Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 416

the reservoir during flushing periods is also released. In general a conical scour hole in front of the outlet
is formed during the flushing. Sediment from the upper portion of the reservoir is transported towards the
dam during draw down, but only material in the scour hole can be flushed out.
Fig. 7.16 Sediment concentration of mud flow during empty flushing of the Zhuwo Reservoir experiment
Draw-down flushing or pressure flushing was used to flush sandy bed material in the Tarbela Reservoir
in Pakistan (Lowe and Fox, 1995). The Tarbela Dam is a 137 m-high embankment dam on the Indus
River mainly used for hydropower (3,750 MW installed capacity) and irrigation. Its total capacity was
14.3 billion m 3 at closure in 1974 but it had decreased 17.4 percent due to sedimentation by 1992. The
original project was designed for a 50-year economic life with no previsions for the eventual management
of the inflowing sediment load of about 208 million tons per year. The inflowing sediment consists of 59%
fine sand, 34% of silt, and 7% clay. Approximately 99% of the inflowing load is trapped and accumulates
primarily in the form of a delta deposit which is advancing toward the dam (Fig. 7.17). Delta top-set beds
have a slope of about s = 0.0006 to 0.0008. Most sediments are deposited on the top-set bed as the reservoir
fills and levels rise during the wet season, but when the reservoir is draw down for irrigation deliveries,
the river reworks and transports these deposits downstream, extending the delta toward the dam. Most
sediment is transported to the face of the delta at the onset of the wet season when the pool level is still
low but discharge increases from 1,500 to 4,500 m 3 /s.
Fig. 7.17 Advancement of delta deposits toward Tarbela Dam, Indus River, Pakistan (after Lowe and Fox, 1995)