Debris Flows - Environmental Geomechanics - page 12

the materials involved in the debris flow, a levee can have various shapes. In most
cases, the cross-section reveals a curved profile and, when the deposit is dry, it is
characterized by strong cohesion. In other cases, the cross-section has a straight free
surface and even when it is dry, the deposit displays minor cohesion and looks like
a sand or gravel heap. Formation of levees is not systematic. Many observers have
noticed that, after a debris flow has passed through a channel, the channel bottom and
sides have been swept clean of debris.
The alluvial fan is the preferential area for debris flow deposition owing to the
decrease in bed slope and widening of the channel. The slope decrease usually leads
to the sudden stopping of the granular front and increase in the flow depth for the body.
In many cases, debris flows overflow the channel banks and spread as broad lobes on
the alluvial fan. As for levees, the morphological features of lobes vary widely. For
instance, the longitudinal profile of a lobe margin can be curved (parabola-shaped),
straight and tilted, or step-shaped. In the latter case, the deposit looks like an alluvial
deposit. Although they move at low velocities on the alluvial fan, debris flows can
impact or bury structures.
1.4. Modeling debris flows
1.4.1. Debris flow classification and rheological behavior
The diversity in the morphological features of debris flows provides evidence
of different families with specific bulk behavior. Several classifications have been
proposed in the past few years. To date, there is no agreement on the chief
characteristics on which classification should rely. Therefore, some classifications are
based on the size distribution of materials involved, others only consider the mode
of release, etc. Here, we are mainly interested in the manner in which a debris flow
propagates and therefore we suggest using a classification based on bulk mechanical
behavior. We shall therefore consider three families [BAR 03]:
- Muddy debris flow . The transported material is usually characterized by a wide
particle-size distribution. It is sufficiently rich in clay-like materials for the matrix
to have a muddy consistency and lubricate contact between coarse particles. Most
of the time, bulk behavior is typically viscoplastic, which means that the material
shows both plastic and viscous properties [COU 97, MAJ 92, PHI 91]. When the
stress level is low, the material behaves as a solid body, but when the stress level
exceeds a critical value (yield stress), it flows as a fluid does. This yield stress confers
specific properties to the material. For instance, when a given volume of material is
released and spreads down a tilted flat plane, the flow depth decreases regularly. When
the flow depth reaches a critical value (depending on the yield stress and the plane
inclination), the driving shear stress is lower than the yield stress and the flow stops
abruptly. In most cases, the yield stress ranges from 0.5 to 15 kPa. Muddy debris flows
can usually propagate over slopes greater than 5%. The limits of deposits are sharp