Debris flows in the Alps are becoming more intense and unpredictable

On 5 August, a large rockfall broke away at 3’900 metres from the south face of the Matterhorn, on the Italian side of the mountain that towers over Zermatt. The episode fits into a trend researchers have been observing for some time: as the climate warms, Alpine debris flows are becoming more intense and harder to predict.

Italian-speaking Switzerland knows the consequences well. In June 2024, violent storms triggered devastating debris flows and heavy damage in the Maggia Valley, including the Bavona Valley. That summer, similar events also struck the Aosta Valley, Piedmont, Austria and Germany.

Several mechanisms are at work. According to MeteoSwiss data, downpours could become up to 30% more intense, with rainfall concentrated in ever shorter and more violent episodes. "Quell'acqua ha il potenziale di erodere più materiale" (that water has the potential to erode more material), explains Markus Stoffel, holder of the new chair in mountain natural-hazard research at the University of Geneva: the energy of debris flows could therefore increase. Over the course of the century, the lower limit of permafrost could also rise by 200 to 750 metres, depending on slope exposure, soil composition and ice thickness. Swiss glaciers, for their part, have lost almost 40% of their volume since 2000, a retreat that destabilises slope sediments.

The calendar is changing too. Debris flows used to be mainly a summer phenomenon; today they are observed in May and even April, and sometimes continue into October. In January 2018, heavy snowfall followed by mild temperatures and strong rain produced an unprecedented scenario: "Ci sono state enormi valanghe in diverse aree delle Alpi e poi improvvisamente colate detritiche nel pieno dell'inverno. Qualcosa che non avevamo mai visto prima" (there were huge avalanches in several areas of the Alps and then suddenly debris flows in the middle of winter, something we had never seen before), Stoffel recalls. In his view, basins prone to debris flows could generate entirely new kinds of events.

Climate does not explain everything, however. "Stimiamo che l'influenza del cambiamento climatico sia tra un terzo e il 50% di quanto osserviamo" (we estimate the influence of climate change at between one third and 50% of what we observe), the researcher points out: the rest of the risk stems from the expansion of infrastructure. Between 1950 and 1970 the Alpine population grew by more than 30%, and valleys that had been largely isolated filled up with roads, public transport and ski lifts.

Things are not getting worse everywhere, though. Sediments are not unlimited: a very large debris flow can empty a basin down to the bedrock, and the system needs time to recharge through new rockfalls. In the meantime, some areas may temporarily become safer.

A view across the entire Alpine arc

To better understand these dynamics, since 1 May Stoffel has been leading a five-year project with six specialists, funded with one million euros (920’000 francs) by the AXA Fund for Human Progress. While institutions such as the Swiss Federal Institute of Technology in Zurich and the WSL carry out in-depth studies at specific sites, such as the Illgraben torrent basin in Valais, the Geneva group will build large historical databases to identify regional trends, hotspots and dynamics common to the whole of the Alps. By combining field observations, remote sensing and modelling, the researchers want to understand when and where slopes fail and how far destructive flows can travel, in order to develop strategies to limit future losses and damage.