Respective impacts of global warming, permafrost thaw and urban sprawl on debris-flow risk: Insights from the Dorfbach Torrent, Swiss Alps

Debris flows pose significant hazards to mountain communities worldwide. In the European Alps, they annually damage infrastructure and settlements, often resulting in fatalities. Climate warming is projected to locally and temporarily shift debris-flow frequency, intensity, and spatial extent by enhancing rain-on-snow events, precipitation extremes, glacier melt, and permafrost thaw—processes that alter sediment availability in alpine catchments. At the same time, human exposure to debris-flow hazards has intensified as urban expansion increasingly encroaches on torrential cones, driven by limited development space in alpine valleys. Despite their central role in shaping future risk and economic losses, climate and land-use changes are rarely considered together in quantitative risk analyses. This study investigates the depositional cone of the Dorfbach torrent (Valais, Switzerland), drawing on scenarios of past, present, and future debris-flow frequency and intensity. Hazard maps were combined with elements at risk, buildings and infrastructure, from 1880 to 2020, based on historical topographic maps and aerial photographs. Analyses employed the Swiss EconoMe framework, designed to quantify risks from gravitational hazards, to disentangle the influence of climate and land-use on total risk. Results indicate that climate warming and permafrost thaw alone would expand the debris-flow-affected area by 228 %, reducible to 61 % with protection dams. Historical data show settlements and infrastructure on the cone increased by 227 % and 203 % respectively since 1880. Integrating hazard scenarios with risk elements across four time steps (1880, 1940, 1980, and 2020) reveals that land-use changes have led to a 118 % in risk costs between 1880 and 2020. In contrast, the 2010 hazard mapping projected a 238 % increase, highlighting that climate-driven hazard changes contribute more than twice as much as land-use change to rising risk costs. Given widespread urban expansion on Alpine torrential cones and rapid climate shifts, our findings from this archetypal Swiss site underscore the need for similar integrated studies across other mountain regions.