Geotechnical News • June 2019
31
GEOHAZARDS
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A methodology to assess rock glacier destabilization at the
regional scale: example from the French Alps.
M. Marcer, X. Bodin, A. Brenning
Prologue
Rock glaciers are creeping landforms
that are commonly found in mountain
ranges rich in permafrost. Although
their displacement rates normally
don’t exceed 1-2 m/y, in the past
decade several studies showed that
these values can be largely exceeded
if the landform undergoes a “desta-
bilization” phase (Roer et al, 2008;
Delaloye et al, 2013). The destabi-
lization process consists of a rapid
acceleration of the landform that may
last several years and causes the rock
glacier to reach displacement rates of
several tens of meters per year. The
acceleration is preceded by the rapid
development of surface features that
are typically found in rotational land-
slides such as crevasses and scarps
(Eriksen et al, 2018). The region of the
rock glacier downslope of these sur-
face features is the area that is rapidly
accelerating, while upslope areas show
undisturbed behaviour. Due to the
high displacement rates and surface
deconsolidation linked to fracturing,
destabilized rock glaciers may trigger
or precondition mass movements of
unexpected magnitude. It is important
to assess the occurrence of these land-
forms and integrate them into hazard
assessments in regions affected by
cryosphere processes.
Introduction
In the summer of 2006, a destabilized
rock glacier in the Southern French
Alps collapsed causing a landslide
of 250,000 m
3
(Bodin et al, 2016).
Although the landform was in a
remote area and did not cause dam-
age to human infrastructure, this rock
glacier was unknown to the local
authorities, highlighting an inadequate
knowledge of the spatial footprint and
characteristics of rock glaciers in the
region. As a consequence, the RTM
(National Environmental Protection
Agency), in collaboration with the
research laboratories PACTE and
EDYTEM, started a GIS mapping
effort at the national scale to inven-
tory all the rock glaciers in the French
Alps. This effort lasted until 2015
and resulted in the first rock glacier
inventory of the region. More the
3,000 rock glaciers were identified and
almost 500 of them were designated
as potentially creeping (Marcer et al,
2017).
This inventory unlocked the pos-
sibility of analyzing rock glacier
characteristics at a regional scale by
visual inspection using orthoimages.
Efforts focussed on the identifica-
tion of landforms that were in high
consequence settings such as those
topographically connected to human
activities and/or infrastructure. The
systematic inventory revealed that
there were several rock glaciers show-
ing geomorphological characteristics
that are typically linked to destabili-
zation processes, suggesting a high
incidence of the phenomenon. Hence,
the local research institutions focused
the efforts to better characterize this
phenomenon, as described in Marcer
et al (2019) from which this document
is adapted.
The characterization of rock glacier
destabilization in the French Alps was
done using an approach similar to
those in common landslide character-
ization practice: identification, model-
ling and susceptibility mapping (Goetz
et al, 2011). A systematic identifica-
tion of landforms showing evidence
of destabilization was undertaken to
obtain a basic assessment of the phe-
nomenon in the region. Destabilization
evidence was then used to understand
typical topo-climatic characteristics of
occurrence. This ultimately allowed
the authors to model the local suscep-
tibility to destabilization in relation to
the terrain characteristics. Each of the
three steps is described in the follow-
ing sections.
