EP007-09
The Role of Hillslope Failures and Internal Climate Variability in Climate Change Impact Assessment on Debris Flows and Sediment Yield in an Alpine Catchment

Monday, 7 December 2020: 10:54
Virtual
Jacob Hirschberg1, Simone Fatichi2, Georgina Bennett3, Brian W McArdell4, Nadav Peleg5, Stuart N Lane6, Fritz Schlunegger7 and Peter Molnar5, (1)Swiss Federal Research Institute WSL, Birmesdorf, Switzerland, (2)National University of Singapore, Department of Civil and Environmental Engineering, Singapore, Singapore, (3)University of Exeter, School of Life and Environmental Sciences, Geography, Exeter, United Kingdom, (4)WSL Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland, (5)ETH Zurich, Institute of Environmental Engineering, Zurich, Switzerland, (6)University of Lausanne, Lausanne, Switzerland, (7)University of Bern, Institute of Geological Sciences, Bern, Switzerland
Abstract:
Sediment production and transfer processes shape river basins and networks and are driven by variability in precipitation, runoff and temperature. Changes in these hydrological and geomorphological processes are especially difficult to predict in temperature-sensitive environments such as the Alps. In this study, we use a chain of climate-hydrology-geomorphology models to quantify possible impacts in a debris flow-prone catchment in the Swiss Alps (Illgraben). To this end, we combine a stochastic weather generator1 with downscaled and bias-corrected climate change projections2. These climate simulations are fed to a hillslope-channel sediment cascade model3, which is calibrated against observed debris-flow magnitudes estimated from force plate measurements4.

The results highlight the role of hillslope landslides, supplying sediments to the channel, where they can be re-mobilized if sufficient surface runoff is generated. In supply-unlimited conditions, a rather uncertain rise in precipitation combined with a certain rise in air temperature leads to an increase in sediment yield of ~50% by the end of the 21st century. In contrast, if sediment production is considered with a simplified frost-weathering mechanism, future sediment supply is reduced and hence also sediment yield (~ -50%) and the annual number of debris flows (~-25%). We further demonstrate the elevation dependency of the frost-weathering mechanism: at higher elevations a shorter season with snow cover could increase the time bedrock is exposed to freezing temperatures and accelerate sediment production. Although results underlie major uncertainties, we show that these uncertainties can be attributed mainly to irreducible internal climate variability. Therefore, our findings have important implications for the assessment of natural hazards and risks in mountain environments.

REFERENCES

1 Fatichi et al., 2011: Simulation of future climate scenarios with a weather generator

2 National Centre for Climate Services, 2018: CH2018 - Climate Scenarios for Switzerland

3 Bennett et al., 2014: A probabilistic sediment cascade model of sediment transfer in the Illgraben

4 McArdell et al., 2007: Field observations of basal forces and fluid pressure in a debris flow

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