An Investigation Of The Pollutants Discharge Control Strategies In Xiangshan Bay Based On 3D Hydrodynamic And Water Quality Numerical Modelling

Because of obvious industrial diversification,abundant marine resources,superior natural conditions such as large tidal range,small bay mouth and deep port,Xiangshan Bay has high development and utilization value and prospect.With the development of the coastal economy in recent years,the emmision of industrial wastewater,domestic sewage,fertilizers and pesticides in the process of agricultural production and wastewater of ship in Xiangshan Bay is increasing gradually,which lead to the deterioration of water quality.The nitrogen and phosphorus concentration is lower than the standard IV throughout the year,that has caused great harm to the marine environment and ecosystem of Xiangshan Bay.In this study,a three-dimensional hydrodynamic numerical model was established to simulate the spatial-temporal variations of tidal level,tidal currents,temperature and salinity in Xiangshan Bay,using Delft3D.The simulated results agreed well with the in-situ measurement,which shows the model can accurately simulate the hydrodynamic characteristics in Xiangshan Bay.Then,a three dimensional water quality model was established for further calculate the migration and transformation process of pollutants in Xiangshan Bay.The model contains the process of phytoplankton growth and death,nutrient cycle,DO(Dissolved Oxygen)and CBOD(Carbon Biochemical Oxygen Demand).It considers not only the physical diffusion process of pollutants in the water,but also the biochemical process.The simulated results are in good agreement with the measured data in Xiangshan Bay.Finally,it was concluded that the amount of DIN(Dissolved Inorganic Nitrogen)reduction was estimated to be 3.82 t/d for achieving the near-term control target in Xiangshan Bay.By calculating the response coefficient field of the sewage outlet in the bay,it was found that the 11 sewage outlets in the inner-bay had the greatest impact on the water quality environment of Xiangshan Bay.The reduction of DIN by 80%can meet the requirements of the near-term control target.The amount of DIN reduction was estimated to be 2.65 t/d.The verified water quality model was applied to analyze the influence of three discharge strategies,discharge during ebb tide,reduction of discharge amount and integration of sewage outlets on the water quality in Xiangshan Bay.The simulation results showed that the water quality environment of Xiangshan Bay was obviously affected by the sewage outlets,three sewage disposal strategies could directly reduce the DIN concentration in the bay.Different sewage disposal strategies showed different effects.Discharging during ebb tide can more effectively reduce the concentration of DIN in the outport where water exchange is strong,and through the integration and reduction of sewage outlets number,it can more effectively alleviate the situation of high DIN concentration in inner port with weak water exchange capacity.Considering the two optimization measures devised:integrating 11 sewage outlets in the inner bay and moving them to the central-bay(first condition)and the outer-bay(second condition),discharge during ebb tide,the calculation results showed that the first condition can meet near-term control target while the second condition can meet long-term control target,but neither can achieve the water quality standard of functional zoning requirements,because the background DIN value is higher in Xiangshan Bay.The effect on improving the situation of nitrogen exceeding standard in this area is not significant simply using the local optimization methods.