National-scale debris-flow hazard indication modelling for Swiss railway infrastructure

Abstract. Debris flows are among the most destructive alpine hazards, threatening settlements and critical infrastructure with average annual damages in Switzerland of CHF 321 million. Hazard indication modelling of debris flows, among other hazards, offers the possibility to identify hazard hot spots at a regional and national scale. Additionally it lays the foundation for future scenario modelling. We present the Large-Scale Hazard Indication Modelling (LSHIM) framework for debris flows as commissioned by the Swiss federal railway (SBB) with guiding value return periods of 30, 100 and 300 years. For disposition, we combine high-resolution digital terrain models, climatic and geologic layers, and statistical modelling with a Random Forest approach. Disposition is cross-validated against the Swiss event database (StorMe) with an average accuracy of 81.4 % and application results in 16,756 possible release areas. Dynamic runout is calculated with a tailored version of RAMMS::Debrisflow, where we define hydrograph volume from MeteoSchweiz-modelled extreme precipitation. Landscape erosivity is assessed by geology and geomorphology. Results indicate 74, 99 and 110 km of railway affected by debris flow hazard for a 30, 100 and 300 year return period, respectively. The highest intensity class shows the highest relative increase under greater return periods. This approach enables a large scale hazard indication assessment based on high-quality input data. The results are a valuable base for further planning and investigations that incorporate climate change. Limitations in the methodology lie in the static definition of release area and deterministic parameterization.