Climate change impacts on rainfall- and snowmelt-triggered debris flows in an Alpine catchment
Authors: Hirschberg Jacob , Sadowski Yoann , Michel Adrien , McArdell Brian W. , Molnar Peter 
Published: February 2023

Debris flows are surging mixtures of water and sediments and can threaten humans and infrastructure. In alpine catchments, debris flows are often triggered by runoff events as a response to intense rainfall, snowmelt, or a combination thereof. Therefore, debris-flow triggering is expected to be sensitive to future changes in temperature and precipitation. Quantifying these changes is, however, challenging. While changes in temperature are relatively certain, future precipitation characteristics have lower signal-to-noise ratios (e.g., Hirschberg et al., 2021). Furthermore, how such changes influence the seasonal snowpack is not trivial. For example, snowmelt is predicted to start earlier in the year, but at lower rates (Musselman et al., 2017). Quantifying climate change impacts on debris-flow triggering runoff events in high-alpine catchments, therefore, requires studying the complex interactions of changes in precipitation, temperature and the snowpack.Our study focuses on the Grabengufer rock glacier and the gully below, which is located above the municipality of Randa in the canton of Valais, Switzerland. The rock glacier front regularly delivers mobile sediments to the gully (1900 m a.s.l.), where debris flows are frequently triggered after rain and/or snowmelt. We use ALPINE3D, which is a spatially distributed version of the multi-layer snowmodel SNOWPACK (Lehning et al., 2002), to simulate the snowpack evolution and runoff in the Grabengufer basin. Due to the small size and the steepness of the basin, the discharge can be simplified as the snowmelt and liquid precipitation in each pixel and in each timestep. A debris-flow record consisting of 34 events between 1985 and 2016 allowed for calibrating a debris-flow triggering discharge threshold. Ultimately, we plan to use the AWE-GEN stochastic weather generator (Fatichi et al., 2011) and the CH2018 climate scenarios to study changes in debris-flow triggering discharge at hourly resolution. Preliminary results show clear relations between snowmelt, rainfall and debris-flow triggering for the calibration period and provide a proof of concept. Although we cannot address the full complexity of such geomorphic systems leading to debris-flow triggering (e.g., rock-glacier dynamics), we study changes in extreme discharge, which is a key variable for future debris-flow hazards. Furthermore, the studied basin is representative of high-alpine debris-flow torrents and the outcome will be useful for researchers and authorities interested in climate change impacts on alpine mass movements.REFERENCESCH2018 Project Team 2018: CH2018 - Climate Scenarios for Switzerland. National Centre for Climate Services.Fatichi, S., Ivanov, V. Y., & Caporali, E. 2011: Simulation of future climate scenarios with a weather generator. Advances in Water Resources, 34(4), 448-467.Hirschberg, J., Fatichi, S., Bennett, G.L., McArdell, B.W., Peleg, N., Lane, S.N., Schlunegger, F., Molnar, P. 2021: Climate Change Impacts on Sediment Yield and Debris-Flow Activity in an Alpine Catchment. J. Geophys. Res. Earth Surf. 126.Lehning, M., Bartelt, P., Brown, B., and Fierz, C. 2002: A physical SNOWPACK model for the Swiss avalanche warning: Part III: meteorological forcing, thin layer formation and evaluation, Cold Reg. Sci. Technol., 35, 169&#8211;184.Musselman, K. N., Clark, M. P., Liu, C., Ikeda, K., & Rasmussen, R. 2017: Slower snowmelt in a warmer world. Nature Climate Change, 7(3), 214-219.