Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 41

1.2.6 Reservoir Management
In the past decades, China had watched marvelous economic wonders and accelerated dam construction.
Most Chinese rivers have been impounded for the purposes of flood control, power generation, water supply,
irrigation, and/or navigation. On the Yellow River 11 major reservoirs were constructed in 1957-1993.
They are from upstream down the river: Longyangxia Reservoir, Lijiaxia Reservoir, Liujiaxia Reservoir,
Yanguoxia Reservoir, Bapanxia Reservoir, Qingtongxia Reservoir, Sanshenggong Reservoir, Wanjiazhai
Reservoir, Tianqiao Reservoir, Sanmenxia Reservoir, and Xiaolangdi Reservoir. The total capacity of
the reservoirs is 55.8 billion m 3 , equal to the annual runoff of the whole watershed. More than 10 billion
of sediment has been trapped by the reservoirs reducing the total amount of sediment deposited in the
lower Yellow River.
The Xiaolangdi Reservoir is used to supply water to Henan, Shandong, and Hebei Provinces, and also
used to regulate water and sediment. The capacity for trapping sediment is 7 billion m 3 . It is predicted
that the sediment from the Loess Plateau can be trapped by the reservoir for at least 20 years, and,
therefore, the lower Yellow River will be scoured and the flood risk will then be eased. Moreover, the
reservoir is also used to create artificial floods to scour the lower reaches of the channel.
Impoundment of rivers causes many problems. For instance, the Sanmenxia Reservoir was impounded
in 1960. The confluence of the Weihe River flowing into the Yellow River is in the backwater region of
the reservoir. The high pool level caused sediment deposition at the river mouth followed by serious
retrogressive sedimentation. The ground water table raised and the flood discharge capacity of the Weihe
River greatly reduced due to sedimentation in the channel. The ancient city of Xian is endangered by the
reduced channel capacity. At present, hydraulic engineers are discussing decommissioning of Sanmenxia
Reservoir (not removing the dam but opening all the outlets and maintaining the pool at the lowest level
as before the dam construction). Figure 1.36 shows the Yellow River at Sanmenxia dam- the Three Gate
Gorges before the dam construction and the dam (a) and the Sanmenxia Dam impounded in 1960. The
reservoir lost 6 billion m 3 of its capacity in just 10 years of operation due to sedimentation.
(a) (b)
Fig. 1.36 (a) The Yellow River at Sanmenxia - the three gates gorges before the construction of the dam; (b) The
Sanmenxia Dam, which was the first dam on the Yellow River completed in 1960 and reconstructed two times to
mitigate the sedimentation of the reservoir (YRCC, 2001)
For small reservoirs (less than 100 million m 3 capacity) the percentage of capacity loss due to sedimentation
is even higher. The major strategies to control sedimentation and restore the capacity of the reservoirs are:
storing the clear water and discharging the turbid water; flushing by drawdown and flushing by emptying
the reservoir; dredging, and making use of density currents.