Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 418

cost of power generation. The strategy of storing the clear and releasing the turbid was finally applied
with the pool level very low during high sediment concentration period and high pool level during low
concentration period for power generation.
The sedimentation has been controlled and the accumulated sediment deposition volume in two parts
of the reservoir, comprising Tongguan to the dam and the lower Weihe River, remained unchanged since
the operation mode changed to storing the clear water and releasing the turbid water in 1973. Only a slight
increase of accumulated sediment occurred in the upper part of the reservoir extending from Longmen to
Tongguan. The reservoir capacity has remained almost unchanged since 1973.
7.2.2.4 Dredging
Dredging has been used for a long time for sedimentation management in small reservoirs. Different
dredging methods are applied in reservoir management, including mechanical dredging and dumping outside
of the reservoir and agitating sediment with jets so that that sediment can be transported downstream of
the reservoir by currents. Various dredgers have been used: dredge boat; agitating dredger; dipper dredger;
hauling scraper; excavator and bulldozer; and trailer dredger. In general dredging is more expensive than
other strategies of sedimentation control in reservoirs. Nevertheless, jets in combination with turbidity
currents or flushing are more economically feasible and have been applied in large reservoirs.
The sediment in the Sanmenxia Reservoir was scoured by a jet or slurry pump, and it remains in
suspension in the reservoir. Because the suspension has a slightly higher density than the water it flows
to the dam along the reservoir bed in the form of a density current. The density current is released from
the bottom outlet of the reservoir. Jet dredging was conducted in a reach near Tongguan from 1996 to
2003 during the flood season. Figure 7.19 shows the sediment amount scoured from the reservoir bed by
jets. The sediment is composed mainly of silt and fine sand with 20% finer than 0.025 mm and about 40%
finer than 0.05 mm. It is reported that the sediment finer than 0.05 mm may be transported by flow out of
the reservoir but coarser sediment may settle again in the reservoir (Jiao et al., 2008).
Fig. 7.19 Sediment amount scoured from the wake area of Sanmenxia Reservoir by jet dredging
Jet dredging was also used to create artificial turbidity currents to release sediment from the Xiaolangdi
Reservoir. Since the Xiaolangdi Reservoir began to store water in 1999, about 1.5 billion m 3 of fine sediment
had deposited in a 40 km reach upstream from the dam in the reservoir by April 2005. The sediment is fine
with a median diameter of about 0.01 mm. The reservoir sediment was scoured by jet, which resulted in
fine sediment suspension in the reservoir. Because the suspension has a slightly higher density than the
surrounding water, it flows to the dam along the reservoir bed in the form of a turbidity current. Most of the
fine sediment is released from the bottom outlet of the reservoir with turbidity currents.
7.2.2.5 Density (Turbidity) Currents
A density current is the relative motion that takes place in a reservoir between two fluid layers that have