Debris Flows - Environmental Geomechanics - page 3

Figure 1.1. A typical watershed: the Brandy torrent (Savoie, France)
soon as the bed inclination is sufficiently high, gravity has a more pronounced role
on sediment transport. Therefore, on the one hand, bed load transport is more intense,
and on the other hand, a new mode of transport arises: debris flow (see Figure 1.2).
We can define them as follows:
- Debris flows are highly concentrated mixtures of sediments and water, flowing
as a single-phase system. Debris flows look like mudslides and landslides except that
their velocity and the distances they travel are much larger. It is worth noting that in
the literature there are many terms used to refer to slides and/or debris flows, which is
a source of confusion.
- Bed load transport involves transportation of sediment by water. Coarse particles
(sand, gravel and boulders) roll and slide in a thin layer near the bed (called the bed
layer). Generally, fine particles (silts and clays) are brought into suspension as a result
of water turbulence. The system is typically made up of two distinct phases: liquid
phase (i.e. water) and dispersed (solid) phase.
1.2.2. Types of transport
In the laboratory, it is possible to simulate torrential phenomena using an inclined
channel with a mobile bed made up of sand and gravel. Figures 1.3 and 1.4
show two very different situations that can be observed when the channel slope
is increased by only a few percent. Figure 1.3 corresponds to a slope of 17%. At