Debris Flows - Environmental Geomechanics - page 7

(There are many other ways in which water can be provided for the formation of
debris flows: thawing soil, sudden drainage of lakes, dam break, etc., but these are
much less frequent.) A high liquid water content seems to be a necessary condition
for the soil to be saturated, which causes intense surface runoff and an increase in the
pore-water pressure (presumably leading to Coulomb slope failure).
- Sparse vegetation . Vegetation plays a role by intercepting rainfall (limitation of
runoff) and increasing soil cohesion (root anchorage). Vegetation reduces the initiation
potential to a certain extent but does not completely inhibit formation of debris flows.
Many observations have shown that debris flows also occur in forested areas.
1.3.2. Motion
On the whole, debris flows are typically characterized by three regions, which can
change with time (see Figure 1.5):
- At the leading edge, a granular front or snout contains the largest concentration
of big rocks. Boulders seem to be pushed and rolled by the body of the debris flow.
The front is usually higher than the rest of the flow. In some cases no front is observed
because either it has been overtaken by the body (very frequent when the debris flow
spreads onto the alluvial fan) or the materials are well ordered and no significant
variation in the bulk composition can be detected.
- Behind the front, the body has the appearance of a more fluid flow of a rock
and mud mixture. Usually, the debris flow body is not in a steady state but presents
unsteady surges. It can transport blocks of any size. Many authors have reported that
boulders of relatively small size seem to float at the free surface while blocks of a few
meters in size move merely by being overturned by the debris flow. The morphological
characteristics of the debris flow are diverse depending on debris characteristics
(size distribution, concentration and mineralogy) and channel geometry (slope, shape,
sinuosity and width). Debris flow velocity varies very widely but, on the whole, ranges
from 1 to 10m/s [MAJ 96]. The fastest debris flows are reported to move at more than
20m/s [MAJ 96]. Flowing debris can resemble wet concrete, dirty water or granular
material but irrespective of the debris characteristics and appearance, viscosity is
much higher than water. Most of the time, debris flows move in a completely laminar
fashion, but they can also display minor turbulence (or be highly turbulent).
- In the tail, the solid concentration decreases significantly and the flow looks like
a turbulent muddy water flow.
In recent years, many outdoor and laboratory experiments have shed light on
the connections existing between particle-size distribution, water content and flow
features for fixed volumes of bulk material [CHA 09, DAV 86, IVE 97, PAR 01]. In
particular, experiments performed by Parsons et al. [PAR 01] and Iverson [IVE 97]
have shown that the flow of poorly ordered materials was characterized by the
coexistence of two zones, each one with a distinctive rheological behavior: the flow