Circulation And Turbulent Mixing Within The Changjiang River Estuary: Mathematical Modeling And Theoretical Calculation

To gain an insight into the physics of estuarine waters within the North Passage of the Changjiang River estuary, a finite element mathematical model, TELEMAC-3D, is used together with theoretical calculations of the physical parameters to investigate tidal circulation and turbulent mixing there in the dry/flood seasons of 2010, respectively.3D distributions of modeled currents within the North Passage show that:(i) estuarine circulation is apparently present, with current strength in the range of 0.2~0.6 m·s-1 on the neap tide, while it is rather weak on the spring tide in the dry season of 2010;(ii) estuarine circulation dies out in the absence of the baroclinic effect caused by salinity stratification in the flood season of 2010.(iii) lateral circulation(secondary circulation), after taking the baroclinic effect into account, is clearly present at the CSW cross section in the dry season of 2010. Nevertheless, when this baroclinic pressure is omitted in the flood season of 2010, there is hardly any lateral circulation within a tidal cycle.(iv) local horizontal circulation occurs within dike fields of the North Passage in the dry/flood seasons of 2010.To evaluate the contribution of tidal straining circulation to estuarine circulation, a mixing parameter M is estimated at three hydrological gauging stations(the upper reach CS1, the middle reach CSW, and the lower reach CS8) within the North Passage in the dry season of 2010. M at each station is more than 1.0 on the spring tide, and is less than 1.0 on the neap tide. This suggests that tidal straining circulation significantly contributes to estuarine circulation on the spring tide, while gravitational circulation mainly contributes to estuarine circulation on the neap tide.To determine the strength of stratification within the North Passage in the dry season of 2010, spring and neap tidal mean potential energy anomalies(f) are calculated. Results show that tidal mean potential energy anomalies approximately range from 0~30 J·m-3 over a spring tide to 0~90 J·m-3 over a neap tide. Large tidal mean potential energy anomalies mainly occur in the main curved Deepwater Navigational Channel rather than within dike fields. It appears that the stratification of water column there appears to be stronger on neap tide than that on spring tide, and is also stronger in the main channel than within dike fields. Waters in the middle reach are always much more stratified than those within dike fields.To understand the temporal and spatial variability of the physical mechanisms in the dry season of 2010, the time derivative of the potential energy anomalies caused by tidal straining and combined tidal and wind stirring are calculated within the lower reach of the curved Deepwater Navigational Channel of the North Passage at maximum ebb tide. Results show that the time derivative of the potential energy anomalies( f?? /t) caused by tidal straining and combined tidal and wind stirring range from 40 ~ 100 10-′ W·m-3 to 420 ~ 100 10-- ′ W·m-3. Tidal straining increases as combined tidal and wind stirring decreases from spring to neap tides. Tidal straining in the main channel near groynes is much stronger than that within dike fields. Combined tidal and wind stirring is also quite different inside and outside dike fields.The characteristics of the mixing and stratification and their corresponding physical mechanisms are examined at three hydrological gauging stations(CS1, CSW and CS8) within the North Passage in the dry season of 2010. Time series of the potential energy anomaly(f), the time derivative of the potential energy anomaly( f?? /t) caused by tidal straining and combined tidal and wind stirring, the total time derivative of the potential energy anomalies( f?? /t), and the Simpson number(Si) are calculated at each station. Results show that the Si number at CS1(the upper reach) is in the range of 0.3~0.8 from the spring to moderate tides and 0.6~0.9 on the neap tide, suggesting the occurrence of Strain-Induced Periodic Stratification(SIPS). The Si number at CSW(the middle reach) is in the range of 0.2~0.8 from the spring to moderate tides and 1.1 ~1.7 on the neap tide. The Si number at CS8(the lower reach) is in the range of 0.15~0.40 from the spring to moderate tides and 0.90~1.50 on the neap tide. It is suggested that SIPS is present from the spring to moderate tides while persistent stratification is present on the neap tide at both CSW and CS8. The potential energy anomalies at CSW and CS8 increase from the spring to the neap tides while they decrease from the spring to the neap tides at CS1.To evaluate the impacts of mixing and tidal straining on turbulence at three hydrological gauging stations(CS1, CSW and CS8) within the North Passage, plots are made of time series of calculated gradient Richardson number(Ri) and modeled turbulent kinetic energy dissipation rate( e) at each station. Results show that modeled turbulent kinetic energy dissipation rate is approximately in the range of 10-10~10-3 W·kg-1 on the spring tide and 10-10~10-5 W·kg-1 on the neap tide, with the gradient Ri number in the order of 10-4~10-1 near the bottom/surface of the water column, and 10-2 ~101 in the well stratified region of the middle part of the water column at the three hydrological gauging stations. Time series of modeled turbulent kinetic energy dissipation rate follows a predominantly M4 cycle with a flood/ebb tidal asymmetry and a relatively high e near the bottom/surface because of the bottom friction and wind stress, respectively. Tidal straining appears to be responsible for the flood/ebb tidal asymmetric distribution of ε.