USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
The Physics of Debris Flows
-- Richard M. Iverson, 1997,
The Physics of Debris Flows:
IN:
Reviews of Geophysics, 35, 3, August 1997, p.245-296,
published by American Geophysical Union, Paper #97RG00426
Abstract
Recent advances in theory and experimentation
motivate a thorough reassessment of the physics
of debris flows. Analyses of flows of dry, granular solids
and solid-fluid mixtures provide a foundation for a comprehensive
debris flow theory, and experiments provide
data that reveal the strengths and limitations of theoretical
models. Both debris flow materials and dry granular
materials can sustain shear stresses while remaining static;
both can deform in a slow, tranquil mode characterized
by enduring, frictional grain contacts; and both can
flow in a more rapid, agitated mode characterized by
brief, inelastic grain collisions. In debris flows, however,
pore fluid that is highly viscous and nearly incompressible,
composed of water with suspended silt and clay, can
strongly mediate intergranular friction and collisions.
Grain friction, grain collisions, and viscous fluid flow
may transfer significant momentum simultaneously.
Both the vibrational kinetic energy of solid grains (measured
by a quantity termed the granular temperature)
and the pressure of the intervening pore fluid facilitate
motion of grains past one another, thereby enhancing
debris flow mobility. Granular temperature arises from
conversion of flow translational energy to grain vibrational
energy, a process that depends on shear rates,
grain properties, boundary conditions, and the ambient
fluid viscosity and pressure. Pore fluid pressures that
exceed static equilibrium pressures result from local or
global debris contraction. Like larger, natural debris
flows, experimental debris flows of ~10 m3 of poorly
sorted, water-saturated sediment invariably move as an
unsteady surge or series of surges. Measurements at the
base of experimental flows show that coarse-grained
surge fronts have little or no pore fluid pressure. In
contrast, finer-grained, thoroughly saturated debris behind
surge fronts is nearly liquefied by high pore pressure,
which persists owing to the great compressibility
and moderate permeability of the debris. Realistic models
of debris flows therefore require equations that simulate
inertial motion of surges in which high-resistance
fronts dominated by solid forces impede the motion of
low-resistance tails more strongly influenced by fluid
forces. Furthermore, because debris flows characteristically
originate as nearly rigid sediment masses, transform
at least partly to liquefied flows, and then transform
again to nearly rigid deposits, acceptable models
must simulate an evolution of material behavior without
invoking preternatural changes in material properties. A
simple model that satisfies most of these criteria uses
depth-averaged equations of motion patterned after
those of the Savage-Hutter theory for gravity-driven flow
of dry granular masses but generalized to include the
effects of viscous pore fluid with varying pressure. These
equations can describe a spectrum of debris flow behaviors
intermediate between those of wet rock avalanches
and sediment-laden water floods. With appropriate pore
pressure distributions the equations yield numerical solutions
that successfully predict unsteady, nonuniform
motion of experimental debris flows.
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04/04/02, Lyn Topinka