Réf. Huggel & al. 2010 - A

Référence bibliographique complète

HUGGEL, C., SALZMANN, N., ALLEN, S., CAPLAN-AUERBACH, J., FISCHER, L., HAEBERLI, W., LARSEN, C., SCHNEIDER, D. WESSELS, R. 2010. Recent and future warm extreme events and high-mountain slope stability. Philosophical transactions of the Royal Society A, Vol. 368, 2435–2459. [Etude en ligne]

Abstract: The number of large slope failures in some high-mountain regions such as the European Alps has increased during the past two to three decades. There is concern that recent climate change is driving this increase in slope failures, thus possibly further exacerbating the hazard in the future. Although the effects of a gradual temperature rise on glaciers and permafrost have been extensively studied, the impacts of short-term, unusually warm temperature increases on slope stability in high mountains remain largely unexplored. [The authors] describe several large slope failures in rock and ice in recent years in Alaska , New Zealand and the European Alps, and analyse weather patterns in the days and weeks before the failures. Although [they] did not find one general temperature pattern, all the failures were preceded by unusually warm periods; some happened immediately after temperatures suddenly dropped to freezing. [They] assessed the frequency of warm extremes in the future by analysing eight regional climate models from the recently completed European Union programme ENSEMBLES for the central Swiss Alps. The models show an increase in the higher frequency of high-temperature events for the period 2001–2050 compared with a 1951–2000 reference period. Warm events lasting 5, 10 and 30 days are projected to increase by about 1.5–4 times by 2050 and in some models by up to 10 times. Warm extremes can trigger large landslides in temperature-sensitive high mountains by enhancing the production of water by melt of snow and ice, and by rapid thaw. Although these processes reduce slope strength, they must be considered within the local geological, glaciological and topographic context of a slope.

Mots-clés

 

 

Organismes / Contact

 

(1) - Paramètre(s) atmosphérique(s) modifié(s)

(2) - Elément(s) du milieu impacté(s)

(3) - Type(s) d'aléa impacté(s)

(3) - Sous-type(s) d'aléa

 

 

 

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

 

 

 Monte Rosa, Alps [case study c.]

 

 

 

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

 

Observations

 

Modélisations

Future trends based on regional climate model (RCM) simulations:

The following analyses are based on the RCM time series with elevation adjustment and bias correction as described [below]. [The authors] studied eight RCM time series to identify periods with air temperature continuously exceeding a threshold of +5°C for periods of 5, 10 and 30 days, thus representing significant melting conditions. These thresholds are based on the case studies described above, where warm air-temperature anomalies of many days duration were observed prior to each failure. We analysed the change in frequency of these events for the period 2001–2050 when compared with the reference period 1951–2000.

Results from the eight models show a clear increase in the frequency of warm air-temperature events in the next several decades compared with the second half of the twentieth century. For a matrix comprising eight models and three event types, only one model produces a slight decrease (approx. 10%) of the 5 day events. The differences among the model outputs are large, but most models show increases in frequency of extreme events of about 1.5–4 times. Large increases in the frequency of warm extremes, by a factor of 8–10, are projected by three models. In two of these three cases, the increase is for 30 day events. On the other hand, five models show no 30 day extreme warm events, either for the past or the future.

Hypothèses

 

 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

Here, [the authors] consider whether unusually warm periods of a few days to a couple of weeks might increase in the future. […] For the present study, [they] analysed results for air temperature 2m above the ground from eight ENSEMBLES RCM simulations [van der Linden & Mitchell 2009]. [They] chose a mix of different RCMs, driven with different GCMs, to provide a representative selection [see details in the study]. Mean daily temperature results were analysed for anomalously warm temperature events both in the past and the future. The analysis is based on one grid box that represents the longitudes and latitudes of the Jungfrau region. The Jungfrau region was chosen based on the availability of long-term high-elevation observational time series (since 1958) from the Swiss Federal Institute of Meteorology and Climatology (MeteoSwiss), which enables performance analysis and de-biasing of the RCM simulations. Typically, measured air temperatures at high-altitude climate stations are highly correlated. The temperature records at the Jungfraujoch and Gornergrat stations have a correlation coefficient of 0.98 for the common period of record. Therefore, we assume that the Jungfraujoch data are representative also for conditions at or near Gornergrat (including Monte Rosa).

A horizontal resolution of 25 km, which is now the standard for many RCM simulations, is too coarse to represent the topography of a high-mountain region realistically. The selected grid box is referenced to an elevation of 2244m a.s.l., and therefore needed to be adjusted to the level of Jungfraujoch at 3580m a.s.l., applying a basic lapse-rate correction of 0.6°C100m−1 (as an average over the year; Rolland 2003) to adjust the air temperature of the RCM grid box with an elevation of 2244–3580 m, the elevation of the Jungfraujoch climate station. In addition to the elevation adjustment, [the authors] applied a bias correction to each RCM time series (Salzmann et al. 2007a,b). The bias relates to the difference between the mean annual air temperature observed at Jungfraujoch for the available time period 1960–2000 and the respective temperature of the RCM time series. The bias correction involves only one temperature value per RCM; different values for different seasons or months were not used.

 

(2) - Effets du changement climatique sur le milieu naturel

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 

 

(3) - Effets du changement climatique sur l'aléa

Reconstitutions

Monte Rosa case study: The east face of Monte Rosa extends from about 2200m to over 4600m a.s.l., and was the site of two spectacular avalanches in 2005 and 2007 (Fischer et al. 2006). […]. Studies based on sequential historical photographs have shown that the ice cover on the east face of the mountain changed a little during the twentieth century until about 1980, when it began to rapidly decrease (Haeberli et al. 2002; Fischer et al. submitted b). Slope instability involving both ice and rock increased around 1990 and has continued to the present (Fischer et al. 2006). Instability culminated in two large avalanches, one on 25 August 2005, and the other on 27 April 2007.

The 2005 event was a large ice avalanche (1.1 × 106 m3) that initiated from a steep glacier terminating at 3500m a.s.l. and reached the foot of the face, where a large supraglacial lake had formed in 2002, but had drained in 2003. Had the lake still existed, the avalanche would have generated a displacement wave with catastrophic consequences for the downstream community of Macugnaga. The avalanche occurred at night, which probably prevented injuries to tourists who often spend daylight hours on the pasture that was affected.

The 2007 event was a rock avalanche that detached from the exposed bedrock at approximately 4000m a.s.l. near the top of the east face of Monte Rosa. It involved about 0.3 × 106 m3 of rock that fell to the base of the slope, again impacting the area of the former supraglacial lake. […]

Weather in the days and weeks prior to the landslides was very different for the 2005 and 2007 slides. Several warm periods of 5–10 days duration occurred in June and July 2005. Temperatures rose to 5°C above the 1994–2009 average (of the Gornergrat station record). The warm periods were interrupted by temperature drops, with several freeze–thaw cycles during the 20 days before the failure. After the last freezing event 4 days before the landslide, temperatures again increased to 5°C on the day of failure. Much melt water was produced during the warm periods and possibly penetrated to the base of the steep glacier, lowering the strength at its contact with the underlying bedrock. The repeated cycles of melt and refreezing may have also destabilized the bedrock.

Temperatures in April 2007 were extraordinarily warm in central Europe, producing a spring heat wave. April temperatures at Jungfraujoch at 3580ma.s.l. were 5°C warmer than the mean of the previous 49 years. The temperature of −3.5°C, 1 day before the landslide, is in the 98th–99th percentile of the long-term April record (for Jungfraujoch available since 1958). The Jungfraujoch climate record is highly correlated to the record at the Gornergrat station, which is the nearest station above 3000m a.s.l. to the failure site, and which reveals interesting thermal patterns during the weeks before the failure. Exactly 1 month prior to the landslide, temperatures dropped to an estimated −24°C at the failure site, which is about 10°C lower than the long-term average. After this unusually cold period, temperature rose steadily to approximately −5°C near the date of the failure. However, radiation on the east face of Monte Rosa in April is high and cloud cover was generally low in April 2007. [The authors] thus infer that snow and ice melted at the surface in spite of the subfreezing air temperature. In addition, it is possible that thermal energy during particularly warm summer months in 2003 and 2006, when the 0◦C isotherm was above 4000m a.s.l., penetrated into bedrock some metres deep at the level of the rock-slope failure.

Discussion and conclusions: [The] analysis of temperature records days and weeks before several large high-mountain rock and ice avalanches is a start in documenting this little investigated aspect of slope instability. A clear predictive thermal trigger is scarcely discernable for all events. However, examination of the ensemble of temperature conditions prior to failure for our case studies suggests that some thermal patterns are repeated at locations as different and distant from one another as Alaska, New Zealand and the European Alps: (i) unusually warm temperatures over several days during the weeks or days before failure and (ii) sudden drops of temperatures, typically below freezing, after warm periods and hours to days before failure.

All the studied events had warm temperatures, far above freezing prior to failure, except the 2007 Monte Rosa landslide. In most of the cases, temperatures above freezing during summer months are not exceptional, although the observed temperatures were far above normal. At Mt Cook, for example, the peak temperature 3 days before failure was 8.5°C above the long-term average. Temperatures in the days before the April 2007 Monte Rosa rock slide were up to 4–5°C above 1 s.d. of the long-term record. Such unusually warm periods enhance melt of surface snow and ice. The water generated by melting can infiltrate rock slopes via fractures and joints, increasing hydrostatic pressures and thus reducing shear strength (Huggel et al. 2008a). […] Melt water can also penetrate to the base of steep glaciers and reduce their resistance to failure.

Temperature pattern (ii) has been inferred at the sites of the Mt Cook and the 2005 and 2008 Mt Steller events, as well as at the site of a rock slide in permafrost in the eastern Swiss Alps, not discussed here further (Fischer et al. submitted a). It did not occur, however, at the 2005 Monte Rosa ice avalanche and the 2007 Monte Rosa landslide. A sudden lowering of temperature may favour slope failure by refreezing the surface following infiltration of melt water into bedrock during the preceding warm period. Such ‘lock-off’ situations are difficult to quantify owing to a lack of on-site measurements with piezometers and other instruments, but have been invoked in similar conditions (Fischer et al. submitted a,b). In this context, we should also consider the importance of rainfall or melt of fresh snow as potential sources of infiltrating water and slope destabilization. Accurate measurements of precipitation at sites of high-mountain slope failures are rare and difficult to acquire because of the large spatial variation in precipitation in areas of high relief. The Gornergrat meteorological station is less than 10km from Monte Rosa, but [the authors] did not use its precipitation data for this study because precipitation is highly variable in this region (Machguth et al. 2006). […]

Another climate variable that was not examined in this study, but may have an important effect on surface warming and melting, is radiation. Short-wave radiation constitutes a major portion of the energy available for melt during summer and may be close to 200Wm−2 in alpine conditions (Oerlemans 2001). In several of the cases presented here, short-wave radiation may have played a role in melting snow and ice. For instance, air temperatures reached only −5◦C before the failure at the site of the 2007 Monte Rosa landslide, but clear-sky conditions resulted in high short-wave radiation that may have melted surface snow, with possible infiltration of water into the highly fractured bedrock. […]

Observations

 

Modélisations

 

Hypothèses

Based on an analysis of the ENSEMBLE RCM simulations, [the authors] have found that short periods with very high temperatures may increase 1.5–4 times in the next several decades compared with the 1951–2000 reference period. Short duration warm periods may produce a critical input of water into slopes. The projected increase in extended warm periods of up to 1 month is also of concern because such events can lead to substantial thermal perturbation of subsurface hydrology. […] Several aspects of the role of warm extremes in high-mountain slope stability remain unresolved, but this study hopefully will stimulate further discussion and research. As argued [in the study], not every warm extreme will trigger a large slope failure. However, [the authors] think that large slope failures will increase in temperature-sensitive, high-mountain areas as the number of warm extreme events increases.

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

Fréquence

Monte Rosa case study: Two meteorological stations at elevations greater than 3000m are near Monte Rosa—a station at Testa Grigia, Italy (3488m a.s.l.), 15km to the west; and the Swiss station at Gornergrat (3130m a.s.l.), 9km to the northwest. The Testa Grigia station has a temperature record for 1951–2000, but measurements unfortunately were not continued after 2000, while the station at Gonergrat came into operation only in 1994 and has been working properly since then. Based on these data, temperature extrapolations yield a MAAT of −5 to −6°C at the lower end of the ice avalanche failure zone, suggesting a cold glacier front, but probably polythermal to temperate conditions at some distance behind the front. Previous studies show that the lower limit of permafrost is at about 3000m a.s.l. on north- to northeast-facing slopes, and up to 500m higher on east-facing sections (Zgraggen 2005; Fischer et al. 2006; Huggel 2009). Estimates of MAGST for the 2007 rock-slope failure, based on data from the aforementioned meteorological stations and rock temperature loggers deployed on the east face (Zgraggen 2005) suggest temperatures of about −6°C. These conditions compare with those on Mt Steller south, but unlike Mt Steller, the Monte Rosa site was not thermally perturbed by the overlying glacier ice. A more likely destabilization factor is the enormous loss of ice at the base of the failure zone over the past 20 years with a volume of more than 20 × 106 m3, which probably caused significant changes to the stress and temperature fields (Fischer et al. submitted b). In addition, the dip of the foliation in the gneissic bedrock is parallel to the surface slope, adversely affecting slope stability.

Discussion and conclusions: […] [This] analysis concentrated on temperature aspects of large high-mountain slope failures, but this is only one component of a highly complex physical system that, in response to gradual and sudden changes in external and internal controls, produces a slope failure. Several geological factors, including structure and rock type, glaciation, permafrost, topography and seismicity, are important determinants of slope stability in this environment. It is fundamental in this context to consider the time scales involved in causative and trigger factors. […] Geology and topography are typical predisposing factors. The histories [of slopes prior to failure] are characterized by processes that gradually reduce their shear strength over periods of decades to millennia. Short-term events, operating over days to weeks, such as warm extremes, high-intensity rainfall or earthquakes, can rapidly reduce the strength of the slope. However, these events may not necessarily trigger failure, depending on their impact and the shear strength of the slope when they occur. Slope failure only occurs if the potential triggering event reduces the shear strength below a critical threshold. These events will trigger a landslide only if slope stability is already low and near the threshold of failure.

The concept of effective time scales is particularly important when considering bedrock permafrost and slope stability. Thermal perturbations resulting from twentieth century warming, for instance, has now penetrated to depths of a few decametres in high-rock slopes (Haeberli et al. 1997; Noetzli et al. 2007), whereas short warm extremes may have an effect at a few metres depth only one or several years later. Open fractures, however, can facilitate infiltration of water into rock slopes and thus contribute to a much more immediate effect on slope stability. […]

In this paper, [the authors] examine meteorological conditions leading up to several large rock and ice avalanches in south-central Alaska, the European Alps and the Southern Alps of New Zealand. [They] focus on the air-temperature history days and weeks before failure. Long-term ground surface and firn or ice temperatures, geology and topography are also considered, but are not the primary focus of the paper. Based on results from case studies, [they] define appropriate climatic indicators and use the recently completed ENSEMBLES regional climate model (RCM) runs for the central Alps to determine whether critically warm weather, which has the potential to trigger large slope failures, will occur more often in the future. For the analysis of meteorological and climatic conditions of case studies, [they] generally use reference time periods that vary according to the available climate records. For future projections, we consider the period 2001–2050, using the reference period of 1951–2000.

 

(4) - Remarques générales

Conclusive statements on the impacts of extreme weather events on slope stability in high mountains cannot be made at this time because research on this topic is not far advanced and because slope stability in high mountains is controlled not only by thermal conditions but also, and particularly, by geology, topography and hydrology, all of which are highly interconnected (Fischer & Huggel 2008). Also, other climate parameters, such as radiation, are important in complex, high-mountain topography (Salzmann et al. 2007a,b), and may play a role in triggering slope failure. […]

The landslides described in the case studies did not cause any major damage to people or infrastructure, partly owing to fortunate circumstances, partly owing to the remote location of the landslides. In densely populated and developed mountain regions such as the European Alps, however, serious consequences have to be considered from large slope failures. Cascading processes (e.g. landslides impacting natural or artificial lakes producing outburst floods) are of particular concern. With the Monte Rosa case study, it has been indicated that similar landslides as in 2005/2007 would probably have resulted in a major disaster had they occurred during the existence of a large glacier lake in 2002/2003.

 

(5) - Syntèses et préconisations

 

Références citées :

Fischer, L. & Huggel, C. 2008 Methodical design for stability assessments of permafrost-affected high-mountain rock walls. In Proc. 9th Int. Conf. on Permafrost, University of Alaska, Fairbanks (eds D. L. Kane & K. M. Hinkel), pp. 439–444.

Fischer, L., Kääb, A., Huggel, C. & Noetzli, J. 2006 Geology, glacier retreat and permafrost degradation as controlling factors of slope instabilities in a high-mountain rock wall, the Monte Rosa east face. Nat. Hazards Earth Syst. Sci. 6, 761–772.

Fischer, L., Amann, F., Moore, J. & Huggel, C. Submitted a The 1988 Tschierva rock avalanche (Piz Morteratsch, Switzerland): an integrated approach to periglacial rock slope stability assessment. Eng. Geol.

Fischer, L., Eisenbeiss, H., Kääb, A., Huggel, C. & Haeberli, W. Submitted b Detecting topographic changes in steep high-mountain flanks using combined repeat airborne LiDAR and aerial optical imagery—a case study on climate-induced hazards at Monte Rosa east face, Italian Alps. Permafrost Periglac. Process.

Haeberli, W., Wegmann, M. & Vonder Mühll, D. 1997 Slope stability problems related to glacier shrinkage and permafrost degradation in the Alps. Eclogae. Geol. 90, 407–414.

Haeberli, W., Käab, A., Paul, F., Chiarle, M., Mortara, G., Mazza, A. & Richardson, S. 2002 A surge-type movement at Ghiacciaio del Belvedere and a developing slope instability in the east face of Monte Rosa, Macunaga, Italian Alps. Norw. Geogr. Tidsskr. 56, 104–111.

Huggel, C. 2009 Recent extreme slope failures in glacial environments, effects of thermal perturbation. Quat. Sci. Rev. 28, 1119–1130.

Huggel, C., Caplan-Auerbach, J., Gruber, S., Molnia, B. & Wessels, R. 2008a The 2005 Mt. Steller, Alaska, rock–ice avalanche, a large slope failure in cold permafrost. In Proc. 9th Int.Conf. on Permafrost, University of Alaska, Fairbanks (eds D. L. Kane, & K. M. Hinkel). pp. 747–752.

Machguth, H., Eisen, O., Paul, F. & Hoelzle, M. 2006 Strong spatial variability of snow accumulation observed with helicopter-borne GPR on two adjacent Alpine glaciers. Geophys. Res. Lett. 33, L13503.

Noetzli, J., Gruber, S., Kohl, T., Salzmann, N. & Haeberli, W. 2007 Three-dimensional distribution and evolution of permafrost temperatures in idealized high-mountain topography. J. Geophys. Res. 112, F02S13.

Oerlemans, J. 2001 Glaciers and climate change. Lisse, The Netherlands: A.A. Balkema Publishers.

Salzmann, N., Frei, C., Vidale, P. & Hoelzle, M. 2007a The application of regional climate model output for the simulation of high-mountain permafrost scenarios. Global Planet. Change 56, 188–202.

Salzmann, N., Nötzli, J., Hauck, C., Gruber, S., Hoelzle, M. & Haeberli, W. 2007b Ground surface temperature scenarios in complex high-mountain topography based on regional climate model results. J. Geophys. Res. 112, F02S12.

van der Linden, P. & Mitchell, J. 2009 ENSEMBLES, climate change and its impacts, summary of research and results from the ENSEMBLES project. Met Office Hadley Centre, Exeter, UK.

Zgraggen, A. 2005 Measuring and modeling rock surface temperatures in the Monte Rosa East face. Master thesis, ETH Zurich University of Zurich, Zurich, Switzerland.