Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 442

The main strategy to control sedimentation in the Three Gorges Reservoir is storing the clear water
and releasing the turbid water. The Yangtze River transports about 90% of its annual sediment load with
only 60% of its annual in the three months from mid June to mid September. The pool level is drawn down
to 145 m from June to September allowing the turbid water wash through the reservoir downstream. The
reservoir stores water beginning in October when the inflowing water becomes clear. By applying this
strategy a permanent capacity of 22 billion m 3 can be preserved.
The 22 bottom outlets at an elevation of 56 m are used to discharge flood flow during the construction
of the project and will be closed at the end of the third phase of construction. The 23 deep outlets will be
permanently used for discharging flood flow. The elevation of the deep outlets is 90 m and the discharge
capacity at pool levels 130, 140, and 150 m are 51,000, 60,000, and 64,000 m 3 /s, respectively. If the
flood discharge is not over 62,000 m 3 /s, the reservoir is operated for “storing the clear water and
releasing the turbid water” and the pool level is drawn down to 145 m to create a condition favorable
for sediment flushing by letting a large discharge out of the reservoir. When the incoming flood exceeds
this value, water will be stored in the reservoir and the release is adjusted according to a predetermined
schedule aimed at keeping the flood damage in the downstream area to a minimum. After the flood peak
has passed, the reservoir will again be drawn down to 145 m.
Drawing the reservoir down to 145 m during the majority part of the flood season in most years will
keep the upper limit of deposition below the backwater curve with its lower end at the FCL (145 m), as
shown in Fig. 7.38 (Curve 1). During the low-flow season, the river carries little sediment, but still conveys
39% of the annual runoff, i.e. 171 billion m 3 of water at Yichang. Water will then be stored in the reservoir
for power generation and navigation. The amount of water needed to fill up the reservoir to its normal
pool level varies with the scheme adopted but is generally less than 22 billion m 3 , which is only a small
portion of the runoff in the low flow season. Eventually a new alluvial channel will form in the reservoir
under the backwater curve connecting the FCL at the dam and the natural flow upstream, shown as Curve 2
in Fig. 7.38. This part of the reservoir storage can be preserved indefinitely.
Fig. 7.38 Schematic diagram of the pool levels and deposition curve of the TGP reservoir: NPL—normal pool level
(175 m); FCL—flood control level (145, 135, or 150 m); Curve 1 is the upper limit of sediment deposition and Curve 2
is the backwater profile for the case of the reservoir at the FCL
How much of the original flood regulating capacity can be preserved depends, among other things, on
the morphology of the reservoir. The reservoir looks like a ribbon in the plan view. The 700-km long
reservoir is quite uniform in width and is for the most part less than 1000-m wide. Since the estimated
width of the equilibrium channel corresponding to the hydrological conditions of the reservoir is 1,300 m,
little flood plain is expected to form along the main channel in the reservoir, Thus, large percentages of