Evolution of rockfall risk following changes in hazard and exposure: Application to a road section in the Zermatt valley, Swiss Alps

Due to rapid and intense socio-environmental transformations in rockfall-prone areas, risk assessment accounting for non-stationary conditions becomes a crucial issue for supporting long-term land-use zoning and adapting mitigation strategies. During the past few decades, several studies have examined the impacts of global warming on rockfall activity, especially at high-elevation sites. By contrast, concomitant changes in exposure, such as modifications in road traffic with changes in mountain tourism and social practices, have received little attention, making the overall changes in risks largely unknown. This study therefore proposes a quantitative risk assessment (QRA) of the cantonal road in Balmatten (Zermatt Valley, Swiss Alps), which evaluates the respective and overall effects of rockfall frequency and traffic density changes on rockfall risks at three time steps: 1960, 2010 and 2060. The approach relies on strong, yet reasonable assumptions that minimize computational burden. Results demonstrate the significant impact of road traffic fluctuations (+818%) on risk estimates, which largely outweighs the effects of climate change (+100%) on rockfall risk over the 1960–2060 period. Despite several challenges inherent to the implementation of non-stationary QRA approaches, these findings demonstrate their value to support adaptation of mountain territories in a time of global change. • Non-stationary analyses are essential to capture socio-environmental evolutions. • Scenario-specific variables influence the primary driver of risk change. • Risk analysis was addressed by adapting hazard frequency and traffic exposure. • Integrating dynamic conditions in QRA can improve risk management strategies.