Environmental Engineering Reference
In-Depth Information
Figure 1.9.
Levees left by a debris flow in the Dunant river in July 2006 (Valais, Switzerland)
(courtesy of Alain Delalune)
surface. Lateral levees were formed by the granular front and confined the ensuing
muddy body. Note that the levee formation is probably not induced by particle
segregation since it is also observed for dry granular flows involving spherical equal-
size particles [FÉL 04].
1.3.3.
Deposition and effects
The distance that a debris flow can travel depends a great deal on the mechanical
characteristics of the debris as well as the total volume, channel geometry and bed
inclination. For instance, it is generally observed that a debris flow moving over a
flat tilted plane thins by spreading laterally and stops suddenly, seemingly when the
thickness reaches a critical value. In contrast, if the debris flow is channelized, it may
travel quite a long distance over gentle slopes. In European alpine countries, debris
flows (of sufficient volume) generally begin to decelerate when the slope ranges from
10 to 25%. For some torrents (e.g. Illgraben in Switzerland and Boscodon in France),
debris flows can propagate over gentle slopes (of
<
5%). In volcanic soil areas, it has
been also demonstrated that lahars (debris flows involving water-ash mixtures) can
propagate over very slight slopes (
<
1%) [MAJ 96].
For some debris flows, constant deposition occurs all along the channel and forms
levees on the lateral boundaries of the torrent. Depending on the size distribution of
