Environmental Engineering Reference
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than water in the same gully. The results indicate that the resistance of debris flow cannot be approached
using the viscoplastic constitutive equations.
For the dilatant model of two-phase debris flow, an essential shortcoming is the omission of the
interaction between the two phases and identification of different roles of different materials in the debris
flow. The constitutive equation can be applied only if all parts of the flow behave the same rheologically,
which is not true for most debris flows. Another important shortcoming of the theories is neglecting the
unsteadiness of the flow. In unsteady flow, the shear stress is not balanced by the driving force and the
inertia or the kinetic energy of the flow plays a role in the motion, especially at the initiation stage, and in
maintaining the motion for a distance in a region of very gentle slope.
Debris flow normally occurs in gullies and rivers, which have drainage areas of 1-100 km 2 . If plenty
of loose solid materials are available on a slope the so called slope debris flow may occur on a slope
which has an extremely small drainage area. Slope debris flow is defined as the phenomenon that a
high-concentration mixture of debris and water flows down the slopes for a short distance and then stops
at the toe of the slope (often at highways and river banks). The slope debris flow is very different from
the normal debris flows and is discussed in this chapter as well.
4.3.2 Mechanisms of Two-Phase Debris Flows
4.3.2.1 Initiation of Two-Phase Debris Flows
Debris flow is often initiated during channel bed erosion by rainstorm floods flowing down the gullies.
The initiation of two-phase debris flows was studied experimentally in a tilting flume 10 m-long and
50 cm-wide with glass-sided walls (Wang, 2002; Wang and Zhang, 1989). Five kinds of gravel were used
for the experiments with diameters ranging from 5-10 mm to 50-90 mm. The liquid phase was a suspension
of water and clay with a concentration of about 100 kg/m 3 . Before the experiments the gravel was put on
the bed forming a mobile bed 20 cm deep. Then, clear water or the clay suspension flowed down the flume
from the upstream entrance. The water content, gravel concentration, size distribution, and the rate of
gravel transport were obtained by analyzing the samples. The initiation and movement of the debris
flows were observed by two video-cameras from the top and the glass side-walls of the flume.
As shown in Fig. 4.33 if the slope of the flume bed was small or the discharge of the liquid phase was
small, no debris flow was initiated. In this case water flowed over the bed and individual particles were
carried by the flow, in the motion of normal bed load (Fig. 4.33(a)). The front was low and propagated
down the slope at a relatively high velocity. There were no or very few particles in the front. The gravel
concentration was only 0-80 kg/m 3 . The velocity of the flow was higher than that of the debris flow and
the front velocity was close to the surface velocity of the main flow because a much smaller number of
particles was carried by the flow and much less energy was consumed by the solid phase. This is the
normal bed load-laden flow.
As the slope and the discharge of the liquid phase increased and became large, however, particles were
removed from the bed and rolled in the front of the flow (Fig. 4.33(b)). Individual particles move faster
than the front, and thence more and more particles moved to the front, forming a head consisting of
rolling particles (Fig. 4.33(c)). Particles in the head collided with each other and with the bed, consuming
a lot of energy, therefore, the head moved at a lower velocity than the liquid and particles in the main
flow. The particles in the main flow caught up with and rolled over the head. The head became so high as
to be several times the diameter of the large stones and stopped growing. The head rolled down the flume
like a bulldozer (Fig. 4.33(d)). Particles in the head collided with each other and made noise. A high
concentration of particles, up to 1,100-1,600 kg/m 3 , was carried down the flume with the head. This is
debris flow. Many debris flows in nature are initiated by storm rainfall and turbulent runoff and exhibit
the same physical pattern as in the experiments.
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