Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 417

7.2.2.3 Storing the Clear and Releasing the Turbid
Sediment transportation in many Chinese rivers occurs mainly in 2-4 months of the flood season, that is,
80%-90% of the annual sediment load is transported with 50%-60% of the annual runoff. The Three
Gorges Project on the Yangtze River is planned for flood control, power generation, and inland navigation.
For these purposes, it is important to maintain adequate storage in the reservoir. The main strategy to
control sedimentation is to draw down the pool level from 175 m to 145 m in the flood season from June
to September when the sediment concentration is high and allow the turbid water to wash downstream
through the reservoir. The reservoir starts to store water in October when the inflowing water becomes
clear (i.e. has a lower sediment concentration). Figure 7.18 shows the typical variation process of sediment
concentration at Yichang—the dam site—and the operation scheme of pool level for sedimentation control.
By storing the clear water and releasing the turbid water, less sediment deposits in the reservoir while the
reservoir is still able to store enough water for power generation in the low flow season. The details of
the sedimentation management in the Three Gorges Reservoir are given in the Section 7.4.
Fig. 7.18 Typical variation process of sediment concentration at the dam site of the Three Gorges Project and the
operation scheme of pool level for sedimentation control
The strategy of storing clear water and releasing turbid water has also been successfully used in the
management of sedimentation behind the Sanmenxia Dam on the Yellow River after several failures of
other sedimentation management strategies. The Sanmenxia Dam, 105-m high and 739-m long, was the
first large dam on the Yellow River. The crest elevation of the dam is 353 m and the designed reservoir
capacity is 35.4 billion m 3 with a normal pool level of 350 m. The operation scheme of the Sanmenxia
Reservoir has been substantially changed to achieve a balance between sediment inflow and outflow in
the following three reservoir operation modes: (i) storage from September 1960 to March 1962, the reservoir
was operated at a high storage level the whole year round; (ii) detaining flood water and sluicing sediment
from March 1962 to October 1973, the reservoir was operated at a low storage level throughout the year,
detaining floods only during flood seasons and sluicing sediment with the largest possible discharges;
and (iii) storing the clear water and releasing the turbid water from November 1973 to the present, the
reservoir has been operated at a high level (315þ320 m) to store relatively clear water during non-flood
seasons (NovemberþJune) and at a low level (302þ305 m) to release high sediment concentrations
during flood seasons (JulyþOctober). Serious sediment accumulation occurred during the first operation
mode period. The operation pool level had to be changed to 303 þ 318 m in 1962 þ 1973, allowing flood
water scoured sediment to flow through the reservoir. Sediment was sluiced out of the reservoir but at the