Recognition, classification and mechanical description of debris flows
P. Coussot & M. Meunier
Summarized by Mark Salvatore
23 February 2009
Goal: To review the present knowledge of debris flow recognition, classification, and
propogation.
What makes debris flows unique from…:
- Hyperconcentrated stream flows?
o Transient nature of debris flows
 Debris flows are sharp, transient phenomena that occur almost as periodic
surges
o Relative velocity between the liquid and solid is small in debris flows
 Deformation is approximately continuous and occurs on large scales
 Debris flows can be considered one-phase flows of viscous fluids
o Debris flow deposits
 The structure of the deposits of debris flows differ only slightly from the
flow itself
 There is no significant grain sieving due to the high density of the material
o Solid fraction range of debris flows and hyperconcentrated stream flows:
 Hyperconcentrated stream flows = 1% - 25% solid material
 Debris flows = 50% - 90% solid material
- Landslides?
o Velocity
 Debris flows generally flow between 0.5 – 10 m/sec
 Landslides generally flow at rates less than a few centimeters per day
o Motion type
 Landslides generally experience small internal deformation
 Often the original structure can be observed in final landslide
deposits
 Typically contain a clay fraction that ensures the cohesion of the
bulk
 Landslides also occur solely on steep slopes and generally travel short
distances
 Debris flows experience significant internal deformation, resulting in final
deposits that look nothing like the original assemblage of materials
 Either contain a small clay fraction, or the clay fraction plays a
minor role
 Debris flows can also flow over long distances of highly variable slopes
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Flow is typically confined to constricted corridors such as stream
channels
o Deposit aspect
 Debris flows exhibit unique features, including lobes, snouts, lateral
levees, mechanical deposits in channels, and deeply incised channels
Debris avalanches?
o Velocity
 Debris flows generally flow between 0.5 – 10 m/sec
 Debris avalanches flow at rates greater than 10 m/sec
o Motion type
 Water plays a minor role in debris avalanches because of grain dilation,
creating a small ratio of water to air
 Debris flows require water between grains to provide as a lubricant
 Also results in relatively smooth and continuous deformation
Submarine debris flows?
o Submarine slumps typically behave similar to landslides and retain evidence of
their original structure in their deposits
o Turbidity currents are chaotic, behaving similar to hyperconcentrated stream
flows
 Also exhibit grain sieving – coarse grains are deposited first and fine
grains are deposited last
Debris flow initiation processes:
- The progressive transition of a landslide into a debris flow due to an energy increase
following a slope increase or due to a water supply
o Can result in deposits of granular material accumulated in a zone after upstream
erosion or waterlogged upper layers of a slope or a bank.
- The generalize erosion of the surface of the stream basin
o Likely due to the conjunction of small-scale bank slides or collapse, bed erosion,
and solid transport
o Likely the most common initiation process because it is closely associated to
normal stream activity
Debris flow propagation
- Best described using non-Newtonian fluid mechanics
- Characterized as a mass of a single viscous material undergoing large homogeneous
deformations without significant changes to its mechanical properties
- Consists of free surface flows undergoing simple shear
o Can be modeled as fluid layers, in the form of parallel planes, gliding over one
another
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Debris flow material belongs to the class of suspensions
Two main courses of research:
o Takahashi (1978) considered debris flow material as a suspension of force-free
particles in a viscous fluid, undergoing rapid flow
 Found that Bagnold’s theories of concentrated suspensions was unable to
correctly characterize debris flow mechanics
o Considers debris flows as viscoplastic fluids
 Particles form networks through the material that must be broken for flow
to occur
 The most meaningful physical parameter is the yield strength, τc, which
increases exponentially as the solid fraction increases
 Determines the form that the deposits take upon settling
 Can be inferred from field measurements
 Can also be used to provide an estimation of viscous dissipations
during flow
Two main types of debris flows based on their flow type:
o Muddy debris flows
 The clay fraction is large enough (> 10% or so) for the fine particle-water
mixture to form an interstitial fluid that lubricates grain motions
o Granular debris flows
 The clay fraction is low enough for direct grain contacts to play a major
role on the mass behavior
o The behavior of debris flows at the intersection of these two types is poorly
understood
Questions pertaining to debris flows and application to Mars:
- What role does ice play in the formation and development of debris flows? Can debris
flows form as the result of grains interacting with water/ice mixtures?
- Do the presence of salts or salt “slurries” influence the morphology, form, or deposits of
debris flows? What implications might this have for the mineralogies of martian debris
flows when viewed from orbit?
- It seems as if debris flows only occur when the ground surface is at or near saturation. At
what level of saturation must the original surface be at before debris flows can be
initiated? Does this have any implication to the occurrence and nature of martian debris
flows?