Lagrangian Residual Current And Inter-tidal Transport In The Nonlinear Estuary Or Coastal Sea System, With An Application To The Bohai Sea

With the development of the economy, the pollution in the ocean, especially in the coastal zone is becoming more and more severe. The typical characteristic of the ocean environment is the ever moving sea water. Thus, except the input/output between the interfaces and the biogeochemical effects, the fate of all matters in the sea, ranging from the physical characteristics of the ocean itself, e.g. temperature and salinity, to the nutrients, the pollutants and the plankton, is mostly determined by the movement of the sea water. The intra-tidal movement is determined by tidal currents, while the inter-tidal transport is controlled by the"coastal circulation". So the study on the coastal residual circulation becomes one of the key problems in the coastal environmental oceanography.Circulation refers to the quasi-stationary current in the ocean. Prof.Feng, R.T.Cheng and their colleagues have developed the Lagrangian residual current and mass-transport theory in a three-dimensional weakly nonlinear coastal system which has been applied in many places and yields good results. While for the very shallow water area, or at the places with great gradients in topography or complicated coastal line, the nonlinear effect is enhanced, the assumption"weakly nonlinear"can not be satisfied, then the accuracy of the simulations in such places can not be guaranteed. Therefore it is necessary and important to development the Lagrangian residual current and inter-tidal transport theory in a nonlinear coastal system either for the sake of the application or of the theory system itself. In this thesis, Lagrangian mean velocity is strictly proved to be the velocity of the circulation in a tidal dominant, dual time scale, nonlinear thermal-dynamical coastal system. Firstly, under the assumption that the scale of the net displacement of the water parcel should be smaller than the characteristic length of the current field, the Lagrangian mean velocity can be treated as an instantaneous velocity in the time scale of circulation. Secondly, the Lagrangian mean velocity is proved to satisfy the continuity equation for an incompressible fluid. The third and also important result is that the Lagrangian mean velocity in nonlinear system has been proved to satisfy the conservation equation of material surface. Thus, the Lagrangian mean velocity can be termed as"Lagrangian residual velocity", and can be used to describe the velocity field of circulation in coastal seas. It should be emphasized that in the nonlinear system, the coastal circulation generally contains infinite temperal-spacial fields of velocities, each of which corresponds to a value of initial phase.Inter-tidal transport process is closely related to or even nonlinearly coupled with the circulation. So a new inter-tidal transport equation is deduced by Lagrangian time average in the nonlinear system. Similar to the fields of circulation, the inter-tidal transport processes also contain infinite temperal-spacial fields of concentration, each of which corresponds to a value of initial phase.By constructing model seas with different topography and shapes, the nonlinear Lagrangian residual velocity is calculated by its definition in a mono-tidal constituent system. After the comparison of the Lagrangian residual velocities and concentration fields with different initial phases, it can be deduced that when the nonlinear effect becomes stronger, the changes of the Lagrangian residual velocity and concentration fields with the initial phases become more and more prominent. Thus the nonlinear theory has been verified by the simulation test. The parameterκis used to indicate the nonlinearity of the system in the weakly nonlinear theory(Feng,1987), while in the nonlinear system, it is not appropriate to regard this parameter as an indication for the nonlinearity, especially for the area with great gradients in depth or complicated coastline. A new parameter,κ·λ/L is introduced, in whichλis the wave length of the tide and L is the scale of the local current field.Much research work has been done to understand the circulation of the Bohai Sea, while until now there is still no convincing conclusion on the Bohai Sea circulation. In this thesis, a new topography with high resolution of 1'×1' is used to simulate the Lagrangian residual velocity field driven by only M2 tide in the Bohai Sea. The results show that there is an anticlockwise gyre in the central Bohai Sea which coincides with the early results under the weakly nonlinear theory. It is the first time to find a small clockwise eddy in the central of the anticlockwise gyre in the Bohai Sea over an undersea bank for the use of the topography with high resolution.The Lagrangian residual velocity has three dimensional distribution in both the Bohai Bay and the Liaodong Bay, e.g. the current goes out at the upper layers while comes in at the lower layers at the south of the Bohai Bay.