The 2023 Fluchthorn event (Tyrol, Austria): The transition from rockfall to debris flow

The 2023 rockfall event at the Fluchthorn massif in the Silvretta Mountain Range, Austrian Alps, presents a valuable case study in understanding the cascading processes from rockfall to debris flow in a high alpine, glacial to periglacial environment. This study aims (1) to characterise and better understand the post-failure evolution of the 2023 Fluchthorn rockfall (Silvretta Mountain Group, Austria/Switzerland) into a debris flow through the analysis of the geomorphological, geological, and meteorological conditions of the study area prior to the event, as well as DEMs and hydrological data; and (2) to discuss the impact of the rockfall–debris-flow on the hydrological system of the valley. Approximately 1 million m3 of rock mass detached from the southern Fluchthorn summit, initiating a debris flow that spread into the Futschöl Valley, temporarily blocking the creek and forming a transient lake. A total of 1.8 million m³ of material was transported and deposited along the Futschöl Valley. Hydrological monitoring captured significant discharge changes, with a flood wave recorded shortly after the event, and ponding due to river blockage, thus capturing the rapid propagation of debris-laden runoff through the Futschöl Creek and the temporary formation of a lake in the deposition zone. The findings emphasize the importance of a multi-disciplinary monitoring frameworks for assessing evolving hazards in high-mountain environments impacted by climate change.