Turbulent Mixing, Stratification And Tidal Straining Within The North Passage Of The Changjiang River Estuary In The Dry Season: Data Analysis And Mathematical Modelling

Field measurements were made of tidal elevation, current velocity, salinity, and suspended sedimentconcentration at the three gauging stations CS1, CSW and CS8within the North Passage of the ChangjiangRiver estuary on spring, moderate and neap tides in January2010. They qualitatively display spring/neap andflood/ebb tidal variability in the stratification within the water column.Time series of the shape and rotational directions of tidal ellipses within the North Passage show:（i） at StationCS1they rotate anti-clockwise in the upper layer and clockwise in the lower layer on spring tide; anti-clockwiseon moderate tide; clockwise within the whole water column on neap tide.（ii） at Station CSW they rotateanti-clockwise in the upper layer and clockwise in the lower layer on spring and moderate tides; anti-clockwisein the whole layer at the turning of tidal flow from flood to ebb and clockwise within the whole water column atthe turning from ebb to flood.（iii） at Station CS8they rotate clockwise in the upper layer and anti-clockwise inthe lower layer on spring, moderate and neap tides. The different rotational directions in the upper and lowerlayers suggest that there may be a pycnocline within the water column.To evaluate the potential of vertical turbulent mixing within the North Passage, the overall Richardson number（Rio） is estimated using the density of water after including suspended sediment concentration. Results showthat:（i） Rioat Stations CS1and CSW can be of the order of101to102, suggesting stronger stratification, at theturning of the tide, and of the order of10^-2to10^-1, suggesting stronger mixing, at the maximum flood and ebbtides.（ii） Rioat Station CS8ranges from0.25to5at the flood tide, suggesting moderate stratification, and can beof the order of10^-2at the ebb tide, suggesting strong mixing.（iii） stratification appears to be stronger at the floodtide than at the ebb tide. Stronger stratification is present on spring tide; while stratification lasts longer on neaptide than spring tide at the three gauging stations CS1, CSW and CS8.To determine the possible location of a pycnocline within the North Passage, the vertical gradient Richardsonnumber （Ri） is estimated. Results show that:（i） peak values of suspended sediment concentration occur at themaximum flood/ebb tides because of the strong mixing (Ri: in the order of0¹0^-2) at the bottom. Thepycnocline becomes thinner and moves upward even to the surface. Suspended sediment diffuses upwards to theplace beneath the pycnocline at the three gauging stations （CS1, CSW and CS8）.（ii） stratification is enhanced（Ri: in the order of101¹02） and the pycnocline becomes thicker and moves downward at the turning of thetidal flow at the three gauging stations （CS1, CSW and CS8）. Apparent rotation of the tidal ellipse appears andupper and lower tidal ellipses with different rotational directions are separated by the pycnocline. Bottom mixingis weakened by strong stratification and suspended sediment is restrained beneath the pycnocline which mayenhance siltation. Based on those observations, a conceptual model is cautiously proposed, where the pycnoclinemay suppress the vertical transport of momentum and energy, and change the rotational directions between theupper and lower layers. The suspended sediments are restrained under the pycnocline, causing spring/neap andflood/ebb tidal variability of the suspended sediment transport.To elucidate the mechanisms for mixing and stratification within the North Passage, estimates are made of thevariability in the potential energies caused by tidal straining, estuarine circulation and tidal stirring followingSimpson et al.（1990）. Results show:（i） tidal straining is present at the three gauging stations CS1, CSW, andCS8. Straining-induced periodical stratification is predominant at these locations.（ii） tidal straining is the keymechanism for stratification. The rate of change in potential energy due to estuarine circulation is smaller thanthat due to tidal straining and tidal stirring by the order of102and103.（iii） the competition between tidalstraining and tidal stirring causes the flood/ebb tidal variability in stratification. Mixing is enhanced by tidalstraining and tidal stirring at the maximum flood tide. The water is still mixed because the rate of change inpotential energy due to tidal stirring is stronger than that due to tidal straining although tidal straining tends toaccelerate water column stratification at the maximum ebb tide. Nevertheless, mixing is not strong as at themaximum flood tide. Strong stratification occurs when the rate of change in potential energy due to tidal stirringis weak at the turning of the tidal flow, this may be enhanced by lateral tidal straining.To reproduce the tidal variability of mixing and stratification within the North Passage, mathematicalmodeling is undertaken using the three dimensional baroclinic hydrodynamical model GETM. Modeled tidalelevations are validated against those data at Niupijiao. Modeled current speed/direction and salinity arevalidated against measured data on spring and moderate tides at Station CS8. Modeled current speed and salinityare in agreement with measured data at the bottom, while they are less than measured data at the surface. Thismay be due to the fact that artificial jetties and spur dikes are not taken into account. Preliminary results showthe flood/ebb tidal variability in mixing and stratification. Strong stratification occurs at the turning of the tide,while strong mixing occurs at the maximum flood and ebb tides.