Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 443

both the flood control and low flow season regulation storage may be preserved indefinitely.
The sedimentation process of the TGP reservoir has been simulated by using one-dimensional numerical
models. The computation was made for the reservoir operational levels schemes with hydrological time
series. As the recurrence period of the 1954 flood is approximately 40 years, the stream record of 1954 was
inserted into the hydrological series in such a manner that it appears three times in 106 years. The time
series, thus, consists of the hydrological records of the flowing sequence of years:
2 cycles of 1961-1970 plus 1954, 1955;
2 cycles of 1961-1970 plus 1954, 1955;
4 cycles of 1961-1970 plus 1954, 1955 and
2 cycles of 1961-1970
Figure 7.39 shows the accumulation of sediment in the reservoir from 0 to 100 years for various schemes
calculated by the Yangtze River Conservancy Commission, in which 160-135 means that the normal pool
level is 160 m and the flood control pool level is 135 m. Curve 4 in the diagram is for the scheme with
NPL = 175 m and FCL = 145 m (but in the first 10 years NPL = 156 m and FCL = 135 m). After 80 years
of operation, the accumulated sedimentation amount is nearly in equilibrium and increases very slowly.
The total sedimentation in 100 years is about 16 billion m 3 .
Fig. 7.39 Accumulated sedimentation in the reservoir during 0-100 years of operation of the reservoir for various
reservoir operational level schemes (Sedimentation Panel for TGP, 2002)
In order to reduce the rate of sedimentation in the reservoir and develop the hydro-power of the river,
the Xiangjiaba and Xiluodu Reservoirs will be constructed after the completion of the TGP reservoir. The
Xiangjiaba Dam site is located 1,020 km upstream of the TGP dam with a capacity of 5.06 billion m 3 .
The reservoir can be used to trap sediment for 60 years before it reaches equilibrium. The Xiluodu Dam
is located 1,180 km upstream of the TGP dam with a total capacity of 11.57 billion m 3 . The reservoir can be
used to trap sediment for 90 years before it reaches equilibrium. Bed load and coarse suspended load from
upstream reaches of the Yangtze River can be trapped by the two reservoirs. Thus, the rate of sedimentation
of the TGP reservoir can be reduced in the first 90 years. Figure 7.40 shows the accumulated sedimentation
of the TGP reservoir for the three scenarios: Scheme 1 = without upstream reservoirs; Scheme 2 = with
the Xiangjiaba Reservoir; and Scheme 3 = with the Xiluodu Reservoir. The operation of the Xiluodu or
Xiangjiaba Reservoirs will reduce the sedimentation volume by 2-4 billion m 3 in the period of 30-80
years of TGP reservoir operation.
In the reality, the Three Gorges reservoir began to impound in June of 2003. Sedimentation in the first
years of operation has received great attention. In the first ten years of operation the pool level rose
gradually following the completion of the dam in the ranges of 135-141 to 145-156 and 145-175 m. From