Application Of The Lagrangian Residual Current Theory To A Model Bay And The Xiangshan Bay

The shallow seas are faced with a serious problem of environmental pollution anddamage due to the uncontrolled exploitation by human beings. The frequentecocatastrophes have become a great threat to the balance of the ecosystem as well asthe sustainable development of the human society. Considering the long-termtransport of the pollutants in shallow seas being determined by the circulation, there isno doubt that the research on the circulation should be the basis of the solution to theenvironmental problem in shallow seas.With a depth-averaged numerical model, the tidally induced Lagrangian residualcurrent in a model bay was studied. Based on the long-term mass transport, it isappropriate to use the Lagrangian residual velocity rather than the Eulerian residualvelocity or the Eulerian residual transport velocity to describe the residual current.The parameter κ, which is defined as the ratio of the typical tidal amplitude at theopen boundary to the mean water depth, is considered to be a conservative indicatorof the nonlinear effect in the system. It is found that the feasibility of making the masstransport velocity approximate the Lagrangian residual velocity is strongly dependenton κ. The error between the mass transport velocity and the Lagrangian residualvelocity tends to increase with a growing κ because of the invalid perturbation methodin this case. An additional error will come from the various initial tidal phases due tothe Lagrangian drift velocity when κ is no longer small. According to the residualvorticity equation based on the mass transport velocity, the Coriolis effect is found toinfluence the residual vorticity mainly through the curl of the tidal stress. A significantdifference in the flow pattern indicates that the Lagrangian residual velocity issensitive to the bottom friction in different forms. Different from the linear bottomfriction, the one in the quadratic form shows a more complicated variation both intime and space. As a result, the nonlinear effect of the system will be greatly changedto modify the pattern of the residual current. By analyzing the in-situ observational data collected during three investigations,the Lagrangian residual current and its dynamical mechanism in the Xiangshan Baywere studied. It was the first time to use the Lagrangian drifters to observe the surfaceresidual current in this area. The results indicate that the surface residual current in thedeep channel tends to flow towards the mouth of the bay with an average speed ofabout0.09m/s. Moreover, the observational data were used to calculate the masstransport velocity. It is found that the bottom residual current in the deep channeltends to flow towards the head of the bay with a maximum speed lower than0.02m/s.Thus, the vertical circulation with a two-layer structure in the Xiangshan Bay can berevealed. It is also found that the depth where the residual current changes itsdirection seems to be shallower during the spring tide than that during the neap tide,implying a temporal difference in the strength between the surface and bottomcurrents. By computing each term in the dynamical governing equations, thecirculation in Xiangshan Bay is found to be dominated by the barotropic pressuregradient force, the turbulent stress and the tidal body force. By contrast, the baroclinicpressure gradient force, which is one order of magnitude smaller, can be neglected inthe momentum balance. As a result, the vertical circulation in the Xiangshan Bay canbe viewed as the barotropic tidally-induced circulation rather than the baroclinicgravitational circulation.