and lower reaches of the Channel on a neap tide,with the value of squared shear(S2)exceeding 10-3 s-2.Calculated gradient Richardson number(Ri)is small at the salt-fresh water interface,indicating the occurrence of Kelvin-Helmholtz instability there.Secondly,the possible effect of stratification on the vertical structure of the semidiurnal tidal ellipse in the Changjiang River estuary is analyzed based on observation data.A pycnocline occurs at each station along the North Passage on a neap tide only.Calculated potential energy anomalies within the North Passage are 100?200 Jm-3 larger on a neap tide than on a spring tide.The tidal ellipses degenerate to rectilinear motion at each layer of the water column at each station along the North Passage,while they rotate clockwise at each station over the Hengsha Shoal.The surface-to-bottom inclination angle difference of the tidal ellipse at a seaward station is 400 on a neap tide but a few degrees on a spring tide.The vertical phase shifts of the tidal ellipses range from 200 to 500 at most stations along the North Passage on a neap tide,while they are less than 100 on a spring tide.Rapid changes in the inclination angles and phase shifts of the tidal ellipses occur at the pycnocline.An abrupt reduction in the vertical eddy viscosity at the pycnocline seems to be the major cause for the vertical variability of ellipticity,inclination angle and phase of the tidal ellipses on a strongly stratified neap tide.The surface-to-bottom ellipticity difference appears to have a positive linear relation with the overall Richardson number(Rio)mainly along the North Passage on a neap tide.A one-dimensional coastal model with vertical turbulence closure scheme seems to be able to reproduce the major features of the vertical structure of the tidal ellipses at a seaward station on a spring tide.Finally,the three-dimensional finite element mathematical model,TELEMAC-3D,incorporating a stability function,and the potential energy anomaly equation(?-equation),are used to analyze neap-spring tidal and intratidal variability of intermittent mixing and stratification within the North Passage of the Changjiang River estuary in the wet season.Eight terms in the ?-equation are used to examine physical mechanisms and the relative importance of each term for the lower reach of the North Passage.As revealed by the gradient Richardson number(Ri),the Simpson number(Si)and the potential energy anomaly(?),weak mixing and persistent stratification appear on a neap tide,while strong mixing and periodic stratification on a spring tide within the main channel in the middle and lower reaches of the North Passage.The landward subtidal flow is much stronger on a neap tide than that on a spring tide.Within the main channel in the lower reach,large magnitude of longitudinal ?-advection(Au)reflects the important effect of saltwedge movement on stratification Large magnitude of lateral ?-advection(Av)may be enhanced by large lateral gradient of ? due to the complex bathymetry and artificial structures.Both longitudinal(Au)and lateral ?-advections(Av)are temporally and spatially intermittent.Large longitudinal depth-mean straining(Bu)overlays the combined effect of tidal straining,circulation and river discharge.Large lateral depth-mean straining(Bv)is generated by large lateral density gradient interacting with the shear flow.The magnitude of integrated vertical turbulent buoyancy flux mainly depends on tidal stirring at the bottom,while wind stirring at the surface and shear instability at the pycnocline are secondary contributors.The magnitudes of the other physical mechanisms including longitudinal non-mean straining(Cu),lateral non-mean straining(Cv)and vertical advection are relatively smaller than those above.Neap-spring tidal variability of mixing and stratification mainly results from combined effect of three principal physical mechanisms,i.e.tidal stirring,longitudinal(Bu)and lateral depth-mean strainings(Bv).Intratidal variability of mixing and stratification is apparent on a spring tide.It seems that Advection,Straining and Stirring Induced Periodic Stratification(ASSIPS),rather than Advection and Straining Induced Periodic Stratification(ASIPS)and Straining Induced Periodic Stratification(SIPS),controls intratidal variability of mixing and stratification within the North Passage.