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Increasing water content
t e r i a l
Mudflows
Landslides
Hyperconcentrated
flows
Debris
flows
Water
Stream flows
Stability
Solid
(Granular flows)
ial
Debris avalanches
Rock falls
Increasing solid fraction
Fig. 2.10 Classification of mass movements and flows on steep slopes as a function of solid debris fraction and material type.
(Source: Coussot & Meunier 1996; redrawn from Earth Science Reviews , Cousset, P. & Meunier, M. (1996) Recognition, classification
and mechanical description of debris flows, 40 , 209-27, with permission from Elsevier.)
its usefulness for understanding environmental
sedimentology is limited because most of the
important events are large-scale mass move-
ments which are highly episodic and inevitably
site specific. Examples include localized flash
floods, rock slides and volcanically triggered
debris flows.
activity and sedimentation (Case Study 2.2 Mount
St Helens); and sediment delivery in tectonically
unstable regions (Case Study 2.3 Waipaoa, East
Coast, New Zealand). Furthermore, these types
of events have common characteristics in terms
of very high rates of sediment production often
coupled to catastrophic or rapid releases of
water or runoff. This leads to a consideration of
sediment-water flows because once a large mass
of water, ice, snow or sediment is released on a
slope it will immediately begin to flow (Pierson
1988). Such flows can rapidly entrain further
material along their paths.
The behaviour of the flowing mass depends
on the ratio of sediment to water. A very broad
spectrum of flows is possible ranging from rela-
tively dry (non-liquefied) granular flows of rock
debris (debris avalanches) to large flood flows
involving mostly water (Fig. 2.10). Eisbacher
& Clague (1984) have classified destructive
mass movements of this kind into five groups
(Table 2.7):
2.2.4 Sediment transfer processes
The magnitude and frequency of different sedi-
ment transfer processes operating in mountain
environments vary according to the local or
regional tectonic and geological setting. It is
beyond the scope of this chapter to provide a
detailed review of the environmental sediment-
ology of volcanic, seismic and tectonic studies.
However, the coincidence between these variables
and mountain areas is undeniable. For example,
case studies later in this chapter clearly demon-
strate links between seismic activity and debris
flows (Case Study 2.1 Huascaran, Peru); volcanic
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