Environmental Purification

After completion,the TGP will have an annual electricity output of 85 billion kwh,which is equivalent to a yearly saving of 50 million tons of coal,14 thermal power plants with an installed capacity of 1. 3 million kw each and 3 coal fields with an annual 15 million tons as well as an equivalent of the 800 km double-track railway from Qinhuangdao to Datong,which would occupy a lot of farmland as well. The latter alternative would leave us contending with a large amount of waste water,waste gases and residues in addition to 120 million tons of carbon dioxide,2 million tons of sulphide dioxide,10,000 tons of carbon monoxide, 370,000 tons of compound oxygen and mitorgen a year. All these show that the Project is an excellent environmental purifier for the mankind .

Research of Sediment

 The sediment issue of the TGP is one of the major technical problems needing to be tackled. Since 1950's,many domestic research institutes and a lot of famous sediment experts have conducted extensive studies on this issue . The successful experience of Gezhouba Hydroelectric Project on the sediment treatment has laid solid foundation for the sediment study of the Three Gorges Project.

Comprehensive research methods as prototype observation, mathematical model computation, physical model tests, and analogue analysis on existing projects have been adopted for the sediment study of the project. The conclusion is that the sediment problem of the TGP is resolvable based on the examinations and authentication organized by sediment experts at home and aboard.

1. Sediment Load

 According to the observations of Yichang Hydrologic Station over 40 years, the sediment discharge of suspended load on average is about 526 million tons a year, with median diameter of 0.033 mm. The annual bed load is about 8.6 million tons, including 0.76 million tons of cobbles, which takes up only a small proportion of the total sediment load.

The water discharge at TGP' s dam site is large, reaching 451 billion m3 annually with average sediment rate only about 1.2 kg/m3, whereas, the sediment discharge at Sanmenxia Project on the Yellow River is 1.6 billion tons with average sediment load of 37 kg/m3. The water discharge at the TGP dam site is 10 times that at Sanmenxia Project, but the annual sediment load is only one third of it. Therefore, the sediment problem of the Yangtze River is quite different in nature from that of the Yellow River.

In recent years, some experts deemed that the sediment yield in the Yangtze River is in a tendency of increase based on the data of the early 1980's. However, it is not a fact. According to the long term observation and checking results, the sediment load in the Yangtze River changes periodically below or above the average level on a random basis, and there is no obvious tendency of increase. The value of sediment load at the dam site was far below the average several times in the late 1980's and early 1990's. This is an evident proof.

2. Permanent Use of the Reservoir

 The Yangtze River has abundant runoff. The Three Gorges Dam is to be equipped with 23 bottom discharge outlets with low elevation (90 m) and large dimension (7x9 m) . Meanwhile, the reservoir level in the flood season will be maintained at a fairly low elevation (145 m) . These two characteristics allow the application of a strategy of reservoir operation summed up in China as "discharging the turbid and impounding the clear". During the flood season from June to September, when the river carries 84 % of the annual sediment load and 61% of the annual runoff, the reservoir draws down to FCL (145 m) that creates a condition in favour of sediment sluicing, letting a large discharge of sediment out of the reservoir from the bottom outlets. At the end of the flood season (October) when there is less sediment content in water, the reservoir will be impounded to NPI. (175 m) for enhancement of power generation and navigation. In this operation manner, most of the sediment will be moved out, the large part of the deposit of sediment will remain in the dead storage of the reservoir, and most part of effective storage can be preserved permanently.

The length of the TGP reservoir is over 600 km and the average width is only about 1,100 m. It looks like a ribbon in the plain. So it is still a river, and not a lake-like reservoir. The whole reservoir is quite uniform in width and for the most part less than 1,000 m wide, only one seventh of the reservoir is about 1,000 to 1,700 m wide.The morphology of the reservoir is favourable for the removal of sediment . Since the estimated width of the equilibrium channel corresponding to the hydrological conditions of the TGP reservoir is 1,300 m little flood plain is expected to be formed along the main channel in the TGP reservoir. Thus, large percentage of effective storage can be preserved permanently.

The deposition in the TGP reservoir has been analyzed numerically by mathematical models. The result is quite satisfactory. According to the calculation results, after a century operation when the reservoir sedimentation already comes to the balanced state, the reservoir's effective storage capacity could still remain at 86-92%.

It is believed that the conclusions are reliable, not only because the mathematical model is developed in a large number of researches and observations and verified by many experts at home and abroad, but also it is based on several successful engineering practices.