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Sediment Cascades in Alpine Mass Movements

 

A sub-project of the WSL research program “Climate Change Impacts on Alpine Mass Movements”

Mountain regions are sensitive to climate change because of altered snow and glacier melt, permafrost thawing and an expected increase in the frequency of intense precipitation. Climate Change Impacts on Alpine Mass Movements (CCAM) is a strategic initiative to research these processes aiming at developing optimum adaptive strategies to these types of changes.

Climate change impacts in alpine regions are already apparent in glacier recessions and permafrost thawing. As a consequence, cascading processes involving rock slope failure and debris flows are favored and threaten valley floors. The interconnection of these events were recently exemplified by a severe rock fall at Piz Cengalo, ensuing debris flows which destroyed big parts of the village of Bondo(GR).

Within the CCAMM research program, we focus on projections of changes in the sediment disposition, frequencies and magnitudes of debris flows by making use of the recently released CH2018 scenarios, which provide climate projections up to 2100. The outcome of our work will provide projections of debris-flow activity in selected Alpine catchments. Furthermore, the assessment of sediment deposits, frequencies and magnitudes of mass movements are required for hazard mapping and provide useful information for numerical runout models like RAMMS.

 

Controlling factors for debris-flow magnitudes

Understanding meteorological and geomorphological conditions controlling debris-flow magnitudes and frequency is essential for their reproduction and robust climate change projections. For the Illgraben debris-flow torrent, we have investigated the role of various precipitation characteristics in the debris-flow formation and found that while debris flows can be triggered by relatively small rainfall amounts, magnitudes experience positive feedbacks from antecedent wetness (Fig. 2).

 

Climate change impacts on debris-flow activity

In a recent study (Hirschberg et al., 2021), we used the sediment cascade model SedCas (Bennett et al., 2014) to investigate climate change impacts on the debris-flow activity in Illgraben (Fig. 1). By combining SedCas with climate change projections and the AWE-GEN weather generator (Fatichi et al., 2011), we were able to quantify changes in temperature and precipitation and how it affects the sediment generation and sediment yield. We found that the warming decreases sediment supply by frost-weathering from hillslopes and thus also reduction in both the annual sediment yield (-48%) and the number of debris flows (-23%) by the end of the century (Fig. 3). Furthermore, the study provided insights into the effects of elevation on sediment production as well as the major sources on uncertainty for climate change projections on sediment fluxes.

 

This work is conducted within the Mountain Torrents group from WSL and in collaboration with Prof. Dr. Peter Molnar (ETH Zürich) and Dr. Georgina Bennett (University of Exeter).

 

 

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