16 March 2020
An analysis of the Piz Cengalo landslide
In August 2017 a large landslide initiated on the Piz Cengalo mountain in Switzerland. It travelled 6.5 km before striking the village of Bondo. In total it was responsible for eight fatalities.
In an article just published in the journal NHESS, which is open access, Mergili et al. (2020) provide a detailed description and simulation of the landslide. The first part of the paper provides a really good description of the event. The landslide was initiated at 09:31 local time on 23 August 2017 when about 3.6 million cubic metres broke off from the eastern face of Piz Cengalo. The sequence of events is shown in the images below, from Mergili et al. (2020):-
After detaching, the landslide impacted the glacier below and entrained about 600,000 cubic metres of ice. In a manner that was similar to the Mount Haast landslide, the mass behaved initially as a rock avalanche. However, downstream it evolved into a debris flow that swept down the valley to strike the village of Bondo. Over the next day or so a series of nine debris flow surges struck the village.
Interestingly, two different scenarios have been proposed to explain the behaviour of the landslide, and in particular the formation of the first debris flow. These are described in the schematic diagram below, from Mergili et al. (2020):-
In the first scenario (S1), the front of the rock avalanche became highly saturated, and transitioned into the first debris flow. In the second (S2), the rock avalanche stalled mid-valley, but was overtopped by an avalanching flow of entrained ice and rock fragments, to form the debris flow.
Mergili et al. (2020) have attempted to simulate these two scenarios. Interestingly, the models indicate that both are plausible, and the simulations do not allow us to determine which is likely to be correct.
The behaviour of these complex landslides remains challenging to understand and model, but is really important if future hazards are to be understood. The impacts of global heating, which are increasing the frequency of these large events, means that this remains a really important research question.
Mergili, M., Jaboyedoff, M., Pullarello, J. and Pudasaini, S. P. 2020. Back calculation of the 2017 Piz Cengalo–Bondo landslide cascade with r.avaflow: what we can do and what we can learn. Natural Hazards and Earth System Sciences, 20 (2), 505-520. DOI: 10.5194/nhess-20-505-2020.