Environmental Engineering Reference
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in which S v can be estimated for Yellow River sediment by S v = S /2650 (where S is in kg/m 3 ); Z is the
fall velocity of a single particle of diameter equal to D 35 , the diameter for which 35 percent of the
sediment is equal to or finer than.
The method of Wang and Dittrich (1992) for differentiating bed load, suspended bed material load and
wash load has been introduced in Chapter 1:
Bed Load > Z = 3 > Suspended Bed Material load, > Z = 0.06 > Wash Load (5.56)
With this method and Eq. (5.55) a lot of relatively coarse sediment becomes wash load in hyperconcentrated
flow because the fall velocity is very small and the value of the Rouse number is smaller than 0.06.
For hyperconcentrated flows in narrow and deep channels, the group fall velocity of sediment decreases
quickly with sediment concentration and the Rouse number becomes very small. For instance, the fall
velocity reduced by 50% compared to a single particle for a concentration of about 240 kg/m 3 , and
becomes 5 times smaller for a concentration of about 530 kg/m 3 , 10 times smaller for 740 kg/m 3 , and 30
times smaller for 1,000 kg/m 3 . All sediment becomes wash load and the sediment delivery ratio is equal
to one during hyperconcentrated floods. On the other hand, in a wide and shallow channel, especially on
the floodplain, the shear velocity may be 10 times smaller than the main channel and the Rouse number
is large, and, thus, the sediment becomes bed material load. The turbulence intensity is not sufficient to
balance the fall velocity. Coarse particles settle down and the concentration reduces, thence the fall
velocity of fine particles increases and these may settle as well. As a result, serious sedimentation occurs
in a wide and shallow channel and the sediment delivery ratio is small.
The upper part of the lower Yellow River is wide and shallow and the channel is wandering. The Weihe
River and the North Luohe River are narrow and deep and the channel is meandering. The sediment
delivery ratio is high in a meandering channel and is low in a wandering channel, as shown in Fig. 5.36.
Although the maximum daily-average discharge in the lower Yellow River is larger than that in the
Weihe River and the North Luohe River, the sediment delivery ratio is much smaller than the later two.
Fig. 5.36 Sediment delivery ratio ( P ) as a function of the maximum daily-average discharge for wandering and
meandering channels (data from the Hydrological Data of the Yellow River, Yellow River Conservancy Commission)
5.2.3.4 Intensive Scour by Hyperconcentrated Floods
As the discharge of a hyperconcentrated flood is higher than the bank full discharge, the sediment
suspension flows on the floodplain and serious sedimentation occurs on the floodplain because the
roughness of the floodplain is high and the velocity is low. In the meantime the hyperconcentrated flood
scours the main channel. Sometimes, a phenomenon of the so-called “ripping up the bottom” occurs
(Wan and Wang, 1994). The peculiar phenomenon of 'ripping up the bottom' is described by local
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