Environmental Engineering Reference
In-Depth Information
unknown. It is worth noting that in the field, debris flows can also form from landslides
[IVE 97]. In this case, the transformation mechanisms are similar to soil liquefaction
processes (rapid creep of saturated soils). In the following, we will tackle the problem
of debris flows, which are intrinsic to mountain torrents and steep slopes. Other
chapters in this topic deal with bed load transport.
1.3. Initiation, motion and effects of debris flow
1.3.1. Initiation
The torrential activity of a watershed depends on many parameters. Debris flows
are common in some areas and uncommon in others. In areas prone to debris flow
formation, their frequency also varies. In some watersheds, several debris flows occur
each year, while for other torrents they are rare. Conditions for initiation of most debris
flows usually include [COE 08, IVE 97, MON 09, STA 98, TOG 97]:
- Steep slopes . In the Alps, slopes in excess of 70% are liable to surface erosion
(sediment transport induced by runoff) and landslides (soil failure leading to large
masses of saturated materials coming loose).
- Abundant supply of unconsolidated materials . Debris flows originate either from
the simultaneous contributions of many material sources or from a single source
(landslides):
- Slow and continuous erosive processes on slopes in the drainage basin form
deposits of materials in the torrent bed. Such deposits can be subsequently mobilized
during intense floods and then transform into debris flows. In this case, debris flows
originate as a slurry, primarily of water and fine particles, which erodes its channel
and grows in size. Presumably, instabilities in the bed load transport (such as those
observed in the laboratory) arise and enable debris flow initiation. Usually the volume
produced every year by erosion over the whole drainage basin is small and thus the
amount of sediment that can be involved by a single debris flow is limited ( < 10 5 m 3 ).
In the field, the absence of failure surfaces and the presence of rills in the drainage
basin are generally the evidence that a debris flow has picked up coarse materials
from the bed.
- Old ill-consolidated deposits (moraines, massive rockfall deposit, etc.) can
mobilize into landslides to form debris flows. In this case, the volume of materials
involved can be very large ( > 10 5 m 3 ) depending on the total volume made available
by the source. Likewise, certain soils (e.g. gypsum) are very liable to landslides and
can supply materials to debris flows. Presumably, initiation is due to a combination
of several mechanisms: rapid creep deformation, increase in pore pressure, increase
in load, erosion at the foot of the landsliding mass, etc. In the field, the presence of a
failure surface can clearly serve to identify the source of material.
- Large source of moisture . Most debris flows occur during or after heavy and
sustained rainfalls. In some cases, snowmelt can be sufficient to form debris flows.
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