Geosciences Column: Hazard perception – how great is the risk of a rockfall?

In this month’s Geoscience’s column, Sara Mynott discusses the geological hazards associated with climate warming and how recent research sheds new light on our understanding of rockfall frequency.

Rockfalls are the free-falling movement of bedrock material from a rock face, a phenomenon also encompassed by the terms ‘landslide’, ‘rockslide’ and ‘rock avalanche’. They range from small debris falls of only a few cubic-metres to large ‘bergsturz’ events of over 1 million metres-cubed. The number of rockfalls reported has increased in recent years and is often attributed to global warming, despite the lack of research in this area. The debate among scientists regarding the effect of climate change on geomorphic hazards has led to a lot of confusion among the media and hence, the public.

Climate change is expected to have numerous consequences for natural hazards and the IPCC has predicted that geomorphic hazards will increase in alpine regions as a result. However, recent research published in Natural Hazards and Earth Systems Science suggests that this may not be the case. In a recent assessment of Austrian rockfalls over the period 1990-2010, Oliver Sass and Manfred Oberlechner investigated how temperature influences their frequency. Their dataset was compiled from events that were large enough to be recorded in the media, restricting it to events that have the capacity to affect people and/or infrastructure. The Huben rockslide that occurred in 1999, which resulted in both the loss of alpine road access and the destruction of a local sawmill, presents one such example.

Rockfall in Huben, Austria, that occurred on 11 March 1999, far below the permafrost limit. This rockfall resulted in both the destruction of a sawmill and loss of road access (Source: Sass and Oberlechner, 2012).

Historical records of rockfalls are scarce, making predictions for the future a challenge and, until recently, little research on the temporal frequency of rockfall events had been carried out. This is partly due to the research focus on areas of permafrost, which cover less than 4 % of the Austrian Alps. Consequently, the relationship between surface temperature and rockslide frequency in permafrost regions is well-known. Permafrost, which exists at sub-zero temperatures, cements sediment together and gives it stability. Unsurprisingly, warming causes permafrost to degrade, leading to a loss of sediment stability and an increased risk of geomorphic hazards. The likelihood of these hazards occurring is a function of substrate type. However, areas of public interest (those with infrastructure) tend to be permafrost-free. In fact, 91% of the events studied were below the permafrost limit (less than 2100 m elevation).

Contrary to the IPCC’s predictions, the study found that there was no relationship between temperature and the number of rockfall events below the permafrost limit, nor was there any correlation between precipitation and rockfall frequency. The increasing settlements and infrastructure within the Alps means there is a greater risk of a geomorphic hazard occurring and the increase in availability of information means there appears to be more events than 20 years ago. Thus, the apparent increase in rockfall occurrence in recent years is likely to be due to a reporting bias.

Whilst there is no evidence for warming increasing the annual number of rockfalls, changes in seasonal weather patterns have resulted in a shift in their occurrence throughout the year. Rockfalls are generally more common in spring than at any other time of year as both the increase in water supply (through snowmelt and high precipitation rates) and high degree of freeze-thaw activity (also known as cryoturbation) destabilises the sediment. However, in recent decades, a greater proportion of rockfalls have occurred during the summer months, leading to a more even distribution of these hazards throughout the year.

Below the permafrost limit there is insufficient evidence to support the notion that increasing rockfall events are associated with climate warming. In fact, the study reveals that milder winters may even reduce the number of rockfalls outside areas of permafrost. Whilst Sass emphasises that these results are preliminary, they highlight the complexity of predicting the impacts of climate change and expose an alternative way in which it can affect hazardous earth processes.

By Sara Mynott, EGU Communications Officer

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