| Bohai Bay is a half-closed inland shallow sea. It was surrounded by some economic cities such as Beijing city, Tianjin city and Tanggu city. Because of its predominant geographic location, Bohai Bay plays an important role in country's economy, national defence and foreign trade. Thus more and more attentions are being paid to its environment. In 2002 the Bohai Bay topography was investigated. The result of the investigation showed that the Bohai Bay topography had been changed due to the activition of enclosing the sea for farmland in recent years. Through the comparison with the Bohai Bay topography in 1980', the tide-induced residual current is diagnostically calculated by POM sea model. On that basis, the paper studies the water exchange time and primarily discusses the pollutants' transportation mechanism in Bohai Bay. Finally according to the inspected data of pollutants in Bohai Bay, the paper evaluates the water quality, and gains the environmental capacity and the advisable discharge mounts of pollutants in Bohai Bay depending on the optimal controlling theory. All the works above lay a foundation for further understanding the pollution mechanism in Bohai Bay and putting porward the method to scientifically control and deal with the pollution.The study of hydro-dynamical characteristics is presented by using the POM sea model, driven by tidal water elevation in bay mouth as open boundary. The result shows that because the open boundary is near the amphidromic point, to witch tide-induced current is sensitive, some illusive effect is appeared in the result to some extent. Based on that, the method using the embedded model driven by current speed is put forward to simulate the Bohai Bay current. That is to say, the current speed at the bay mouth obtained through the hydrodynamical simulation using POM sea model of Bohai Bay is used to driven the embedded model. The result is testified by current speed surveyed data of fixed points. So, the simulation by use of the embedded method with current speed as the open boundary condition can lead to better and reliable results.The effect of variation in topography on the circulation of Bohai Bay is analyzed throughthe simulation in both old and new topography. The result shows in new topography the clockwise circulation in northwest corner of Bohai Bay disappear. Instead, there exists a strong counter-clockwise circulation. The circulation in bay mouth southern part becomes weak with shorter radius in presence of the strong counter-clockwise circulation in the west part. Through analyzing the pollutants' concentration distribution features based on the surveyed data, it is clear that the structure of the pollutants' concentration distribution is mainly affected by the tide-induced Lagrangian residual current field.Coupled with the hydrodynamic model, the three-dimension water quality model is established. Through the model, the paper calculates the average residence time in both old and new topography. The result shows the water exchange capability in Bohai Bay top is on a low level: under the old topography the average water exchange time is 300 days, while under the new topography it is increased to 316 days, which indicates the variation in topography can weak the water exchange capability. The spatial distributing structure of water exchange time is well similar to the tide-induced Lagrangian residual current, which demonstrates that the tide-induced Lagrangian residual current may be an important dynamic factor in controlling water exchange in Bohai Bay. According to the characteristics of mixing among sub-areas, early mixing among sub-areas is related to geographic distance, and its balance is primary affected by of the characteristics kinetic field.The water quality is analyzed and appraised according to three times investigation of pollutants in 2000. A linear programming model is proposed to calculate the environmental capacity by using the method of sub-area controlling. The model is designed to make the total of allowable amount of discharge in every discharge point reach maximum under the pre-requisite that every kind of water function area achieves the corresponding environmental standards. Through the calculation of responding coefficients, both the optimal layout and the maximal amount of the pollutants discharge are gained.To sum up the above works, there are some innovative points in this paper. Firstly, through the study of hydro-dynamical characteristics in Bohai Bay, compared with the result of driven by tidal water elevation, that of embedded model driven by current speed can relieve the effect of open boundary to the area simulated. Secondly, in recent years the Bohai Bay topography has been changed due to the activities of enclosing the sea for farmland. Theeffect of variation in topography on the circulation of Bohai Bay is analyzed through simulation using embedded model driven by current speed with high-resolution in both old and new topography, for the first time the adjusting of current structure is depicted on the change of the topography. Thirdly, the coupled model was used to simulate the average residual time in both old and new topography. The result shows that the change of topography makes the water exchange with open-sea more easily. Compared with the study in abroad and domestic, the method used is more novel and the result is more reliable. Finally, by using the optimal controlling theory, the environmental capacity in Bohai Bay is calculation. Furthermore, both the optimal layout and the maximal amount of pollutants discharge arebeen presented... |
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