Flood Defense and Water/Sediment Management—With Particular Reference to The Yellow River - River Dynamics and Integrated River Management - page 386

floods can flow through a narrow and deep channel for a long distance without serious sedimentation.
The sediment fall velocity for hyperconcentrated flow is much smaller than for low concentrations.
Figure 6.55 shows a hyperconcentrated flood flowing in an artificial canal. The hyperconcentrated flood
was diverted into a 50 km long irrigation canal without serious siltation of the canal.
Fig. 6.55 A hyperconcentrated flood in the North Luohe River is diverted into an irrigation canal and transported
for over 50 km without serious sediment deposition
Much heavier sediment loads can be carried by hyperconcentrated flow than by low concentration
flow. The Sanmenxia Reservoir was suddenly draw-drawn and emptied in 1963 before the flood season.
Water carrying sediment was released to the lower reaches and the concentration was 300 kg/m 3 . The
artificial hyperconcentrated flow was maintained for 5 days (Zhao, 1996). Artificial hyperconcentrated
floods can be generated by using the Xiaolangdi Reservoir to transport sediment at high efficiency
through a narrow and deep channel to the sea. The total capacity and sediment trapping capacity of the
Xiaolangdi Reservoir are larger than those of Sanmenxia Reservoir. Generation of hyperconcentrated
flow using the Xiaolangdi Reservoir is feasible.
A difficulty for this strategy is how to narrow and deepen the lower Yellow River channel. Li (1992)
suggested to dig a 800 km long deep and narrow channel with dredgers at a cost of $250 million over
3-5 years, then to initiate artificial hyperconcentrated floods by emptying the Xiaolangdi Reservoir and
increase the sediment concentration by mechanical agitation along the river. In this way the average
sediment concentration could be kept over 300 kg/m 3 and 8 billion m 3 water can transport more than 2.4
billion tons of sediment (three times the annual load) to the sea. Hence the problem of accumulative
siltation of the channel could be solved. Wang et al. (1997) indicated that the following technical problems
have to be solved before implementation of the project strategy:
(1) In order to create an artificial hyperconcentrated flow, the size composition and the ways to control
the size composition have to be studied to make the size composition matches the flow intensity; (2) It
takes at least 7-10 days for a hyperconcentrated flood to travel the distance from the Xiaolangdi
Reservoir to the river mouth. How can an artificial hyperconcentrated flood be generated and maintained
for more than 10 days? (3) The sediment carrying capacity of hyperconcentrated flow is high but very
unstable. The fall velocity of sediment particles is proportional to the -4 to -7 power of sediment
concentration. The fall velocity of coarse particles reduces sharply with increasing concentration,
therefore, coarse particles can be carried in suspension by hyperconcentrated flow but if a part of the