Geology Reference
In-Depth Information
Table 9.2 .
Detachment failure volumes, components of mass transfer, and long-term rates of
mass transfer, Fosheim Peninsula, Ellesmere Island, Nunavut, Canada, 1994-1995.
Year
Black Top Creek
Hot Weather
Big Slide Creek
(3.6 km
2
)
(7.6 km
2
)
of
Creek
study
(0.82 km
2
)
Detachment failure
1988
24-40
6.2-13.0
0.2-0.4
volume (10
3
m
3
km
−2
)
1998
0.5-0.8
0.2-0.5
1.4-3.2
Downslope mass transfer
1988
3600-3200
240-280
8.2-8.9
(10
3
Mg m km
−2
)
1998
8.5-6.9
3.6-3.8
190-240
Long-term rates of
downslope mass
transfer
(10
3
Mg m km
−2
) a
−1
40-84
5.9-12.0
17-38
Source: Lewkowicz and Harris (2005). Reproduced by permission of John Wiley & Sons Ltd.
Lewkowicz, 1990, 1992; Stangl et al., 1982). Often, failure is initiated during periods of
rapid spring thaw and/or following periods of summer precipitation. Failures may also
occur following the destruction of surface vegetation by forest fi re (Zoltai and Pettapiece,
1973) or following human-induced terrain disturbance (Heginbottom, 1973).
The magnitude and frequency of active-layer-detachment failures occurring in rela-
tively warm, discontinuous permafrost, and those occurring over cold, continuous perma-
frost terrain, has been analyzed by A. G. Lewkowicz and C. Harris (2005). They conclude
that rates of geomorphic work, when considered over a time period of 100-200 years, are
of the same order of magnitude. However, pre-conditioning of the active layer appears
important because it was found that rapid thaw did not necessarily initiate activity. Most
failures involved elements of both compression and translation of the soil mass. Table 9.2
summarizes some of the volumes and components of mass transfer that were calculated
and from which long-term rates of unit mass transfer were approximated. These data
illustrate not only the considerable amounts of material that can be moved by this process
but also its extreme variability.
9.5.2. Debris Flows, Slushfl ows, and Avalanches
In environments characterized by abundant snowfall, rapid mass movement may occur
through snow and debris avalanches. Such activity is especially favored in areas of
glacially-oversteepened slopes. Most avalanches start as snow avalanches which then pick
up varying amounts of rock debris en route, ultimately becoming debris avalanches or
slides. These are sometimes termed “dirty avalanches” (Rapp, 1960a, p. 127) or “mixed
avalanches” (Washburn, 1979, p. 193). More liquid forms such as slush avalanches and
mudfl ows occur where excessively wet (ripe) snow is subject to rapid thaw.
Debris fl ows (see Figure 9.2A, B) are rapid movements of masses of rock and/or debris,
gliding on slide planes causing considerable friction erosion (Rapp, 1985). They are char-
acterized by a distinct slide scar, and an eroded slide track terminating in a slide tongue
or lobe. Often, debris slides are heavily saturated with water, usually from melting snow,
and quickly become viscous debris fl ows creating lateral debris-fl ow levees that terminate
in debris fans.
The importance of debris fl ows and avalanches depends upon such factors as relief,
climate, and lithology. One of the earliest studies to distinguish between the processes
