Alpine rock instability events and mountain permafrost

Rockfalls, rock slides and rock avalanches in high mountains

The terms rockfall, rock avalanche and rockslide are often used interchangeably. Different authors have proposed definitions based on volume thresholds, but the establishment of fixed boundaries can be tricky. Rockfall can be defined as the detachment of a mass of rock from a steep rock-wall, along discontinuities and/or through rock bridge breakage, and its free or bounding downslope movement under the influence of gravity^[1,2]. Usually, we use this term when the volume is limited, and there is little dynamic interaction between rock fragments, which interact mainly with the substrate. Rockslides involve a larger volume (up to 100,000 m³) and the blocks often break in smaller fragments as they travel down the slope. In both rockfall and rockslide, the blocks move downslope mainly by falling, bouncing and rolling. On the other hand, rock avalanches involve the disintegration of rock fragments to form a downslope rapidly flowing, granular mass demonstrating exceptionally high mobility^[3]. The size of these rock failures can vary from single boulders to several million cubic meters (e.g. the catastrophic failures of Triolet, 1717, and Randa, 1991).

Rock failures are widespread at any latitude and elevation, from mountain environments to coastal cliff, affecting the evolution and stability of different landscapes. They are among the most hazardous geomorphological processes, because of the velocity and the volume that these phenomena can involve. In mountainous areas, infrastructures, local populations and tourism can be directly threatened.

Rock slopes stability varies along time, especially in high mountains landscapes, in which the influencing external factors (e.g. hydrogeological conditions, and ice mass distribution) are changing during glacial cycles. Especially at high elevations, climatic factors are critical for rockfall stability (and others instability phenomena as rock avalanches or rock glaciers destabilisation), since they control weathering processes (mainly through temperature regime and precipitation activity) and because of rock-wall permafrost warming.

Rock instability triggering factors and the role of permafrost

The presence of damaged areas, weakness planes and discontinuities are essential to the development of instability. Focusing on high mountain environments, despite preconditioning factors as lithology and rock mass quality^[4], the main preparatory and triggering factors have been recognized in: seismic activity^[5,6], pressure release following glacier retreat^[7], glacial erosion, including valley steepening, deepening and undercutting^[4], and climate change, also involving permafrost degradation^[8,9].

Permafrost is ground, both soil and rock, which has been frozen (temperature below 0°C) for two or more years^[10]. Its climatically-driven degradation is considered nowadays one of the primary mechanisms in which climate is influencing slope stability^[8]. It can involve ground ice temperature rising, ice-content modification or permafrost disappearance, especially at lower altitudes^[11]. Several studies have highlighted the relationship between rockfall and permafrost degradation, especially in marginal permafrost conditions, albeit the level of dependence is still not clear and the role of other factors (e.g. rock mass quality) cannot be neglected. For instance, among the events occurred during the last century in the European Alps, the Brenva Glacier rock avalanches of 1920 and 1997 in the Aosta Valley (Italy), Cresta Guzza (Val Malenco, Italy) in 2013 (probably reported here for the first time, see details below and Figure 3), the recent instability in the Pizzo Cengalo, in 2012 and 2017, can be related to warm permafrost.

Glacier retreat and changes in permafrost, as related to present atmospheric warming, may destabilise over-steepened rock walls and accelerate cliff retreat in high mountain areas^[12,13]. Ground ice melt, due to temperature increase and related changes in the groundwater regimes, influences the stability of rock walls located within the permafrost altitude belt, i.e. the altitude at which permafrost presence is confirmed^[14,15]. Permafrost can affect rock walls stability through different processes, often acting together, as shear strength reduction due to interstitial ice melting, loss of ice-rock interlocking, increase in rock damage and fracture propagation due to freeze and thaw cycles or frost-cracking processes, changes in hydraulic conductivity and pore water pressure^[16,17]. The presence of permafrost in the affected rock-walls is commonly evidenced by ice directly observable in more than a quarter of the rockfall scars analysed, see an example in Figure 1^[13]. Climatic oscillations have a strong influence on all the processes mentioned above, as they are strictly related to temperature-driven ice dynamics and water availability.

In the last decades, an increased pattern of rockfall activity, on the entire Alpine arc has been observed^[9]. At the same time, permafrost temperatures are showing an upward trend in the Alps^[18], possibly leading to a growth of permafrost-related slope instabilities. Ravanel et al. (2017) explored the relation between increased temperatures and number of rockfalls in the Mount Blanc massif. The yearly number of rockfalls showed a strong increase (three to four times the average) in 2003 and 2015, years characterized by a summer heatwave.

Historical and recent events

One of the first recorded and documented event (Table 1) dates in 1717^[19] at Triolet. During the night of 12-13 September, a massive block of ice-clad rock plunged onto the Triolet glacier from the crest of the towering Mount Blanc massif in northwestern Italy. Travellers accounts reported the event for more than a century after, due to the extensive damage and life losses it caused.

On 18^th of September 2004 a rock avalanche of about 3 million m³ detached from the South-East flank of the Punta Thurwieser ridge (Italian Central Alps). Global warming has been recognised to have influenced this failure, namely through permafrost degradation and changes in glacier extension^[20,21]. In this case, the runout was unusually long and rapid (Figure 2), mainly due to the propagation path, partly involving the Vedretta dello Zebrù glacier, and to the involved material, including rock, debris, ice and snow.

During 3^rd March 2013, a small rockfall has interested Cresta Guzza in the Bernina Range, northern Italy. The rockfall, probably widely unknown, has been only documented by G. Neri and Dr R. Scotti and we can show you a photo of it in Figure 3. This not-well documented rockfall highlights one of the challenges related to these events: the majority of the rockfalls often affect remote areas, and they are only recorded when they threaten touristic routes or infrastructures. Which means that rockfalls may also be occurring at a higher rate, but we don’t have information about them, thus making it harder to forecast or prevent.

More recently, on 23^rd August 2017, a rock failure, of about 4 million m³, occurred on the north face of Pizzo Cengalo (Figure 4), upper Val Bondasca (lateral valley of the Bregaglia Valley, on the Swiss Alps). The onset of the event and the subsequent rock avalanche was recorded on video. The rock avalanche then travelled on the Cengalo glacier, turning into a massive debris flow when it reached the Bondasca river. In 2012 already, a smaller rockfall event took place at Pizzo Cengalo and deposited 1.5 million m³ debris at the foot of the mountain.

Finally, one of the last recorded events occurred at Flüela Wisshorn (Swiss Alps) on 18^th March 2019 (Figure 5). A rockslide, involving about 250,000 m³ of material detached from the northwest flank of the mountain, video here. The detached rock travelled across a very famous ski tour route, but no one was injured, only thanks to the timing of the event, which occurred shortly after midnight. According to a preliminary photo interpretation, the failure surface suggests the presence of preexistent discontinuities and some breakage along the intact rock. However, more accurate investigations are still needed.

To conclude, we summarise the main known rockfall events along the Alps at high altitude in Table 1. However, as we said before many more events may have occurred that we are not aware of and thus we believe this list may not be complete. If someone is aware of other cases, we will be glad to add them! Just leave a comment below, and we will come back to you.

Table 1 – major rockfall event at high altitude reported on the Alps.

Name	Location	Date	Altitude [m asl]	Volume [106 m3]
Triolet	I	1717	3600	16-20
Fletschhorn	CH	1901	3615	0.8
Brenva I	I	1920	4200	2-3
Felik	I	1936	3585	0.2
Jungfrau	CH	1937	3800	0.15
Matterhorn	I	1943	4150	0.24
Miage I	I	1945	3050	0.3
Becca di Luseney	I	1952	3150	11.0
Druesberg	CH	1987	2100	0.07
Tschierva	CH	1988	3280	0.3
Piz Scerscen	CH	1988	3750	–
Randa	CH	1991	2300	30
Miage II	I	1991	3000	0.3
Zuetribistock	CH	1996	2250	1.1
Brenva II	I	1997	3725	2
Matternberg	CH	1996	2720	0.1
Zugspitze	D	2001	2630	0.03
Monte Rosa	I	2001	3100	0.005
Gruben	CH	2002	3520	0.1
Aiguille du Diable	FR	2003	4114	0.020
Pointe du Domino	FR	2003	3648	0.025
Thurwieser I	I	2004	3658	2.5-3
Thurwieser II	I	2010	3658	0.1
Pizzo Cengalo	CH	2012	≈ 2500	1.5
Cresta Guzza	I	2013	≈ 3600	–
Piz Kesch	CH	2014	≈ 3400	0.15
Punta Tre Amici	I	2015	≈ 3600	0.020
Aiguille du Tacul	FR	2015	4248	0.016
Tour Ronde I	FR	2015	3792	0.015
Tour Ronde II	FR	2015	3792	0.002
Aiguille du Plan	FR	2015	3673	0.012
Aiguille du Midi	FR	2015	3842	0.012
Petit Dru – Bonatti	I	2015	≈ 3700	–
Pizzo Cengalo	CH	2017	≈ 2500	4
Flüela Wisshorn	CH	2019	≈ 3000	0.25

 

About the Authors:

Stefano Alberti is a postdoc research fellow at Politecnico di Milano, Italy, and a landslide and geomechanics specialist. He obtained his PhD in Earth Sciences, curriculum in Engineering Geology, at Università degli Studi di Milano-Bicocca, Italy, in collaboration with the Kyoto University. Active collaborator of the Regional Glacial Survey of Lombardy (SGL). He is interested in the interplay among geological structure, the thermo-hydro-mechanical behaviour of rocks, slope instability processes on different spatial and temporal scales and their geomorphological, geohazards and engineering aspects. His research focuses on mechanisms and long-term evolution of rock slope deformation; monitoring and modelling of large landslides (e.g. MPM); laboratory rock deformation; geomorphological aspects in periglacial and glacial environments (e.g. rock glacier and permafrost distribution); rock fall modelling and protection.

Margherita Cecilia Spreafico is a postdoc researcher at the Università degli Studi di Milano-Bicocca, Italy, working on the long-term activity, damage and collapse potential of large slow-moving landslides in rock. She obtained her PhD in Civil, Environmental and Materials Engineering at the Università degli studi di Bologna, Italy, in collaboration with the Simon Fraser University in Vancouver, Canada. Her research interests focus on large landslides mechanisms, kinematics and dynamics; numerical modelling of complex landslides; factors predisposing and controlling slope failures; rock mechanics, including rock mass characterisation from terrestrial laser scanner and photogrammetric data; discrete fracture networks and rock bridges; the influence of the groundwater flow on slope stability

 

 

 

REFERENCES

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Post edited by: Valeria Cigala and Gabriele Amato