Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 290

runoff began to reduce because of increasing water and sediment diversion and construction of numerous
sediment trap dams in the upper and middle reaches of the river. In the 1990s and 2000s a few
hyperconcentrated floods occurred in the Yellow River but the concentration was lower than before.
Since the impoundment of the Xiaolangdi Reservoir, one hyperconcentrated flood has occurred in the lower
Yellow River on Aug. 22-31, 2004. The highest sediment concentration and the crest discharge of the
hyperconcentrated flood flowing into the Xiaolangdi Reservoir were 542 kg/m 3 and 2,960 m 3 /s and the
released concentration and discharge from the reservoir were 346 kg/m 3 and 2,690 m 3 /s. The highest
downstream concentration was to 358 kg/m 3 at Huayuankou (Zhengzhou), which then reduced to 179
kg/m 3 at Sunkou, 177 kg/m 3 at Aishan, and 146 kg/m 3 at Lijin.
Hyperconcentrated floods cause unique fluvial processes in the middle and lower reaches of the
Yellow River. The high concentration and fluctuation in discharge cause some abnormal phenomena and
problems in flood defense. The study of these phenomena and their mechanism may provide understanding
of hyperconcentrated floods, which is essential for river training and flood defense strategies.
As early as the 1960s Chinese hydraulic engineers started field investigations of the hyperconcentrated
flow occurring in the rivers on the Loess Plateau. In the first edition of their monograph published in
1983, Qian and Wan (1983) reported the basic characteristics of hyperconcentrated flows,. Qi and Zhao
(1985) studied the fluvial process of hyperconcentrated floods. The first International Workshop on
Hyperconcentrated Flow was held in Beijing in 1985. Chinese scientists reported on the hyperconcentrated
floods in the Yellow River and its tributaries and the mechanism of hyperconcentrated flows (Qian et al.,
1985; Wang and Qian, 1985) and American scientists reported on the sediment transportation by lahars
and hyperconcentrated flows at Mount St. Helens (Scott and Dinehart, 1985; Janda and Meyer, 1985).
Since then hyperconcentrated flow has attracted the concern of scientists and engineers. Qian (1989)
summarized the main results on the development, flocculation, rheology, hydraulics, and application of
hyperconcentrated flows. In the monograph with title of “Hyperconcentrated flow”, Wan and Wang
(1994) reported the unique features of hyperconcentrated floods, including the phenomena of ripping up
the bottom, river clogging, roll waves and instability of hyperconcentrated flow, and the mechanism of these
phenomena. Julien (1989), Julien and Lan (1991) and O'Brien and Julien (1995) performed laboratory
studies on physical properties and mechanics of hyperconcentrated sediment flows. Wang et al. (1994,
1998) and Wang (2002) revealed the rheological properties of hyperconcentrated flows, mechanism of
surface instability, and drag reduction.
Hyperconcentrated flow is complicated. The hydraulic features of hyperconcentrated floods are
different from those for low concentration floods. The fluvial process induced by a hyperconcentrated
flood is extremely rapid. The river morphology may be changed more by one hyperconcentrated flood
than the changes that may be caused by normal flow and low concentration floods over a decade. The
fluvial process during a hyperconcentrated flood also changes the propagation of flood waves and induces
peculiar phenomena. This section provides a basic picture of these phenomena and mechanisms of these
phenomena.
In hyperconcentrated flow, the presence of a large amount of solid particles remarkably influences or
changes the fluid properties and flow behavior. In many cases of hyperconcentrated flow, sediment
together with water, forming a pseudo-one-phase fluid, moves in its entirety and sediment can no longer
be considered as material carried by the water. Hyperconcentrated flow occurs often in the Yellow River
and its tributaries. In Table 5.1 the maximum and average monthly sediment concentrations of ten main
tributaries in the middle reach of the Yellow River are listed. Hyperconcentrated floods may carry
sediment, concentration higher than 1,500 kg/m 3 . Hyperconcentrated floods cause rapid and severe erosion
and deposition.