3. Sedimentation near the Dam Area and the Fluctuating Backwater Region

 Owing to the complexity concerning hydraulics and sediment deposition problems near the dam area and in the fluctuating backwater region which makes the computation of mathematical model almost impossible, 14 physical sediment models (5 for dam area and 9 for different sections of the fluctuating backwater region) have been established to conduct detailed studies.

he testing of physical sediment model in China made a great progress during the construction of Gezhouba Hydroelectric Project and achieved world advanced level in this field. Now Gezhouba Reservoir has been in operation for 15 years, comparison has been made on real observation vs. model testing results. Generally, they are quite consistent but the results of model have some security. The physical sediment model testing of the TGP has adopted basically the same method of Gezhouba Hydroelectric Project with a model scale generally of 1 by 100 to 1 by 300.

The results of the model tests for the TGP demonstrate that within 30 years' reservoir operation, the cumulative sediment deposition will have no serious impact on the project's normal operation both in the dam area and in the fluctuating backwater region. But several decades later, sediment deposition in the fluctuating backwater region might affect the navigation and harbour operation during extremely dry years when the reservoir' s water level drops down to the lowest level. Even there are such problems, they can be solved by the optimized reservoir operation, proper river taming,regulation works and harbour rehabilitation. In the dam area, the negative effects of sediment deposition on navigation and power generation, after sediment and hydraulic model tests, can be solved by engineering measures such as optimized layout of structures and special facilities for sediment preventing, sluicing and scouring, etc...

4. The Effect on Flood Water Level at Chongqing

 After decades' operation of the reservoir, the flood water level at Chongqing City which is at the tail of the reservoir might be increased due to the sediment deposition. Through computation analysis and model testing, it is estimated that after 100 years' operation, the flood water level at Chongqing City will be increased from present 194.3 m to 199 m in case of a 100-year frequency flood. But because Chongqing is a mountainous city, most of the urban areas are over 200 m in elevation, and the center part about 250 m. Therefore, the increased flood water level will not affect it much.

In addition, some positive impacts such as to-be-built reservoirs upstream of Chongqing and water & soil conservation work 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-size reservoirs will be built 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 Jialing River, Goupitan project and Pengshui project on its tributary Wujiang River, etc... These reservoirs shall be completed within several decades. The completion of these projects, combined with the upstream water & soil conservation works being performed and cultivation of Yangtze shelter belt forestry will decrease the sediment deposition in the Three Gorges Reservoir and lower the flood water level of Chongqing stretch. At the same time, they shall also simplify the tackling measures of the sediment deposition in the Three Gorges Reservoir.