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Rock slope failure on Piz Kesch in February 2014: course of events and causes

 

In February 2014 a pillar of rock collapsed on Piz Kesch (canton Grisons) and deposited large boulders on the Porchabella Glacier. Nobody was injured and no infrastructure was damaged. As shown by the SLF rockfall database, an event of this nature is not unusual in mid-winter. Although major rock slope failures are known to occur at all times of the year in mountain permafrost, the reasons have not been extensively researched. Assessments of the causes of the rockfall on Piz Kesch were no more than speculative. Given that the event was not directly observed, both the exact timing of the pillar's collapse and the quantity of rock that crashed down the mountain were unknown. Furthermore, the Swiss Seismological Service did not record any tremors, which regularly accompany similar events.

Timing and course of events established

Together with geologists working at the ETH, the SLF has now investigated the causes of the rock slope failure on Piz Kesch in the context of a research project. On the basis of a layer of Saharan dust that had been deposited in the snow covering the deposit, the team headed by Marcia Phillips was able to establish that the collapse occurred during the first half of February. Using photographs taken before and after the event, and laser scan images, the researchers were also able to determine that approximately 150,000 m3 of rock had been released, which is the volumetric equivalent of around 150 houses.

 

A geological analysis of the pillar on Piz Kesch revealed that the rockfall was triggered when the rock bridges, which had been keeping the rock mass in equilibrium, fractured. It was unclear, however, whether the mass collapsed in one single event or a series of smaller slides. With the aid of the SLF computer program RAMMS, which simulates alpine mass movements, the researchers modelled a variety of scenarios for the rock slope failure on Piz Kesch. When they simulated the event as a debris flow, the program successfully reconstructed the kilometre-long deposit that was actually observed on the Porchabella Glacier. The researchers assume that snow and glacial ice were entrained by the loose rocks and melted during the course of the event. The resulting slurry of stone and water then flowed down the mountain with the consistency of a debris flow. This would explain the long, tongue-shaped deposit. The hypothesis is corroborated by the deep channels carved in the glacier by the rocks along their flow path.

Preparatory phase of several millennia

While investigating the rockfall on Piz Kesch, the researchers made an especially interesting discovery. The permafrost ice which had been inside the collapsed pillar of rock contained organic material which carbon dating showed to be approximately 6,000 years old. In other words, fractures were probably forming in the rock pillar as far back as 6,000 years ago – aided by the climate prevailing at the time. Temperatures then were about 0.7° C higher than now on average, the glaciers were less extensive, the winters colder, and the summers warmer. It is therefore reasonable to conclude that the process triggering a rock slope failure can develop over a period which spans several thousand years. The occurrence of a major rockfall not only depends on current conditions, but also on the interaction of geology and climate over several millennia.

On Piz Kesch, the Porchabella Glacier covered the pillar of rock several times over the course of the last millennia. This exposed the rock to varying degrees of loading. With the recent retreat of the glacier, a final supporting element ceased to exist. Ice wedging may be a possible explanation for the collapse of the pillar in February 2014. Winter conditions are ideal for this process. A similar sequence of processes developing over millennia and acting in conjunction with current climatic conditions is likely to be taking place in the permafrost in locations comparable to Piz Kesch, so that similar rock slope failures could occur elsewhere in future.