|
|
|
characteristics allow the application of a strategy of reservoir operation Summed up in China as "discharging the turbid and impounding the clean' 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 (145m ) that creates a condition in favour of sediment sluicing, allowing 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 NPL (175 m) for enhancement of power generation and navigation. In this operation manner, most of the sediment will be discharged, the large part of sediment deposited will remain in the dead storage of the reservoir, and most part of the effective storage can be preserved permanently. The length of the TGP reservoir is over 600 km and its average width is only about 1, 100 m, looking like a ribbon in the plain. So it is still a river-like, rather than a lake--like reservoir. The whole reservoir is quite uniform in width and for the most part less than 1, 000 m, only one seventh of the reservoir is about 1, 000 to 1, 700 m wide. The morphology of the reservoir is favorable for the removal of sediment. Since the estimated width of the equilibrium channel is 1, 300 m corresponding to the hydrological conditions of the TGP reservoir, 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 silt deposition in the TGP reservoir has been analyzed numerically by mathematical model ling. 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 based on a large amount of research and observation and verified by many experts at home and abroad, but also because it is based on several Successful engineering practices.
Shadow Play Gorge
4.3 Sedimentation in the Dam Area and
in the Fluctuating Backwater Region Owing to the complexity concerning hydraulics and sediment deposition problems in 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 in the fluctuating backwater region ) have been established for detailed study. A great progress had been made in the physical sediment modeling in China during the construction of Gezhouba Project and world advanced level achieved in this field. Now Gezhouba Reservoir has been in operation for 16 years, comparisons have been made on field observation vs. model testing results and these comparisons, generally speaking, are quite consistent. The physical sediment modelling of TGP has adopted basically the same methods for the Gezhouba Project with a model scale generally of 1 by 100 to 1 by 300. The results of the model tests for 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 such problems, if there are any, can be solved by optimized reservoir operation, proper river training and 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 faciliti -es for sediment preventing, sluicing and dredging, etc.
Wu Gorge
|
