Numerical Study On The Impact Of The Sewage Drainage On Water Quality Of Shanghai Water Sources

Changjiang river, one of the longest rivers in China, has the largest runoff,and its estuary has a special geographical environment with the interaction of the runoff and the tidal current. The Yangtze River was bifurcated by Chongming Island to south and north branch from Xuliujing, most of the flow go down through the south branch. The south branch was divided into south and north channel by Changxing Island and Hengsha Island. The south channel was bifurcated by Jiuduansha Shoal to south and north passage, therefore three bifurcation areas and four estuarine outlets were formed.With the rapid growth of economy,demand for basin development and the amount of industrial and agricultural wastewater emission increased sharply, the water environment pollution becomes worse and worse.The main water source of Shanghai is from the Huangpu River.Neither the quanlity nor the amount was suitable for drinking, so the Yangtze River was taken as the second water source and how to make full use of that was seriously considered.The main water source to Shanghai from the Yangtze River is Baogang,Chenxing and Qingcaosha reservoir.Qingcaosha reservoir is located below the south and north channel bifurcation.As the planned strategy water source of Shanghai, the Qingcaosha reservoir which supported the water resources security and is sustainable for the development of Shanghai which has an important significance.So it has profound influence on the theory and practice, and provided evidence for further utilization of the Yangtze River fresh water resource and safety.A 2D current and water quality model was set up with Delft3D-FLOW in this research, which was well calibrated and verified with the observed tidal levels and velocities. Considered with the project of the Qingcaosha Water Source, pollution concentration distribution caused by sewage outfalls in the south bank of the Yangtze River during a dry season and flood season are simulated and the computed results show: a high pollution zone appears near the south shore, and moves upward with flood flow and downward with ebb flow, however, it is bounded a limit region around the outfalls.According to the COD concentration of the monitor points in flood and dry season,it can be seen that: the concentration was affected by the tidal current in flood season,and it was affected by the upper runoff in dry season.Since the sewage discharged from the Zhuyuan and Xinhe outfalls,the COD concentration of monitor points,which set between the two outfalls,maintain a high level.And the COD concentration is low in rest of the monitor points.The COD concentration of Qingcaosha is 10mg/l which can be classified as the second class water,and the outfalls has little effect to the water quality. During the tidal period, the water quality of the Qingcaosha water source can ensure the normal water supply as the Shanghai water source.