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4.4 Effect on Flood Water Level at
Chongqing After several decades' operation of the reservoir, the flood water level at Chongqing which is located at the mouth of the reservoir might increase due to sediment deposition. Through computation analysis and model testing, it is estimated that after one hundred years' operation of the reservoir, the flood water level at Chongqing will increase from present 194.3 m to 199 m in case of a 100--year frequency flood' Chongqing is a mountainous city, most of the urban areas of which are over 200 m in elevation and the center part about 250 m, the increased flood water level, therefore, will exert little impact on it. In addition, some positive impacts, reservoirs to be built and water &. soil conservation works to be conducted upstream of Chongqing for instance, have not been taken into account in the estimation mentioned above. As a matter of fact, according to the overall planning of the Yangtze Valley, a number of large--sized reservoirs will be built in the foreseen future on the upper Yangtze and its tributaries, Such as Xiluodu project and Xiangjiaba project on the mainstream, Hechuan project and Tingzikou project on its tributary, the Jiating River, Goupitan project and Pengshui project on its tributary, the Wujiang River, etc. The completion of these projects, combined with the upstream water & soil conservation works being performed and cultivation of shelter belt forestry on the upstream of the Yangtze Valley will surely decrease the sediment coming into the Three Gorges reservoir and lower the flood water level of Chongqing stretch. At the same time, they shall also simplify the tackl ing measures of the sediment de- position for the Three Gorges Project.
Wu Gorge
5. ENVIRONMENTAL IMPACT ASSESSMENT
(ElA) The impacts of the Three Gorges Project on the environmental and ecological system have been attracting worldwide attention. It is almost 40 years since the ElA of TGP began in 1950s' Since 1980s, the EIA of TGP has been deepened along with the improvement of legislative system of environmental protection and development of environmental science. The ¡°Environmental Impact Statement of TGP¡±(EIS) jointly prepared in December 1991, by the Environmental Assessment Department of the Chinese Academy of Sciences and the Yangtze Valley Water Resources Protection Research Institute had passed the complete legal approval procedures at the beginning of 1992. 5.1 Characteristics of the Reservoir
and Changes in the Hydrological Regime (1) The total capacity of the TGP reservoir amounts to 39.3 billion m3, only accounting for 8.7 % of the yearly runoff at the dam site, and its effective capacity 16.5 billion m3, accounting for 3.6 %. Therefore, the reservoir is of a seasonal regulation one with low run-off regulation capability. (2) The reservoir is of a canyoned and river--like reservoir with a total length of about 600 km and average width of 1.1 km which is less than twice the width of the natural alluvial channel. The surface area of the reservoir will reach 1,084 km2, and the land area to be inundated will be 632 km2. (3) The annual runoff downstream of the dam will keep unchanged after the operation of the reservoir, therefore, the annual flow going into the sea will not be affected. The monthly flow downstream of the dam will basically remain unchanged except for in October when the flow decreases slightly because of impoundment and the period from January to May when the flow increases slightly' The monthly flow after the operation of the reservoir will still be controlled within the natural amplitude, so that the influence on the hydrological regime downstream of the dam will not be significant.
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