Debris Flows and Landslides - River Dynamics and Integrated River Management - page 231

may develop even at constant incoming flow rate if S y is much larger than S vis ; and 2) the free surface is
stable if S y is smaller than S vis . The growth rate of wave height depends on the parameter S y . The larger is
the parameter S y, the higher the growth rate and the higher the waves.
Fig. 4.47 Growth rate per distance of the wave height ' h / L as a function of the dimensionless parameter ´ B /2 ¤ m h
4.3.3.4 Bed-Paving Process
Pseudo-one-phase debris flow often develops into a series of waves. When a wave flows through a
channel, a layer of water-sediment mixture deposits is stuck on the channel bed. The flow wave becomes
smaller and smaller because of the loss of volume of the mixture. when it finally stops it forms a paved
path with a layer of water-sediment mixture. Then another wave follows. This process is termed the
“bed-paving process” by Chinese researchers. In the Jiangjia Ravine, the average paving distance of a
wave is about 50 m.
Pseudo-one-phase debris flow has a yield stress, and flows down an inclined channel only if its depth
exceeds a critical value, H 0 . When a pseudo-one-phase debris flow wave passes through an open channel,
a tail section with a depth less than H 0 stops moving under action of the yield stress. The wave, therefore,
becomes shorter and shorter and finally rests on the channel bed. As long as the matrix of the debris flow
exhibits a yield stress, the first few debris flow waves pave the way and then the following waves can
flow through the channel.
A wave flowing over a channel paved with a debris mixture resulting from the foregoing waves, may
lose a part of its volume (rear part) if its velocity is low, or in other cases it may gain in volume by taking
a part of the mixture from the bed if its velocity is high. A debris flow wave can grow or shrink depending
on the thickness of the debris mixture layer previously deposited on the channel bed. This brings about a
great difficulty in predicting the debris flow discharge. Figure 4.48 shows the variation of discharge of a
debris flow wave along the flowing course in the Little Almakinca River in Russia. The discharge was less
than 20 m 3 /s and increased to 150 m 3 /s after traveling 4 km. It reduced from 150 m 3 /s to about 60 m 3 /s
after traveling 0.4 km farther, and then it increased to about 230 m 3 /s in another 1.6 km of travel.
Fig. 4.48 Variation of a debris flow discharge along the Little Almakingka River