Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 412

flushing; 2) empty flushing or free flow flushing; 3) releasing density currents; 4) storing the clear and
releasing the turbid; and 5) dredging.
7.2.2.1 Empty Flushing
Fan (1985) has classified flushing into two general categories: (1) empty or free flow flushing, which
involves emptying the reservoir to the level of the flushing outlet with riverine flow through the
impoundment, and (2) drawdown or pressure flushing, which requires less drawdown but is also less
effective. The second method is not commonly used. Empty flushing can also be classified according to
whether it occurs during the flood season or the non-flood season. While both strategies have been applied
successfully, flood season flushing is generally more effective because it provides larger discharges with
more erosive energy, and flood borne sediments may be routed through the impoundment.
Flushing scours a single main channel through reservoir bed while floodplain deposits on either side
are unaffected. Profile views of a flushing channel are shown in Fig. 7.10.
Fig. 7.10 (a) Drawdown flushing causes erosion in the upper part of the reservoir and redeposition near the dam,
with pressure flow through the bottom outlets; (b) Empty flushing results in erosion in the whole reservoir, with free
flow through bottom outlets
Some irrigation reservoirs with small capacity are emptied before flood season and flushed during the
first part of the flood season. They are refilled during the latter part of the flood season. Because of the
high sediment concentration that can be released during the flushing period, the downstream irrigation
canals must be designed to be able to transport a high sediment concentration. Seasonal emptying is also
feasible when water demand is seasonal.
Figure 7.11 shows the Sefid-Rud Reservoir in Iran being emptied for empty flushing. After the reservoir
was emptied many fishes were left in the fluid mud layer on the reservoir bed. Local people row boats in
the fluid mud and capture the fishes, as shown in Fig. 7.11(b). Figure 7.12 shows the sediment discharge
and sediment concentration of the inflow and outflow of the Sefid-Rud Reservoir in Iran during the
empty flushing. The sediment concentration and sediment discharge of the outflow was 40 times higher
than those of the inflow due to flushing. Empty flushing is very effective for scouring sediment.
The Hengshan Reservoir on the Tangyu River (a tributary of the Yongding River) in Shanxi Province,
China applied the empty flushing strategy to control sedimentation and restore the reservoir capacity. The
Hengshan Dam is 69 m high with a reservoir capacity of 1.33 million m 3 . From 1966 to 1974, 3.19 million
m 3 of sediment deposited in the reservoir with a thickness of 27 m near the dam. To restore the capacity
of the reservoir the reservoir was emptied to flush sediment in 1974 and 1979. During the empty flushing
period from July 28 to September 4 in 1974, the inflowing sediment was 0.13 million t but 1.19 million t
of sediment were flushed out of the reservoir (Guo et al., 1985). The reservoir was emptied and flushed
again in 1979 from Aug. 9 to September 30, the inflowing sediment was 0.2 million t but 1.55 million t of
sediment was flushed out of the reservoir. A mud flow with a maximum concentration of about 1370 kg/m 3