Numerical Simulation Of Lagrange Residual Current In The Changjiang Estuary, Hangzhou Bay And Their Adjacent Sea

Along with the economy development, the marine pollution in the coastal zone is becoming more and more severe. The coastal circulation theory is the basis for understanding the dynamics of the coastal pollution and environmental changes. As the typical estuaries, Changjiang Estuary and Hangzhou Bay have been of wide interests from scientific communities. Many studies tried to reveal the features of the circulation in this area. However, most of these studies aimed at the instantaneous current field and/or the mean current field during a tidal period. The Lagrangian circulation in the Changjiang Estuary, Hangzhou Bay and their adjacent sea is simulated in this thesis, to investigate the key dynamics for the long-term transport process.The 3D Lagrangian current model is established, with the river flow, wind, East China Sea (ECS) Current and tides (including M2,S2,O1,K1) taken into account. A 3-D linear component tidal wave model is imbedded to calculate tidal residual force, the Splitting Current Method ( SCM ) is used to split the Lagrange velocity and to solve the currents of tide and circulation, and an effective convergent difference scheme on hydrostatic consistent condition is used to solve baroclinic gradient force in sigma coordinate..The simulated tidal components (i.e. M2, S2, O1 and K1) are consistent with the tidal characteristics of this area. Based on the 3D Lagrangian model, the zero-order and the first order circulations in winter and in summer are simulated in Changjiang Estuary, Hangzhou Bay and their adjacent area. The following is the most important results:The zero-order circulation is mainly controlled by the ECS Current. The Taiwan Warm Current (TWC) has two branches all year around, with obvious seasonal variation, which can intrude into the Changjiang Estuary in summer and then turn to east into ECS. The baroclinic effect favors enhancing such pattern. The zero-order circulation has less contribution to the Hangzhou Bay. The wind-driven current, northward in summer and southward in winter, is remarkable in shallow sea area. The wind-driven current is stronger in winter than summer; however, it can weaken the TWC in winter, relative to the surface current. The tidal induced current is more significant in shallow sea area than in shelf sea area. The tidal induced current forms a closed clockwise circulation in the north of Changing Estuary and show southward residual current pattern along the 30m isobath in the Zhejiang coast. The nonlinear interactions of zero-order circulations mainly represent the eastern branch of TWC, by minishing the current and increasing the curl of the branch.The unique geographical features with narrow-within and width-without in Hangzhou Bay can greatly strengthen the tidal current, so the dominant barotropic circulations are the tidal induced current and the wind-driven current. The basic pattern of tidal induced current is that the current flows into the Hangzhou Bay along the north area of the Hangzhou Bay and flows out along the south. The tidal induced current is controlled by an anti-clockwise eddy with other eddies. This pattern is against transporting material outward. Although the wind-driven current enhance such effect in winter, the wind-driven current in summer can weaken such effect, due to the different main wind direction. The freshwater from Changjiang flows into Hangzhou Bay, reaches the south at Jinshan and flows out of Hangzhou Bay. Althought in winter the runoff is weakened, the strong wind can enhance the trend. All these are conducive to the formation of Jinshan freshwater-front.