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Rockfalls and climate in the permafrost-affected rockwalls of the Mont Blanc massif L. Ravanel and P. Deline. EGU 11, Vienna, Austria, 03-08 April 2011. Punta Thurwieser, 2002 - PowerPoint PPT Presentation
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Rockfalls and climate in the permafrost-affected rockwalls
of the Mont Blanc massif
L. Ravanel and P. Deline
EGU 11, Vienna, Austria, 03-08 April 2011
Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna
► Many smaller rockfalls during the 2003 summer heatwave in the Alps
B.
Ser
gent
Les Drus, August 2003 V = 6 000 m3
Context: many rockfalls in permafrost-affected rockwalls in the last two decades
► Large recent rock falls/avalanches in high mountain areas worldwide
Punta Thurwieser, 2002V = 2.5 M m3
Fos
sati
Increase in the number of rockfalls: real or perceived?
What correlation between rockfalls and global warming?
1
Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna
Study area: the Mont Blanc massif
40 km
14 km
Seen from the West
North face of the Grandes Jorasses (4208 m a.s.l.)
Aiguille du Midi (3842 m a.s.l.)
► Large permafrost area
► Steep granitic slopes
► Very active morphodynamics
► Good accessibility
► Important vulnerabilities
2
1863 2009
► Overview: Identify morphological and colour changes and date the corresponding rockfalls in two areas of the massif (Drus, Aig. de Chamonix)
► Material: ~ 150 photos
West face of the Grands Charmoz (3445 m a.s.l.)
► Period: End of the Little Ice Age - 2009 = 150 years
M1: Comparison of photographs
Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna
3 methods to collect and process historical and current data on rockfalls
► Results: 50 rockfalls (500 to 265 000 m3)
3
Bisson brothers, 1862
Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna
► Period: 2003
M2: Analysis of a satellite image
► Results: 182 rockfalls (100 to 45 000 m3)
► Overview: Rockfalls identified in the entire massif from their supraglacial deposits
► Material: SPOT-5 image 051/257 taken at the end of the 2003 Summer heatwave
4
Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna
M3: Network of observers
► Results: 139 rockfalls (100 to 33 000 m3)
5
► Period: 2007 to 2009 (2010 data not already processed)
► Overview: A network of observers allows to survey rockfalls in the central area (57%) of the massif
► Material: Data from observations checked and completed on the field during each fall
North face of Les Droites (4000 m a.s.l.)
Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna
Relationship between climate and rockfalls: at a century scale
+ 0.23°C / decade
Ch
amo
nix,
104
0 m
a.s
.l.,
dat
a M
étéo
Fra
nce
+ 0.20°C / decade
+ 1.4 R. / decade
M1: Comparison of photographs (50 R.)
R. have various frequencies
End LIA – mid-20th c.: great stability
Strong activity since two decades
Concomitance between R. and hot periods
6
Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna
Relationship between climate and rockfalls: at a year scale
M1: Comparison of photographs (50 R.)
MAAT > mean 1951-80
Summer period
Particularly hot or canicular period
M2: Analysis of a satellite image (182 R.)
7
0°C isotherm
Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna
M3: Network of observers (139 R.) – The example of 2009 (72 R.)
Aiguille du Midi, 3842 m a.s.l.
Data: Météo-France
Thermal factor: major role in rockfall triggering
Concomitance between occurrence of rockfalls and warmest periods
8
3000
3100
3200
3300
3400
3500
3600
3700
Total N S Total N S Total N S Total N S
Alti
tud
e (
m a
.s.l.
)
2003 2007 2008 2009
► MAAT (°C) ► Mean scar altitude (m a.s.l.)
14
14,5
15
15,5
1616,5
1717,5
18
18,5
3300
3400
3500
3600
3700
3800
3900
4000
4100
4200
3100
3150
3200
3250
3300
3350
3400
3450
3500
2003 2007 2008 20097,6
7,7
7,8
7,9
8
8,1
8,2
8,3
2003 2007 2008 2009 2003 2007 2008 20092003 2007 2008 2009
► Summer mean (°C)
Chamonix Chamonix
► Uppermost scar (m a.s.l.)
Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna
Rockfall altitude as indicator of climate conditions
255 m 175 m
235 m320 m
9
The relationship between temperature and altitude confirms the role of climate in rockfall triggering
Rockfalls and climate in the Mont Blanc massif – L. Ravanel and P. Deline – EGU 2011, Vienna
Conclusions
10
► Hypothesis: global warming is affecting the stability of rock mountain via the permafrost degradation
3 methods of rockfall identification in the Mont Blanc massif
nearly 380 surveyed rockfalls
Permafrost degradation = probably the main triggering factor
IPCC, 2007
Strong climatic (thermal) control on the rockfall triggering
degradation measured and consistent with global warming
► Now almost all the rockfalls occurred in the context of permafrost (according to the TEBAL model)
