Controls on rockfalls in mountain environments – GWC Mag

The Landslide Blog is written by Dave Petley, who is widely recognized as a world leader in the study and management of landslides.

The availability of high temporal and spatial resolution monitoring technologies, most notably terretrial laser scanning (Lidar) and doppler radar, has transformed our ability to understand rockfalls. These systems can be deployed to measure rockfalls in real time, allowing researchers to examine in detail where and when rockfalls develop. My research group at the University of Durham was a pioneer in these approaches (see Rosser et al. 2007 for example), and indeed those researchers remain at the forefront, but the technologies have come a long way since then.

A new, open access, paper has just been published in the journal Natural Hazards and Earth System Sciences (Schneider et al. 2023) that uses these technologies to get into the detail of rockfalls. The site is Brienz/Brinzauls in Switzerland, the location of the large rockslide earlier this year, although this data was collected in 2018 to 2022. At that time, the slope was creeping, so the mass was deforming and therefore generating rockfalls.

This is how the site looked in October 2022:

Over the period of monitoring, between January 2018 and October 2022, a total of 6,743 rockfall events were recorded. For each one, the location of the source zone, the volume of the detachment, the track of the debris and the final position of the debris were recorded. The team also recorded meteorological data. The outcome is a remarkable dataset.

There is huge richness in this important research, so I will highlight just a couple of key takeaways. The first is the role of different climatic controls at different times of the year. In the winter months, rockfall activity increased when temperatures warmed and thaw was initiated. The figure below illustrates this really well – in the bottom left graph, note the increase in the number of events in the middle part of the day, when temperatures typically peak:-

The middle pair of graphs illustrate the other climatic signal, in the summer months. As panel (c) demonstrates, at this time of year, the number of rockfalls per day is related to the rainfall intensity, although in this case the relationship is weaker. So, overall, different drivers affect the process at different times of the year.

The second fascinating insight is that rockfall activity clusters in time in particular locations across the rock mass. The best illustration of this occurred in February 2020, as shown in the graph below:-

Whilst the temperature was fluctuating around zero degrees Celsius, rockfall activity did not correlate with meteorological triggers. But in this case, a 600 m² area of the rock mass was creeping to failure, and the rockfall activity increased in the period leading up to the collapse in that particular area.

This is a brilliant piece of research, made all the better by the publication in an open access format. I shall greatly look forward to reading about the work that was undertaken in the period leading up to the major collapse event on 15 June 2023, which I highlighted on my old AGU blog site.

References

Rosser, N., Lim, M., Petley, D., Dunning, S., and Allison, R., 2007, Patterns of precursory rockfall prior to slope failure, Journal of Geophysical Research-Earth, 112, F04014, https://doi.org/10.1029/2006JF000642.

Schneider, M., Oestreicher, N., Ehrat, T., and Loew, S.: Rockfall monitoring with a Doppler radar on an active rockslide complex in Brienz/Brinzauls (Switzerland), Nat. Hazards Earth Syst. Sci., 23, 3337–3354, https://doi.org/10.5194/nhess-23-3337-2023, 2023.

Text © 2023. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

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