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 European Alps. We used a model chain to quantify possible impacts of climate change on sediment transfer and hazard in a debris flow-prone catchment in the Swiss Alps (Illgraben). We combined a stochastic weather generator1 with downscaled and bias-corrected climate change projections2 to generate climate simulations. These climate simulations then feed the hillslope-channel sediment cascade model, SedCas3, which is calibrated against observed debris-flow magnitudes estimated from force plate measurements4, to make predictions of sediment transfer and debris flow hazard in the Illgraben over the 21st century5.
The results demonstrate the complex interplay between hydrology, sediment production and elevation in alpine catchment response to climate change. The hydrological potential to transport sediment and generate debris flows will increase, driven by increases in precipitation and air temperature. Indeed, if sediment supply to the channel by landslides were unlimited, this would result in an increase in future sediment yield of 48% by the end of the century. However, sediment transfer is also a function of sediment supply by landslides at the head of the catchment, driven by highly temperature sensitive freeze-thaw processes6. At the elevation of the Illgraben (<2000 m), freeze-thaw processes and thus sediment supply will decrease in a warming climate resulting in a decrease in sediment yield of 48% by the end of the century. This result and the competition between hydrological debris flow triggering potential and sediment supply is highly elevation dependent. As we increase mean catchment elevation, sediment production increases due to decreased snow cover and increased exposure of bedrock to freeze-thaw weathering, with implications for the application of findings to other catchments. Although uncertainties in our results are large, we show that these can mostly be attributed to irreducible internal climate variability. Our findings have important implications for the assessment of natural hazards and risks in mountain environments.
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
5Hirshberg et al., 2021: Climate change impacts on sediment yield and debris flow activity
6 Bennett et al., 2013: Patterns and controls of sediment production, transfer and yield in the Illgraben