The rock slide on Pizzo Cengalo and the immediately following debris flow to Bondo on August 23 were a very rare chain of natural events worldwide. Ice and water pressure caused the rock of Pizzo Cengalo to fall. A group of experts has analysed the event and drawn initial conclusions from it. The risk of further landslides and debris flows will persist in the coming years.
On August 23, 2017, more than three million cubic meters of rock fell from Pizzo Cengalo onto the glacier below. The rock slide removed, pulverized and partly melted a lot of ice in a matter of seconds. Together with water from the surrounding area, the released water set the fallen rock masses in motion and formed a stream of debris that flowed through Val Bondasca to the valley floor near Bondo. This interlinking of processes is very rarely observed worldwide, says Dr.
Jürg Schweizer, head of the WSL Institute for Snow and Avalanche Research SLF in Davos.Schweizer is part of a group of experts brought together by the canton of Grisons a few days after the rock slide and debris flow. She analysed the events, drew initial scientific conclusions and identified topics that should be researched in more detail as a result of the events of this late summer. These include the importance of thawing permafrost and water in rock cliffs, the role of glaciers in rock slides, and the development of water content in deposits from rock falls.
Large rock falls are usually foreshadowed
Measurements made it possible to know that a major event was imminent, says geologist Prof. Florian Amann. Due to the increased displacements, the mountain was under surveillance. The northeast flank, from which more than three million cubic meters of rock crashed, was very quiet in the days leading up to August 23. This is astonishing, as major landslides were usually announced by minor rock falls. More than one million cubic meters of rock are still in motion on Cengalo, underlines Amann. Based on the experiences at Pizzo Cengalo, it must be expected that further rock falls will occur.
The assessment of the current geological situation suggests that up to three million cubic meters can come out of the wall in the long term.
Risk of debris remains
Below Pizzo Cengalo, in Val Bondasca, there are around one and a half million cubic meters of rockfall material, which is mobilized by enough water and can penetrate as a debris flow as far as Bondo, explains debris removal expert Dr. Christian Tognacca. Smaller and larger debris flows must therefore be expected in the next few years in the event of heavy rainfall or renewed rock falls. In August 2017, as a result of a process chain, around 500,000 cubic meters of material reached Bondo in several debris flows. This is more than was assumed in 2012 when designing the existing protective structures, says Tognacca.
Even in future, an extreme event like 2017 would not be the basis for preparing hazard maps and measuring protective structures; it would be classified as a residual risk. The Canton of Grisons wants to learn from the Bondo events and also make this knowledge available to other regions of the Alps, underlines Councillor Dr. Mario Cavigelli. Such a group of experts was set up for the first time and had proved to be very effective as part of event management.
The aim is to prepare for future events even better. The municipality of Bregaglia is preparing for further events, explains parish president Anna Giacometti. The future protection concept is currently being developed together with experts from the canton. Until its implementation, the provisionally raised dams at Bondo, Spino and Promontogno remained in place. They would give villages and their inhabitants security.
The members of the expert group:
External experts
- Florian Amann, Prof. Dr., Chair of the Department of Engineering Geology and Hydrogeology, RWTH Aachen University
- Yves Bonanomi, geologist, head of Bonanomi AG - Geological Consultations, Igis
- Martin Funk, Prof. Dr., Head of the Department of Glaciology at the Laboratory of Hydraulic Engineering, Hydrology and Glaciology (VAW), ETH Zurich (in the expert group until 28.10.2017)
- Matthias Huss, Dr., lecturer at the Laboratory of Hydraulic Engineering, Hydrology and Glaciology (VAW), ETH Zurich (in the expert group since 29.10.2017; instead of Martin Funk)
- Christoph Graf, Techn. Staff Mountain Hydrology and Mass Movement, Torrents and Mass Movements, Swiss Federal WSL Birmensdorf Research Institute
- Nils Hählen, Head of Department of the Department of Natural Hazards, Canton of Bern KAWA
- Andrew Kos, Dr., CEO of Terrasense Switzerland AG - Geological Risk Prevention, Buchs SG
- Marcia Phillips, Dr., Snow and Permafrost, Permafrost and Snow Climatology Group Leader, WSL - Institute for Snow and Avalanche Research SLF, Davos
- Hugo Rätzo, Dr., responsible for promoting protective structures in the Landslides, Avalanches and Protected Forest Section of the Risk Prevention Department, Federal Office for the Environment BAFU Bern
- Jürg Schweizer, Dr., Head of the WSL Institute for Snow and Avalanche Research SLF, Davos
- Christian Tognacca, Dr., Head of Beffa Tognacca GmbH - Water Management and River Engineering, Claro
Cantonal expert committee
- Reto Hefti, Head of Department for Forest and Natural Hazards (AWN)
- Urban Maissen, Deputy Head of Department, AWN
- Christian Wilhelm, Dr., Head of Natural Hazards, AWN
- Andreas Huwiler, geologist, AWN
- Martin Keiser, Natural Hazards Specialist, Region 5, AWN
- Roderick Kühne, Project Manager Local Natural Hazards Advisor (LNB), AWN
- Eva Lunz, tech. Natural Hazards Employee, AWN
- Marcel Roth, Head of Hydraulic Engineering Department, Civil Engineering Department (TBA)
Christian Gartmann supported the Office for Forest and Natural Hazards (AWN) in the design and implementation of the media conference on 15.12.2017.