Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 292

in which J is energy slope, which is equal to bed slope s if the flow is steady and uniform.
Figure 5.28 (a) shows the velocity profile and the plug zone. If the yield stress is high the plug velocity
is approximately equal to the average velocity of the flow. Many rivers and gullies that frequently carry
hyperconcentrated flows occur in the Loess Plateau in northwest China. The hyperconcentrated flows
usually consist of clay and silt and have a very high yield stress. The scale of the plug zone reflects the
magnitude of the yield stress. Witnesses described them as intermittent flows with larger waves between
smaller waves. The flow plug can be also seen on the surface velocity distributions. Figure 5.28 (b) shows
the surface velocity distribution of a hyperconcentrated flow in the Luohui Irrigation Canal, northwest
China, through which hyperconcentrated floods were diverted from the North Luohe River for irrigation.
About 80% of the width was in the plug zone and a velocity gradient existed only in zones close to the
banks.
Fig. 5.28 (a) Velocity profile of hyperconcentrated flows; and (b) Surface velocity distribution of a hyperconcentrated
flow in the Luohui Irrigation Canal (photo by Z.Wang)
5.2.3.2 Concentration Distribution and Resistance
In a hyperconcentrated flood, the fall velocity of sediment is nearly zero and the concentration
distribution is uniform. Figure 5.29 shows the ratio of the concentration measured at 0.2 h from the water
surface ( S 0.2 ) to the concentration measured at 0.8 h ( S 0.8 ), S 0.2 /S 0.8 , as a function of the average
concentration of the flows, in which h is the flow depth. The numbers by the points in the figure indicate
the median diameter of the suspended sediment in mm. When the concentration is lower than 200 kg/m3
the ratio is in the range of 0.4 - 0.9; and when the concentration is higher than 200 kg/m3 the ratio is
near 1.0. Even for the case of median sediment diameter around 0.1 mm, the concentration distribution of
hyperconcentrated floods is quite uniform.
The resistance of hyperconcentrated flow can be represented by Manning's roughness coefficient, n ,
although hyperconcentrated flow exhibits higher viscosity than water flow or even becomes non-
Newtonian (Qi and Han, 1991). Figure 5.30 shows the roughness coefficient n of hyperconcentrated
floods and low concentration floods at the Xiaolangdi Hydrological Station on the Yellow River as a
function of concentration, in which the points with concentrations higher than 200 kg/m 3 are regarded as
hyperconcentrated. The discharge of the hyperconcentrated floods was in the range of 2,880 - 9,720 m 3 /s
and the discharge of the low concentration floods was in the range of 4,150 - 9,400 m 3 /s. The resistance
for the hyperconcentrated floods and low concentration floods is almost the same.