Lateral Secondary Flow,Mixing And Stratification Within The Curved Channel Of The North Passage In The Changjiang River Estuary:Data Analysis And Mathematical Modeling

To gain an insight into the temporal and spatial variability of lateral secondary flow, mixing, and stratification and their physical mechanisms within the Curved Channel of the North Passage in the Changjiang River estuary, both field observational data analysis and mathematical modeling are used to study them. Firstly, time series measurements of water level, current velocity, salinity and suspended sediment concentration were made along three cross-channel lines(CS6, CSW, and CS3; three stations for each line) on 25 to 26 February(spring tide/dry season) and 23 to 24 July 2013(spring tide/wet season), respectively. Secondly, quantitative analyses of those data show that lateral secondary flows are present along three cross-channel lines and lateral secondary circulation is also present along the cross-channel line CS3. Circulating Eulerian residual flows are present along the cross-channel line CS6 in the dry season and the cross-channel line CS3 in the wet season. Along the cross-channel line CS3 in the dry and wet seasons, the baroclinic pressure gradient is larger than both the centrifugal and Coriolis accelerations by an order of 1~2, while the latter two terms are nearly the same with an order of 10-4. The Rossby number is around 1. It is suggested that lateral secondary flow is jointly driven by the lateral baroclinic pressure gradient together with the centrifugal and Coriolis accelerations, and the former is relatively more significant than the latter two ones. Thirdly, further quantitative analyses of those data show that tidal mean potential energy anomalies along three cross-channel lines are about 54.23 and 66.56 J·m-3 over the spring tide in the dry and wet seasons, respectively, suggesting that stratification in the dry season is weaker than in the wet season. Tidal mean potential energy anomaly over the flood tide is generally smaller than that over the ebb tide in the dry season, while it is reverse in the wet season, suggesting that tidal asymmetries in turbulent mixing occur. Time derivatives of the potential energy anomalies caused by the cross-channel and along-channel depth-mean strainings are approximately in the range of-67~37×10-3 to-7~11×10-3 W·m-3 in the dry season, and-45~30×10-3 to-14~13×10-3W·m-3 in the wet season, respectively. The cross-channel depth-mean straining is larger than the along-channel one, having a greater impact on lateral mixing and stratification than the latter there. Along three cross-channel lines, the absolute tidal mean values of the along-channel advection(ΦA-x), the cross-channel advection( ΦA-y), the along-channel depth-mean straining(ΦS-x), and the cross-channel depth-mean straining(ΦS-y) account for 26, 33, 18 and 23 percentages of the sum of the four terms during the spring tide in the dry season, respectively. The absolute tidal mean values ofΦA-x, ΦA-y, ΦS-x and ΦS-y account for 13, 9, 22 and 56 percentages of the sum of the four terms during the spring tide in the wet season, respectively. It is suggested that the advection term(maximumΦA-y) may be dominant in controlling mixing and stratification in the dry season, while the straining term(maximumΦS-y) may be dominant in the wet season. Finally, simulations were perfomred with the three-dimensional finite element TELEMAC-3D hydrodynamic model. Modeled results show that lateral secondary flows are present. Stratification is weaker at the flood tide than at the ebb tide and successively decreases from the cross-channel lines CS6 to CS3. Lateral baroclinic pressure gradient and the cross-channel advection(ΦA-y) dominate in the physical mechanisms of lateral secondary flow, mixing and stratification, respectively. Modeled results are reasonably in agreement with findings inferred from observed ones.