Debris Flows and Landslides - River Dynamics and Integrated River Management - page 218

4.3 Mechanisms of Debris Flows
4.3.1 Pseudo-One-Phase and Two-Phase Debris Flows.
Debris flows may be classified into pseudo-one-phase debris flows and two-phase debris flows. Pseudo-
one-phase debris flow occurs if there is a large portion of clay, silt and sand. In pseudo-one-phase debris
flows, water and solid particles make up a fluid mixture, in which gravel, cobbles and even boulders are
moving with the surrounding clay, silt and sand suspension. There is no visible relative movement between
the solid particles and suspension. Viscous and sub-viscous debris flows are pseudo-one-phase flow. The
pseudo-one-phase debris flows can be approached by employing a constitutive equation. Johnson (1970)
and Yano and Daido (1965) postulated that debris flow material behaves as a homogeneous viscoplastic
continuum. Many scientists have employed their models to study pseudo-one-phase debris flows (Julien
and Lan, 1991; Iverson and Denlinger, 1987). Pseudo-one-phase debris flow has a plug in velocity
profiles, which explains that the upper part of the flow moving at a uniform velocity like a solid plug.
Pseudo-one-phase debris flow can develops from a continuous flow into an intermittent flow composed
of a series of roll waves (Wang et al. 1990; Wang, 2002). Because the loose solid materials in dry valleys
have a large portion of fine and cohesive particles, the debris flows are mostly pseudo-one-phase debris
flow and often develop into intermittent flows (Kang et al., 2004).
Two-phase debris flow occurs if the fractions of clay, silt and sand in the solid materials are little. In
two-phase debris flows boulders, cobbles and gravel compose the solid phase and water or suspension of
clay, silt and sand is the liquid phase (Wang et al., 1999). Typical two-phase debris flows exhibit a high,
steep head consisting of rolling boulders. There is obvious relative movement between the liquid phase
and the solid phase and solid particles collide with each other, which consumes a lot of energy. The debris
flows in the Wenchuan earthquake area were mostly two-phase debris flow and some were pseudo-one-phase
debris flow.
Bagnold (1956, 1954) made the most prominent early efforts to construct a theory that accounts for
particle collisions. The core of the theory is the concept of dispersive force. The theory postulates the
debris is a mixture of a dilatant fluid but shear stress is generated mainly by collision between particles.
Scientists have employed this model to study two-phase debris flows (Takahashi, 1981; Savage and
McKeown, 1983). Two-phase debris flows, even with very high concentration of solid materials, were
not very killing because the velocity is not so high. Large pseudo-one-phase debris flows, however, were
extremely disastrous because the velocity was high and people had no time to escape even a warning
signal was issued as the debris flow was detected in the upstream section.
Figure 4.31(a) and (b) shows the deposits of a pseudo-one-phase debris flow in the Jiangjia Ravine.
The pseudo-one-phase debris flow mixture looks like concrete. Figure 4.31(c) shows the deposits of a
two-phase debris flow in the Doufu Ravine in the Xiaojiang River basin in Yunnan. The two-phase debris
flow picture was taken just two days after the debris flow event. Stones and gravel deposited in the gully
and the liquid continued flowing down after unloading of the coarse grains. Therefore, the gully surface
is covered with stones and gravel.
The debris flows in Sichuan, southwesten China, are mostly two-phase debris flow, because the solid
materials in the area consist of a lot of boulders, cobbles and gravel. Sand, silt and clay made up only a
small portion of the loose solid materials. In the two-phase debris flows cobbles and boulders collide
with each other and consume most of the flow energy. The velocity of the two-phase debris flows was
much smaller than pseudo-one-phase debris flows. Figure 4.31 (d) shows the deposit of a two-phase
debris flow occurring in the Zoumaling Gully in the Mianyuan River basin in Sichuan in 2008. About
90% of the solid phase was cobbles and boulders with a median diameter about 0.4 m. The liquid phase
was suspension of silt and clay. The velocity of the debris flow was estimated at only about 1.5 m/s