长江口水动力及污水稀释扩散模拟

NUMERICAL SIMULATION OF THE HYDRODYNAMICS AND SEWAGE DIFFUSION IN THE CHANGJIANG RIVER ESTUARY

利用Delft3D数学模型对长江口水动力条件、上海市现有及拟建排放口污水排放的稀释扩散场进行了模拟。模型利用潮汐数据和排放口附近的现场实测数据进行了率定和验证 ,分别对丰水期、枯水期的大、小潮情况下的流速场、潮位场及污水扩散场进行了研究。结果表明 :长江口的水动力条件有利于污水的稀释扩散 ;现状排放量对长江口水环境影响不大 ;但如果污水在排放前不进行一定的处理 ,规划排放量排入长江口将会严重恶化其水质 ,特别是枯水期小潮时最为严重。建议根据长江口和杭州湾的环境容量 ,结合排放口的稀释扩散能力 ,合理确定污水排放方案。

Shanghai, China's largest metropolitan area with a population of about 14 million, is situated at the mouth of the Changjiang River estuary, and bordered by the East China Sea to the east, Hangzhou Bay to the south, and Zhejiang and Jiangsu Provinces to the west. In the past two decades, rapid economy and population growth has resulted in serious surface water pollution problems due to discharges of large quantities of untreated wastewater into the rivers and other watercourses. Since the 1980s, great efforts have been made to improve the water environment by proper sewerage treatment and the establishment of disposal systems. Two submerged outfalls at Zhuyuan and Bailonggang have been constructed and are now discharging more than 3.0 million m 3/d of sewage to the Changjiang River Estuary. The Third Shanghai Sewerage Project (SSPIII) is at its planning and design stage, for which a outfall is proposed downstream of Bailonggang at Xinhe to cater for a discharge of 1.5 million m 3/d. For a better design for SSPIII, it is essential to study the hydrodynamics of the estuary and the environmental impact of SSPI and SSPII as well as the projected loads of the three stages combined. 2-D hydrodynamic simulations of the well-mixed Changjiang River estuary were undertaken using the DELFT3D model. Depth-averaged unsteady flow and transport in the estuary is solved for representative conditions on a curvilinear, boundary-fitted grid by ADI method. The model is calibrated and validated using tidal data available for 9 stations. The calculated periods cover spring and neap tides; both the magnitude and phase of the water elevation and velocity are reasonably reproduced. The velocity fields, water elevation and far field sewage dilution fields from the existing and proposed outfalls are studied for the spring/neap tides and the dry/wet seasons. The results show that the hydrodynamic condition of the Changjiang River estuary is favorable to sewage dilution and diffusion. The far field sewage dilution fields resulting from the planned sewage flows on the spring tide of wet season show that the sewage is mixed with the estuarine flow over a large area, and is transported towards the outer sea with the Changjiang River runoff; the mixed sewage moves towards Hangzhou Bay during a short duration of the tidal cycle. The area covered by the pollutant patch (as defined by the dilution contour of 1000) is large, reaching Changxing Isle and Hengsha Sand and extends to East-Jiuduan in the outer sea, whereas the dilution contour of 200 still encloses the vicinity of the outfalls. During neap tides in the dry season, serious pollution in the Changjiang River estuary takes place, which is a direct reflection of large sewage flows, weaker river flows and small tidal current velocities. Further, the discharged effluents through the outfalls at present in dry season have little impact on the water environment of the Changjiang River estuary, as indicated by the sewage dilution contours under different conditions. For the planned sewage flow through 4 outfalls, the water quality of the estuary will deteriorate significantly without treatment, especially on neaps of dry seasons. Hence, the Shanghai sewerage project should be designed according to the self-purification capacity of the Changjiang River and Hangzhou Bay.