Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 295

which means both erosion or sediment deposition may occur in the reach between Aishan and Lijin if the
concentration is lower than 50 kg/m 3 .
In general the sediment delivery ratio is high in narrow and deep channels. Figure 5.33 shows the
relation of the sediment concentration at the downstream station with the concentration at the upstream
station on rivers with narrow and deep channels. For hyperconcentrated floods with concentrations in the
range from 100 to 800 kg/m 3 , the delivery ratio is around 1 and the sediment concentration measured at
the downstream stations is quite close to the concentration at the upstream stations. Hyperconcentrated
floods may transport all of the sediment for more than 100 km although the riverbed slope is less than or
equal to 10 -4 . Hyperconcentrated flow may even deliver sediment in a channel with a slope as low as
0.3×10 -4 (Qi and Ru, 1995). A hyperconcentrated flood has a much higher sediment carrying capacity
than a normal flood.
Fig. 5.33 Relation between the sediment concentration at the downstream station, S L , with the concentration at the
upstream station, S up in narrow and deep channels
As shown in Fig. 5.29, the vertical concentration distribution is uniform if the average concentration is
over 200 kg/m 3 . When the average concentration of sediment is below 200 kg/m 3 , the distribution may be
quite non-uniform. In a reach downstream from the Aishan station the concentration near the surface is
130 kg/m 3 , while the concentration near the bottom is 300 kg/m 3 . The non-uniform sediment distribution
caused a non-uniform velocity distribution. The velocity distributions in the lower Yellow River were
measured during floods with different average sediment concentrations. K v is defined as the ratio of the
velocity at y = 0.8 h to the velocity at y = 0.2 h in each case measured from the bottom. For clear water flow,
K v =1.4. K v increases with increasing sediment concentration from zero to about 200 kg/m 3 . For an average
sediment concentration of 200 kg/m 3 , K v reaches its maximum value of around 2. For further increases in
concentration, K v reduces and becomes smaller. For concentrations in the range of 300-900 kg/m 3 , K v
reduces to 1.4. This phenomenon demonstrates that a sediment concentration around 200 kg/m 3 causes the
greatest consumption of turbulent energy. Moreover, the critical discharge for non-siltation varies with
concentration and reaches its maximum for a concentration around 200 kg/m 3 . For instance, the non-siltation
critical flow discharge in the lower Weihe River is about 500 m 3 /s if the concentration less than 100 kg/m 3
or higher than 300 kg/m 3 , and the critical discharge is 800 - 1000 m 3 /s for concentrations around 200 kg/m 3 .
This fact illustrates that a sediment concentration around 200 kg/m 3 is the most difficult for the flow to
transport.