Linkages between abundant rockfall and debris-flow activity at Täschgufer, Swiss Alps

Debris-flow risks are increasing in many regions worldwide, partly as a result of climate change, posing a significant threat to human lives in mountainous and hillslope environments. An increase in the sediment supply rate in debris-flow initiation zones due to climate warming is one of the key factors controlling debris-flow activity, especially in cold regions. However, the sequential process from bedrock weathering and rockfall activity to moisture variations within unstable sediment and the subsequent occurrence of debris flows remains inadequately understood. This study investigates the climatic controls on rock weathering and subsequent rockfall activity, as well as the spatial distribution and frequency of debris flows, based on integrated field monitoring at Täschgufer (Swiss Alps), a large rockwall-talus slope system within a seasonal frost environment. Multiple mechanisms of crack opening, including increases in hydraulic pressure, heating-cooling cycles, volumetric expansion of crack-filling ice due to freeze-thaw cycles, and ice segregation, were monitored on rockwalls at two different elevations. Rockfall activity exhibited spatial variability, potentially influenced by lithology, crack density, and elevation. Debris-flow frequency was higher in areas with active rockfall. Many debris flows were initiated in sediment deposition areas near rockwalls due to direct water supply from the rockwalls and the rapid replenishment of sediment storage following erosion. Our study suggests that both rising temperatures and changes in rainfall patterns associated with climate change will influence rockfall and debris-flow activity in the future.