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EGU 11, Vienna, Austria, 03-08 April 2011

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

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EGU 11, Vienna, Austria, 03-08 April 2011

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  1. 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

  2. Punta Thurwieser, 2002 V = 2.5 M m3 • Large recent rock falls/avalanches in high mountain areas worldwide Fossati Les Drus, August 2003 V = 6 000 m3 • Many smaller rockfalls during the 2003 summer heatwave in the Alps B. Sergent Increase in the number of rockfalls: real or perceived? What correlation between rockfalls and global warming? Context:many rockfalls in permafrost-affected rockwalls in the last two decades 1 Rockfalls and climate in the Mont Blanc massif– L. Ravanel and P. Deline – EGU 2011, Vienna

  3. Large permafrost area • Steep granitic slopes • Very active morphodynamics • Good accessibility 40 km • Important vulnerabilities 14 km Seen from the West Aiguille du Midi (3842 m a.s.l.) North face of the Grandes Jorasses (4208 m a.s.l.) Study area: the Mont Blanc massif 2 Rockfalls and climate in the Mont Blanc massif– L. Ravanel and P. Deline – EGU 2011, Vienna

  4. M1: Comparison of photographs • Period: End of the Little Ice Age - 2009 = 150 years • Overview: Identify morphological and colour changes and date the corresponding rockfalls in two areas of the massif (Drus, Aig. de Chamonix) • Material:~ 150 photos 1863 2009 West face of the Grands Charmoz (3445 m a.s.l.) 3 methods to collect and process historical and current data on rockfalls • Results: 50 rockfalls (500 to 265 000 m3) Bisson brothers, 1862 3 Rockfalls and climate in the Mont Blanc massif– L. Ravanel and P. Deline – EGU 2011, Vienna

  5. 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 M2: Analysis of a satellite image • Period: 2003 • Results: 182 rockfalls (100 to 45 000 m3) 4 Rockfalls and climate in the Mont Blanc massif– L. Ravanel and P. Deline – EGU 2011, Vienna

  6. 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.) M3: Network of observers • Period: 2007 to 2009 (2010 data not already processed) • Results: 139 rockfalls (100 to 33 000 m3) 5 Rockfalls and climate in the Mont Blanc massif– L. Ravanel and P. Deline – EGU 2011, Vienna

  7. 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 Relationship between climate and rockfalls: at a century scale + 0.23°C / decade + 0.20°C / decade Chamonix, 1040 m a.s.l., data Météo France + 1.4 R. / decade 6 Rockfalls and climate in the Mont Blanc massif– L. Ravanel and P. Deline – EGU 2011, Vienna

  8. M1: Comparison of photographs (50 R.) MAAT > mean 1951-80 Summer period Particularly hot or canicular period Relationship between climate and rockfalls: at a year scale M2: Analysis of a satellite image (182 R.) 0°C isotherm 7 Rockfalls and climate in the Mont Blanc massif– L. Ravanel and P. Deline – EGU 2011, Vienna

  9. M3: Network of observers (139 R.) – The example of 2009 (72 R.) Concomitance between occurrence of rockfalls and warmest periods Thermal factor: major role in rockfall triggering Data: Météo-France Aiguille du Midi, 3842 m a.s.l. 8 Rockfalls and climate in the Mont Blanc massif– L. Ravanel and P. Deline – EGU 2011, Vienna

  10. MAAT (°C) • Summer mean (°C) • Mean scar altitude (m a.s.l.) • Uppermost scar (m a.s.l.) 8,3 Chamonix 18,5 Chamonix 3500 4200 18 4100 3450 8,2 17,5 4000 3400 8,1 17 3900 3350 8 3800 16,5 3300 16 3700 7,9 3250 15,5 3600 7,8 3200 15 3500 7,7 14,5 3150 3400 3100 3300 14 7,6 2003 2007 2008 2009 2003 2007 2008 2009 2003 2007 2008 2009 2003 2007 2008 2009 2003 2007 2008 2009 3700 3600 3500 320 m 235 m 3400 Altitude (m a.s.l.) 3300 175 m 255 m 3200 The relationship between temperature and altitude confirms the role of climate in rockfall triggering 3100 3000 Total N S Total N STotal N STotal N S Rockfall altitude as indicator of climate conditions 9 Rockfalls and climate in the Mont Blanc massif– L. Ravanel and P. Deline – EGU 2011, Vienna

  11. 3 methods of rockfall identification in the Mont Blanc massif nearly 380 surveyed rockfalls Strong climatic (thermal) control on the rockfall triggering • Now almost all the rockfalls occurred in the context of permafrost (according to the TEBAL model) degradation measured and consistent with global warming Permafrost degradation = probably the main triggering factor IPCC, 2007 Conclusions • Hypothesis:global warming is affecting the stability of rock mountain via the permafrost degradation 10 Rockfalls and climate in the Mont Blanc massif– L. Ravanel and P. Deline – EGU 2011, Vienna

  12. Thanks! Ludovic.Ravanel@univ-savoie.fr

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