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Therefore, the total duration of construction will last 17 years taking into account the time for preparation. The main course of the river will be closed in 1997, and the initial two turbine-generating units will be put into operation in 2003 (the 11th year). The whole project will be completed in the year of 2009. See master construction schedule in Table 2 attached. The yearly construction peak intensity is as the following: --23 million m3 of earth--and-rock excavation appears in the 3rd year; --8. 0 million m3 of earth-and-rock embankment in the 5-- 6th year; --4. 5 million m3 of concrete placing in the 8th year; --46 thousand tons of metal works installation in the 10th year; --unit installation in the 12--17th years, 4 sets with 2,800MW annually.
XiLing Gorge
4. RESEARCHES ON SEDIMENT Sediment is one of the major technical problems of the Three Gorges Project to be tackled. Since 1950's, many research institutes and experts at home have conducted extensive studies on this issue. The successful experience gained from the Gezhouba Project on the sediment management has laid solid foundation for the sediment study of the Three Gorges Project. Comprehensive research methods, Such as prototype observation, mathematical model computation, physical model tests, and analogue analysis on existing projects have been adopted for TGP's sediment study. The results show that the sediment problem of TGP is resolvable based on the examinations and authentication organized by sediment experts at home and aboard.
4.1 Sediment Load
Based on the 40 plus years' observation at the Yichang Hydrologic Station, the sediment discharge of Suspended load averages about 526 million tons per year, with a median diameter of 0.033 mm. The annual bed load including 0.76 million tons of cobbles is about 8.6 million tons, which takes up only a small proportion of the total sediment load. The water discharge at TGP's dam site is large, amounting to 451 billion m3 annually with a average sediment concentration of only about 1.2 kg/m3. While as, the sediment discharge at Sanmenxia Project on the Yellow River is 1.6 billion tons annually with a average sediment concentration of 37 kg/m3. The water discharge at TGP dam site is 10 times that at the Sanmenxia Project, while the annual sediment load is only one third of it. The sediment problem of the Yangtze River, therefore, is quite different in nature from that of the Yellow River. In recent years, some experts deem 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 true' According to the long term observation and the checking results therefrom, 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 amount of sediment load at the dam site was far below the average for several times in the late 1980's and early 1990's' This is an evident proof. 4.2 Permanent Use of the
Reservoir The Yangtze River is abundant in runoff' The Three Gorges Dam is to be equipped with 23 bottom discharge outlets with low elevation (90 m) and large dimension (7¡Á9 m). Meanwhile, the reservoir level in the flood season will be maintained at a fairly low elevation (145 m). These two
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