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present in the central part of the channel because after has the front has passed, the rear of the debris flow
deposit left in the center of the gully, consists of small particles.
Fig. 4.38 “Stone street” phenomenon of two-phase debris flow in the Xiaojiang River basin in Yunnan Province, China
4.3.2.5 Velocity and Velocity Profiles
From the two-phase debris flows experiments, Wang (2001) obtained the following formula for the
velocity of debris flow head:
JJ
J
q
u
2.96
s
(4.7)
d
ª
º
J J
J
s
s
Ch
120 12.6
J
«
»
vd
d
¬
¼
s
In which, u d is the velocity of debris flow head, q is discharge of water per width, h d is the height of
debris flow head, C vd is the ratio of volume of solid particles to that of debris flow, which is obtained by
measurement. The formula is useful for debris flows consisting of gravel, cobbles, boulders, and very
limited concentrations of clay and silt.
The velocity profiles of solid particles were analyzed by digitizing the video record of the debris flow
experiments. In the experiments, a rolling head moved down the flume and following the head was the
main flow. Figure 4.39 shows the velocity profiles in the head and the main flow of two experiments
Nos. 15 and 17. The particle velocity profiles of the main flow are similar to the grain velocity profile of
debris flow experiments by Tsubaki et al. (1983). It is striking that the velocity profiles of the particles in
the head are quite different from those in the main flow (Fig. 4.39). Particles in the head move at a much
lower velocity than those at the same relative elevation in the main flow. The particle's velocity in the
main flow is about 2 times those in the head. The shapes of the profiles also are different. The head
profiles are nearly linear and those of the main flow are more curved.
The mechanism of the velocity difference between the particles in the head and the main flow is
perhaps that the particles in the main flow receive energy from the flowing liquid and accelerate to a high
velocity. They catch up with the head and collide with and transfer their energy to the particles in the
head, then decrease in velocity. The concentration of particles in the head is much higher than that in the
main flow. A lot of energy is consumed by collisions between the particles and with the bed, so that the
head is subjected to great resistance and moves at a much lower velocity.
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