Climatic triggering of landslide sediment supply in the Alpine Rhine

The AlpRhineS2S project, a collaboration between ETH Zurich and the University of Bern, researches the interplay of geological, geomorphological and hydrological processes within the sedimentary system of the Alpine Rhine in the canton of Grisons in Switzerland. Mechanisms of sediment erosion, transport and deposition determine the pathways of sediment from sources to sinks in a river basin. Long-term basin-averaged denudation rates serve to characterize the geomorphic properties of a catchment and to derive a sediment budget (Garipova et al., 2024), while specific hotspots of erosion considerably contribute to the short-term sediment supply into the fluvial system. Accordingly, mass wasting events play a crucial role in an Alpine geomorphic context by intermittently providing considerable amounts of sediment for transport in the river network. A large part of this sediment is transported in suspension, producing a complex turbidity signal at the outlet (Agostini et al., 2024) that features distinct tracers of source material composition (Garipova et al., 2024). In this contribution, we investigate the effects of precipitation as a triggering factor for frequent mass wasting events in the Alpine Rhine catchment. We correlate records of shallow landslide, debris flow, and rockfall events from the Swiss natural hazard database (StorMe, Swiss Federal Office for the Environment FOEN) to the gridded daily precipitation product RhiresD (Swiss Federal Office of Meteorology and Climatology MeteoSwiss). We estimate rainfall thresholds for those events by classifying consecutive rainfall days as either triggering or non-triggering events and performing jackknife cross-validation to assess the temporal bias of the event data following Leonarduzzi et al. (2017; 2020). We characterize the regional and seasonal effect of heavy precipitation events on increased sediment supply available for transport in the fluvial system. Finally, we also identify individual erosion hotspots and their link to sediment connectivity and slope stability assessments. Analyzing external drivers, we hypothesize on the effect of changes in climatic forcing on erosion mechanisms over the past decades, particularly due to increasing temperatures and precipitation intensities. References: Agostini, L., Demmel, S., Garipova, S., Sinclair, S., Schlunegger, F., Molnar, P. (2024): Suspended sediment transport in a river network: testing signal propagation and modelling approaches. EGU 2024. Garipova, S., Mair, D., Demmel, S., Agostini, L., Akar, N., Molnar, P., Schlunegger, F. (2024): Source-to-Sink Sediment Tracing in the Glogn River Catchment. EGU 2024. Leonarduzzi, E., Molnar, P., McArdell, B.W. (2017): Predictive performance of rainfall thresholds for shallow landslides in Switzerland from gridded daily data. Water Resources Research 53(8): 66126625. https://doi.org/10.1002/2017WR021044. Leonarduzzi, E. and Molnar, P. (2020): Deriving rainfall thresholds for landsliding at the regional scale: daily and hourly resolutions, normalisation, and antecedent rainfall. Nat. Hazards Earth Syst. Sci., 20, 29052919. https://doi.org/10.5194/nhess-20-2905-2020.