Environmental Engineering Reference
In-Depth Information
Fig. 4.36 Growth process of the debris flow head ( L = the distance from the entrance, h d = the height of the debris
flow head)
experiment with gravel of median diameter 7.3 mm was only 4 cm. Whereas the head height of a debris
flow occurring in a debris flow gully with a median diameter of about 200 mm of gravel was about 1.2 m.
The head height of debris flow is proportional to the size of gravel, as shown in Fig. 4.37. An empirical
relation between h d and ˠ 50 can be mathematically formulated as:
(4.6)
It was observed that the water content in the head was low, and sometimes only dry particles moved in
the very front of the head. The velocity of the liquid phase and the particles in the following part of the
head was higher than the moving speed of the head. The instantaneous velocity of small particles was
measured to be higher than that of large particles. Two paradoxes were observed: (1) the small particles
exhibit high velocity but move slower, large particles exhibit low velocity but move faster; (2) particles
in the head consume a lot of kinetic energy due to collisions with each other and between the moving
particles and the bed, therefore, the movement of the head is subjected to an extremely high resistance,
but the debris flow head grows and remains flowing.
h
5.5
D
50
Fig. 4.37 Height of debris flow head ( h d ) as a function of the median diameter of gravel ( D 50 )
4.3.2.4 Stone Street
Two-phase debris flows always exhibit a high, steep, and noisy front composed of large stones. Collisions
of big stones make noise and consume a lot of energy. Hence, the resistance to the movement of the front
is high and the velocity of the stones in the front part is lower than those behind. The mechanism of the
low-velocity head and high velocity of the following flow has been explained by Wang and Zhang (1990)
and Wang (2002). The average velocity of large particles in two-phase debris flows is higher than small
particles. More and more stones finally move to the front of the debris flow. Because more and more
stones come to the front some stones have to go to the two sides of the gully where the driving force is
smaller and the resistance due to collision with the banks is larger. The stones stop at the two sides of the
gully and form a stone street. Figure 4.38 shows the stone street in a debris flow gully in the Xiaojiang
River basin, in which big stones clearly exhibit two lines on the two sides of the gully. No big stones are
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