Flood Defense and Water/Sediment Management—With Particular Reference to The Yellow River - River Dynamics and Integrated River Management - page 372

Fig. 6.41 Discharge capacity of the outlets of Sanmenxia Dam (Wu et al., 2005)

outflow and inflow ratio of sediment reached 105%. In the period from the beginning of the summer in
1970 to the end of the flood season in 1973, about 4.1 million m 3 of sediment in the reach from the dam
to Tongguan was scoured away, and a part of the reservoir capacity was restored. Correspondingly the
bed elevation at Tongguan dropped by 2 m.
After these two stages of reconstruction, the dam can discharge sediment-laden water and causes no
significant detention of large amounts of flood water. However, due to surface abrasion and cavitation,
the bottom sluices were severely damaged, and, therefore, they underwent repairs from 1984-1988. As a
result the total discharge capacity of bottom sluice openings No. 1 to 8 was reduced by about 471 m 3 /s
due to compression. To compensate for the reduction resulting from bottom sluice repairs, two more
bottom sluices, nos. 9 and 10, were opened in 1990. In an attempt to make the most of the dam by fully
utilizing the potential for hydropower generation in the non flood season, penstocks nos. 6 and 7 were
converted back to power generation in 1994 and 1997, respectively. Considering the effectiveness of
sediment flushing by discharge at low levels, the last two bottom sluices, nos. 11 and 12, were also
opened in 1999 and 2000, respectively. To date, there are 27 outlets in Sanmenxia Dam for discharging
flood flows.

6.5.2 Management of Reservoir Sedimentation
Sedimentation in the reservoir depends on the incoming water and sediment, the discharge capacity, and
the operational mode. Reconstruction of the outlet structures has significantly increased the discharge
capacity, providing the dam with the necessary facilities for avoiding significant detention of flood water
which is important for maintaining the sediment balance across the impounded reach in the reservoir. On
top of this, the dam must be properly operated to maintain the reservoir level in order to increase the
benefits of the project and to maintain the sediment balance. For this purpose, the Sanmenxia Reservoir
has adopted three different modes of operation. The average pool levels corresponding to each operation
mode are shown in Fig. 6.42.
(1) Storage. The mode was used during the initial period of reservoir impoundment, from September
1960 to March 1962, when the reservoir was operated at a high storage level throughout the whole year,
according to the original project design.
(2) Flood detention. The mode was applied from March 1962 to October 1973, during which the
reservoir was used for flood detention and sediment sluicing with the water being released without