Environmental Engineering Reference
In-Depth Information
Maintaining a density flow needs a continuous supply of inflow sediment suspension and a force to
overcome any resistance it encounters. If the inflow ceases to supply dense fluid so that it no longer
forms a density current at the immersion point, the already formed density current downstream would soon
stop moving. Since the energy to maintain the flow comes from the density difference and the slope of the
reservoir bed, so a minimum density difference is clearly required. This density difference is much larger
than that required to form a density current. According to field data from Shaver Lake in the U.S. (Bell,
1947), a density current will form if the sediment concentration is higher than 1.28 kg/m 3 . However, the
data from the Guanting Reservoir on the Yongding River in Chine indicates that only if the sediment
concentration in a density current is more than 20 kg/m 3 , can the current continue to flow and finally
reach the dam (Chien et al., 1998).
A continuous turbidity current flowing to the dam is critical for sediment release, which depends on
the inflowing discharge, sediment concentration and the portion of sediment particles finer than 0.01mm.
Figure 7.22 shows the sediment concentration by weight, S , and discharge, Q , of turbidity currents in
2001-2004 (data from Wu et al., 2008). In the figure, the filled circles represent turbidity currents reaching
the dam, in which the portion of sediment particles finer than 0.01 mm is between 25% and 75%; the
black pyramids are turbidity currents moving to the dam but the portion of sediment particles finer than
0.01 mm is higher than 75%; the open triangles are turbidity currents not reaching the dam, although
some of them consists of 90% fine sediment. The curve in the figure is given by:
SQ (kg/s) (7.5)
For high sediment concentration and discharge, or the points above the curve, the turbidity currents
flowed through the whole reservoir and arrived at the dam. For the points below the curve the turbidity
currents stopped in the reservoir, failing to reach the dam.
100, 000
Fig. 7.22 Sediment concentration, S , and discharge, Q , of turbidity currents reaching and not reaching the dam
occurred in 2001-2004 (data from Wu et al., 2008)
In general, the sediment releasing efficiency of turbidity currents is about 20%. Nevertheless, the turbidity
currents in the Xiaolangdi Reservoir had low sediment releasing efficiency because the reservoir bed had
not silted up to the low sill of the bottom outlets. Arriving at the dam the turbidity currents filled the
reservoir and formed a muddy water reservoir. Only after the surface of the muddy water reservoir rose
to the elevation of the bottom outlets, were the turbidity currents released from the reservoir. The average
sediment releasing efficiency, or the ratio of the released sediment to the inflowing sediment load, was
only 6% in the period from 2000 to 2004 (Hou and Jiao, 2003). In 2006, turbidity currents occurred
again in the Xiaolangdi Reservoir by using Sanmenxia Reservoir to create high sediment concentration
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