Evaluation of a downscaled Regional Climate Model for analysing the frequency of debris flows since 1850

Debris flows are a major natural hazard in mountainous regions worldwide and significantly impact the sediment budgets in alpine areas. However, the development of debris flow frequency under climate change conditions has not yet been conclusively clarified, as long-term, comprehensive event records (i.e. not biased towards large events) are scarce. As alpine debris flows are predominantly triggered by high-intensity and short-duration rainfall events, precipitation records can be useful for inferring potential triggers, particularly under transport-limited conditions. As high-resolution precipitation measurements are rarely available over long periods of time, we employed dynamical downscaling of a Regional Climate Model (RCM) based on the Advanced Weather Research and Forecasting model (WRF). This approach resulted in a high-resolution climate model dataset covering most of the Central Alps, with a spatial resolution of 2x2 km and a temporal resolution of 15 minutes. This model enabled us to analyse high-intensity, short-duration rainfall events since the end of the Little Ice Age in 1850.

We compared the RCM with a debris flow record in the Horlachtal catchment in Tyrol, Austria. By analysing remote sensing datasets such as historical and recent aerial imagery, airborne lidar data and lichenometric dates, we identified 991 individual debris flows in the area between 1947 and 2022. Combining the observation dataset with the RCM rainfall data enabled us to take an integrated approach to assessing changes in debris flow frequency in the Horlachtal and their climatic drivers since 1850. The results provide insights into possible future trends in debris flow frequency in a changing climate, showing a weak positive long-term trend for the Horlachtal. The RCM's coverage allows for similar studies in other Alpine regions, offering more detailed insights into the spatial variability of changes in debris flow activity within the Central Alps.