Flood Defense and Water/Sediment Management—With Particular Reference to The Yellow River - River Dynamics and Integrated River Management - page 370

south from Longmen to Tongguan, then makes a 90q turn and goes east. The Weihe River flows into the
Yellow River at Tongguan.
To mitigate the sedimentation, the operation scheme of the dam was changed to detain only flood
water in flood seasons. The primary function of Sanmenxia Dam was originally for flood control. For
this purpose a capacity of 10 billion m 3 was reserved (even after the change in operations ) to cope with
floods that occur only once in a thousand years, such as the 1933 flood. However, the flood-releasing
capacity of the outlet structures was limited. Though the reservoir was operated at a low level during
flood seasons were with all the outlet structures fully opened, the reservoir stage was still high and serious
sedimentation in the reservoir due to the detention of large amounts of flood water was still inevitable.
The net accretion of sediment deposits amounted to 2.04u10 9 m 3 from April 1962 to May 1966. During
this period, 16 floods lasting 89 days in the summer of 1964 caused 0.93u10 9 m 3 of sediment to be
deposited in the reservoir area (Yang et al., 1994).
During the impounding period from September 1960 to March 1962, the “elevation of Tongguan,”
which indicates riverbed variations and is defined as the stage corresponding to a discharge of 1,000 m 3 /s
at Tongguan station, rose 4.5 m (Long and Chien, 1986; Long, 1996), finally reaching 327.2 m in March
1962. Backwater sediment deposition extended over Chishui in the lower Weihe River, about 187 km
upstream of the dam, and extended 152 km in the Yellow River. After the mode of reservoir operation
was changed, the backwater sediment deposition was still rapidly extending upstream, raising the bed
elevation and flood levels in the Yellow River as far as 260 km upstream of the dam. This threatened the
industrial and agricultural bases, and more importantly the capital city of Shanxi Province Xi'an, in the
lower reaches of the Weihe River. In addition, it potentially required the relocation of an additional one
million people. There was much pressure to improve the situation because of the dense population and
the scarcity of farmland in China. In order to alleviate the serious reservoir sedimentation problem and to
achieve a balance between sediment inflow and outflow, a special meeting was held in Xi'an City in
December 1964 to find a solution to the sedimentation problem in the reservoir. The late Premier Zhou
Enlai presided over the meeting, showing the high demand for a resolution to this problem. A policy was
established to “ensure the safety of Xi'an City in the upstream as well as that of the lower Yellow River”
and a decision was made on the reconstruction of outlet structures to increase the discharge capacity.
The reconstruction work was carried out in two stages. In the first stage, two tunnels at an elevation of
290 m were added on the left bank and four penstocks were remolded into outlets for the purpose of
sluicing sediment, as shown in Fig. 6.40. After work on the first stage was in completed in August 1968,
the discharge capacity had been increased from 3,080 m 3 /s to 6,100 m 3 /s at a water level of 315 m. The
reconstructed outlets were put into operation one after another and played a definite role in reducing
sediment deposition in the reservoir area below Tongguan. However, the sills of the outlet structures
were too high and the capability of the reservoir to release floodwater was inadequate. The ratio of
outflow-inflow sediment was 80%. The amount of backwater deposition was still high and the bed
elevation at Tongguan continued to rise.
The work on the second stage commenced in December 1970. In this stage, 8 bottom outlets at an
elevation of 280 m previously used for diversions were reopened to sluice sediment at the lower
elevation and to generate stronger headward erosion. In order to suit power generation at a low altitude
elevation by the river's current during the flood season, the intakes of penstocks No. 1-5 were lowered
from an elevation of 300 m to 287 m, and 5 generation units with a total installed capacity of 250 MW
were installed. The first generating unit started to operate at the end of 1973, and the rest were put into
operation by the end of 1978. After the second stage of reconstruction, the release capacity of all the
outlets increased to 10,000 m 3 /s at an elevation of 315 m (Fig.6. 41). With this capacity, no significant
backwater could accumulate immediately behind the dam in medium or minor flood conditions, and the