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Influence of the summer 2015 heatwave on rock slope failures

 

In the course of summer 2015 an exceptionally large number of rock slope failures was reported in the Swiss Alps, with a concentration in permafrost regions above 2500 m asl. Their dimensions varied from a few hundred to several tens of thousands of cubic metres. Most of these failures occurred near the rock surface at depths influenced seasonal temperature variations in the active layer. There were no large, deep-seated events with volumes of 100 000 m3 and more.

Felssturz in der Westflanke des Piz Cambrena
Figure 1: Rock slope failure in the western flank Piz Cambrena at 3270 m asl at 06.50 a.m. on 27.7.2015. The detachment zone is 108 m high and 45 m wide. Ice and water are visible in upper parts of the detachment zone. The volume was estimated to be 5000 m3 on the basis of a terrestrial laser scan. (Photo: Copyright D. Hunziker, REGA)

Triggering factors: high air temperatures and water

Air temperatures were well above average (2.5 °C higher than the 1981–2010 mean) and the 0 °C isotherm was above 4000 m asl for around 6 weeks. In contrast to the particularly hot and dry summer 2003 during which widespread rock fall activity was also registered, thunderstorms and precipitation occurred at high elevations during the recent heat wave. In addition to permafrost ice, water was often visible in the detachment scars (Figure 1). It is probable that the combination of high air temperatures warming the rock and ice in cracks and sporadic influxes of water into rock discontinuities led to failure through temperature-related strength loss and hydrostatic pressure. Seismic activity can be ruled out as a triggering factor, as none of the events was preceded by an earthquake.

Peak of activity at the beginning of August

The main reported rock fall activity started at the end of July 2015, after air temperatures had been exceptionally high for a month. On July 28th, a shallow rock slope failure with a large surface area occurred in the West flank of Piz Cambrena (Grisons) – releasing 5000 m3 of rock, which corresponds to the volume of about five houses. A series of events followed in various regions of the Swiss Alps, with a peak of activity during the first fortnight of August (Figure 2). The largest incident registered occurred on September 2nd at 3400 m asl in the western flank of the Grande Dent de Veisivi (Valais), with an estimated volume of 80 000 m3. The events took place at all hours of the day or night and in all aspects, except South. Rockfalls of several cubic metres were observed on a daily basis.

Rock slope failures were reported to SLF by mountain guides and mountaineers, hut wardens, helicopter pilots and regional hazard managers. The observations, which often included photographs and films were reported online via www.slf.ch or www.gipfelbuch.ch, by email or SMS. The data are registered in the SLF rockfall database, which contains several hundred events from 1714 onwards.

Verteilung der Jahreszeiten und Volumen der Felssturzereignisse aus dem Hochgebirge im Jahr 2015

Figure 2: Seasonal distribution and volume of rock slope failures in high mountain regions in 2015 (filled symbols). For comparison, the open symbols show the distribution of all other events in the data base. The thick light grey line in the background shows the approximate permafrost limit for steep and snow free rock walls in the Alps, based on measured data and published values.

Large rock slope failures can occur in winter

Analysis of this data shows that whereas in high and cold regions, small and medium events tend to occur mainly in the summer months, the large ones can occur at any time of year (Figure 3). Recent examples of large events in winter include the Bergsturz at Pizzo Cengalo (Grisons) in December 2011 with a volume of around 1.5 million m3 and the collapse of a rock pillar with a volume of 150'000 m3 at Piz Kesch (Grisons) in February 2014. Although these failures were mainly controlled by the structure of the rock mass and its discontinuities, permafrost related processes probably had an influence too. It is possible that another large event may occur in winter 2015/2016, as large rock masses have a delayed reaction to temperature changes.

Felsstürze mit einem Volumen von über 100
Figure 3: Rock slope failures with volumes exceeding 100 000 m3, distributed according to elevation and month of occurrence (1714-2015).

Effects of the 2015 heat wave not yet known

SLF collaborates closely with the Swiss Permafrost Monitoring Network PERMOS (www.permos.ch), which systematically collects permafrost field data such as ground temperature, active layer thickness, ice content and slope deformation. 9 of the 30 SLF boreholes are part of the PERMOS network. The data allow to describe the state of and changes to mountain permafrost in the Swiss Alps. They also provide useful information on the state of permafrost in rock fall detachment areas. Since there is a time lag of around 6 months for the heat from the summer months to reach depths of ca. 10 m and maximum active layer depths are typically reached in late autumn, the main effects of summer 2015 on the permafrost are still developing. It is therefore still too early to determine how the heat wave will have influenced the permafrost. First results are expected at the end of the year.