Numerical Simulation Study Of Hydrodynamics In Bohai Sea

The Bohai Sea is an inner sea of China,surrounded by Tianjin,Hebei,Liaoning and Shandong.With the region around it being populated and economically blooming,the region has become one of the three most important engines of the economy development in China.However,as the fast development of the society and economy in the region,the Bohai Sea has been faced with increasing environment pressure which has been deteriorating the environment in it.In fact,the hydrodynamic condition is one of the key factors determining the evolution of the natural environment in the Bohai Sea and a well understanding of it is important to protect and improve the environment in the Bohai Sea.In this thesis,hydrodynamics in the Bohai Sea was studied using numerical hydrodynamic models,with the assistance of geochemical analysis and field observations.Considering the fact of the natural link between surface water and groundwater,coupled equations for the movements of surface water and groundwater were derived using a control volume containing both surface water and groundwater.Then the equations were discretized using the finite difference method and a refined hydrodynamic model was developed.In this model,the computational domain was naturally extended to the bank and the wet-dry decision to deal with the moving boundary of the surface water was not required anymore.The problem of mass and momentum imbalances caused by the wet-dry decision was improved.Moreover,the equations for surface water and groundwater were unified and the coupled governing equations have the same form as that of the shallow water equations,and the numerical techniques that was used to solve the shallow water equations could be easily applied to this model.In addition,it provides a new way to simulate the interactions of surface water and groundwater.Combining the hydrodynamic model and geochemical analysis,the hydrodynamic condition,sedimentation and the distributions of trace metals in sediments in the Bohai Bay,one of the three bays in the Bohai Sea,and relations among them were studied.Based on the analysis on the concentrations of trace metals in sediments,three regions with high concentrations of trace metals in sediments were found in the north and northwest,the center and the mouth of the Bohai Bay.Simulated results of the hydrodynamic model and the particle-tracking model showed that there also are three vortexes in these regions,which can bring the contaminant including trace metals to the regions with high concentrations of trace metals.Moreover,the hydrodynamic conditions in the vortexes are weak,which contribute to the sedimentation of particles associated with trace metals.Additionally,the analysis on the distributions of radionuclide activities in the sediment cores also showed that the regions with high concentrations of trace metals experienced stable sedimentation.The close correlations among the locations of regions with high concentrations of trace metals and sediment cores with stable decay of radionuclide activities,and the vortexes of water circulation not only indicated the reliability of the distributions of trace metals found in this research,meaning that the hydrodynamic condition is one of the key factors controlling the transports and the distributions of trace metals in sediments in the Bohai Bay,but also certified the credibilities of the numerical models and the water circulation pattern found by them in this research.To study the pattern of water exchange in the Bohai Sea,an average age model was set up using the hydrodynamic model and an existing mass transport model.With the average age model and box model,the circulation and characteristics of water exchange in the Bohai Sea are numerically studied.The distribution of average age of water having entered the Bohai Sea was calculated using the age model and the result showed that ranges of age in the three bays including the Liaodong Bay,the Bohai Bay and the Laizhou Bay are about 2-6 years,10 years and 11 years respectively,which confirmed that the circulation of water in the Bohai Sea is counterclockwise,and the water enters the Bohai Sea through the north part of the Bohai Strait and flows out through the south of it.Box model was used to study the path of water exchange among the parts of the Bohai Sea and the abilities of water exchange of them.The results showed that the central basin is the hub of water exchange among them and the abilities of water exchange of them are rather different.Generally,the Laizhou Bay has a relative strong ability of water exchange among the three bays with the Bohai Bay and the Liaodong Bay experiencing more difficulties in water in them being transported to other parts of the Bohai Sea and to the Huanghai Sea.Using the hydrodynamic model and SWAN model,a model coupling wave and current was set up,with the NCEP CFSR wind dataset being used as wind force data.Then the characteristics,the influencing factors and mechanism of the storm surge in the Bohai Sea were studied numerically.The NCEP CFSR wind dataset was calibrated and validated by measured wind speed.The hydrodynamic model and SWAN model were also calibrated and validated by the measured hydrological data and wave parameters respectively,and then the couple model was validated by measured data of storm surges occurring in the Bohai Sea.The results of calibrations and validations showed that the coupled model is able to simulate storm surges during the cold break invasions.Using the coupled model,the characteristics,the influencing factors and mechanism of the storm surge in the Bohai Sea was studied.The results showed that the storm-induced setups are often large in the regions with shallow water,where the effects of radiation force are more significant than that in regions with deep water.The longshore current caused by wave breaking and dissipation of wave energy is an important contributor to the setup in the south part of the Bohai Sea.The unidirectional radiation force related to the mildly slope topography is another important factor contributing to the large setup in the south of the Bohai Bay,the Laizhou Bay and the north of the Liaodong Bay.Moreover,the results of simulations also showed that the impact of tides on the wave-induced setup is significant,and it should be doubtable to calculate the total water level by adding the storm surge level calculated without considering the effects of tides to a tidal water level.