Permafrost

The circulation of air is driven by the contrast in air temperature between the interior of the
porous terrain and the surroundings. Indeed, the ventilating flow is ascending when the inside
air is warmer – and thus lighter – than the surrounding air. The air stream is descending when
the inside air is colder than the outer. As a consequence, the lower part of the ventilated terrain
experiences a strong negative thermal anomaly comparing to the mean annual air temperature.
The process is thus generating the occurrence of sporadic islands of permafrost at the bottom of
talus slopes and in relict rock glaciers located far below the regional lower limit of mountain
permafrost. Simultaneously, it may conduct to the absence of permafrost in the upper part of
many talus slopes located within the discontinuous permafrost belt in a mountain environment.
The proposed communication will focus on the measurement strategy – especially based
on ground surface temperature mapping and monitoring – that has been developed and applied
a) to detect the occurrence of the mechanism, b) to map the areas affected by the ventilation
process, c) to improve the comprehension of the mechanism and the controlling factors. About
thirty sites have been investigated in the Swiss mountains since 1995.
Main contributions obtained by means of mapping and monitoring techniques will be
presented, as for instance:
- BTS (bottom temperature of the snow cover) mapping has revealed to be a performing
tool – especially on talus slopes and relict rock glaciers, and if performed during or after a
period of cold weather – to detect areas with abnormally warm or cold temperatures that may be
related to the effect of an efficient ventilation process.
- On a 300 m long inactive rock glacier, an 11-year BTS series has allowed to evidence
the occurrence of a probable ventilation mechanism between the frontal part of the landform
and its rooting zone.
- Year-round continuous ground surface temperature monitoring along a longitudinal
transect has permitted on the one hand to define a typology of typical thermal “signature” for
ventilated talus slopes (with 5 main types) and, on another hand, to evidence the spatial
dissymmetry that may occurred between the ascending and descending modes of air circulation
in talus slopes.
- An electrical resistivity monitoring permitted to evidence, by relating seasonal changes
in ground resistivity to variations in temperature, that the air circulation process may induce
rapid temperature changes in the deeper layers of a talus slope by cold weather in winter time.
More generally, the importance of repeating measurements in time (monitoring) is
underlined in this study about internal air circulation. Indeed, many original results arise from
measurements that were repeated occasionally or acquired in an uninterrupted way during
several years. Unlike a simple prospection, monitoring has not only given a fixed image of the
investigated situation, but also an evolutionary insight, which has allowed a better appreciation
of it.
Keywords: air circulation, talus slope, rock glacier, ground surface temperature, BTS mapping,
monitoring
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