implications for permafrost degradation and slope instability in the ...

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R. P. Daanen et al.: Rapid movement of frozen debris-lobes 1527
Temperature ( o C)
Temperature ( o C)
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9/1/08
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1910
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1/1/09
3/1/09
5/1/09
SNOTEL, Air
DG-A, Air
Surface
0.5 m
1.8 m
7/1/09
9/1/09
11/1/09
1/1/10
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Movement of frozen debris-lobes can be observed in the
form of cracks in the surface of these features. These cracks
are up to 25 cm wide, several metres deep and stretch over
tens of metres laterally, and are often responsible for split
tree trunks. Along the terminus of these features, trees are
commonly overrun as the frozen debris-lobe advances. We
observed live trees being pushed over and partially covered
by flowing mud (summer) and by frozen debris slabs (winter).
Frozen debris slabs slide over weak zones, and are observed
during spring protruding out from the surface near
steep slopes. In addition, frozen soil and debris slabs buckle
Degree Days ( o C-day)
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in front of the frozen debris-lobes, forming cavities that collapse
upon spring thaw. This formation seems to be the result
of the feature pushing on the frozen soils in front of the lobe.
Active layer detachment slides and retrogressive thaw
slumps are also observed on these features. Typically these
features mobilize the sediments and accelerate the alluviation
processes. Mobilization of the rocks was also observed
from a steep section of the lobe terminus.
4.2 Ground thermal regime
To understand changes in ground temperature, we collected
both ground and air temperatures on FDL-A (Fig. 5). The
2008–2009 mean annual air temperature is −5.1 ◦ C and the
mean annual ground temperature at a depth of 0.5 m in mineral
soil is −0.3 ◦ C. We compared these data to those from
the Coldfoot Snow Telemetry (SNOTEL) site, which corre-
Figure 5.Air and soil temperatures (surface, 0.5, and 1.8 m) for FDL-A.Air temperatures from lates strongly with a 2008–2009 mean annual air temperature
the SNOTEL
Fig.
weather
5. Air
station
and soil
in Coldfoot
temperatures
are included
(surface,
for comparison.
0.5 and 1.8 m) for FDL-
A.Air temperatures from the SNOTEL weather station in Coldfoot
are included for comparison.
of −7.7
0
2000
-1
-2
Air Temperature (SNAP)
MAAT (Coldfoot)
MAAT (Wiseman)
1000
-3
TDD (Coldfoot)
-4
FDD (Coldfoot)
0
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-6
-1000
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Figure 6.Mean annual air temperature near FDL-A.Reanalysis data is from SNAP; measured
Fig. 6. Mean annual air temperature near FDL-A. Reanalysis data is
data was taken from the SNOTEL site near Coldfoot (60 km south of FDL-A) and near Wiseman
from SNAP; measured data was taken from the SNOTEL site near
(50 km south of FDL-A) (National Water and Climate Center, part of the Natural Resources
Coldfoot (60 km south of FDL-A) and near Wiseman (50 km south
Conservation service); thawing and freezing degree days are for Coldfoot.
of FDL-A) (National Water and Climate Center, part of the Natural
Resources Conservation service); thawing and freezing degree days
are for Coldfoot.
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◦C. The difference of 2.6 ◦C is most likely due to atmospheric
inversion with FDL-A 200 metres elevated over
the valley floor where the SNOTEL site is located. Longterm
changes in ground temperature are mostly a function
of changes in mean annual air temperature. We compared
the short record for mean annual air temperature in the region
to climate reanalysis developed by the Scenarios Network
for Alaska Planning (SNAP) (Fig. 6). This dataset
was downscaled from multiple global circulation models
(http://www.snap.uaf.edu). The short-term trend of decreasing
mean annual air temperature measured near Coldfoot,
Alaska appears contradictory to the regional warming trend
in the long-term record and permafrost temperatures (Fig. 6).
However, the trend in thawing degree days since 2001 indicates
a warming trend for the region in summer, in spite of
relatively cold winters.
Although the soil temperature dataset from FDL-A only
spans two years, it provides some insights into the ground
thermal regime. Soil temperatures collected on FDL-A indicate
an active layer at least 1.8 m deep (see Fig. 5). Movement
of FDL’s causes debris exposure to the atmosphere,
whereas nearby stable areas typically bear continuous cover
of moss and tussocks, resulting in colder soil conditions. Additionally,
the surface on these features is also much better
drained, reducing the thermal offset.
4.3 DGPS surveys
The results of the marker pin survey on FDL-A with a DGPS
demonstrated 1.3 m of movement during four months between
April and August 2008, which indicates an average
rate slightly greater than 1 cm day −1 . Repeat measurements
in 2009 also showed a similar daily movement rate.
A survey of the entire feature during 2009 is presented in
Fig. 2b. The DGPS was carried in a backpack while climbing
over the feature. These data provide a snapshot of the
geometry that can be compared with future datasets.
www.nat-hazards-earth-syst-sci.net/12/1521/2012/ Nat. Hazards Earth Syst. Sci., 12, 1521–1537, 2012