Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 441

7.4.2 Sedimentation and Management Strategies
The main sedimentation problems of concern in the TGP project are the following:
(1) Reservoir sedimentation and permanent use of the reservoir capacity for flood control;
(2) Sedimentation in the navigation channel in the backwater zone;
(3) Sedimentation in the vicinity of the dam;
(4) Degradation of the downstream reaches;
7.4.2.1 Reservoir Sedimentation and Permanent Use of the Reservoir
The normal pool level of the reservoir (NPL) is set at 175 m. Two other characteristic pool levels are
considered, namely the flood control level (FCL), to which the pool will be drawn down at the beginning
of the flood season, generally in June, and the low-flow season control level (LCL), to which the pool may
be drawn down to satisfy the requirements of power generation and to provide adequate depths of flow,
both up- and down-stream of the dam before the next flood season. The reservoir is impounded to the
normal pool level after the flood season, usually from October. There are many alternative flood control
levels for selection, such as 135, 140, 145, and 150 m. the FCL is the major factor affecting the amount
of sediment deposition in the reservoir.
The hydrological station closest to the dam is the Yichang Station. The long term average annual runoff
at Yichang is 450 billion m 3 and the annual sediment load is 532 million t, of which about 0.8 million t
are gravel bed load. The median diameter of suspended load is 0.033 mm and the median diameter of bed
load is 24 mm. More than 88% of the suspended load is finer than 0.1 mm. The long-term average discharge
at Yichang is 14,300 m 3 /s.
The Tsiantan station is located near Chongqing at the upstream end of the reservoir. The annual runoff,
sediment load, and average concentration at Tsiantan are 350 billion m 3 , 462 million t, and 1.32 kg/m 3 ,
Only about 0.3 million t of the sediment load is bed load. The median diameter of suspended load is
0.037 mm and the median diameter of gravel bed load is 51 mm. Figure 7.37 shows the annual runoff,
sediment load, and sediment concentration at Yichang and Tsiantan in the period from 1950-1985. The
annual load at Yichang varies from 350 million to 750 million t and that at Tsiantan from 250 million to
700 million t.
Fig. 7.37 (a) Annual runoff, sediment load, and sediment concentration at Yichang (1950-1985); (b) Annual runoff,
sediment load, and sediment concentration at Tsiantan (1950-1985) where the dashed lines indicate the mean values