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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.

 

Understanding meteorological and geomorphological conditions controlling debris-flow magnitudes and frequency is essential for their reproduction and robust climate change projections. 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).

 

For climate change projections we build up on the probabilistic sediment cascade model SedCas (Bennett et al., 2014) which aims at reproducing debris flow magnitude-frequency distributions. This is achieved through first-order descriptions of the sediment budget and initiation process on a catchment scale. For more robust projections, the model is being extended with physically-based weathering and erosion modules (e.g. frost-cracking, permafrost thawing). The model has been calibrated and forced with climate change scenarios for the Illgraben, a catchment currently producing 3-4 debris flows every year on average, but will be applied at other Alpine catchments. Consequently, the model is supportive in explaining the role of climate variability and sediment history on rare events like debris flows

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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