How is climate change affecting hydro-meteorological triggering for debris flows? An assessment based on convection-permitting models and a bias-neutral procedure

Debris-flow activity is expected to change in the future following the expected changes in sub-daily rainfall rates. In this study, we connect high-resolution climate simulations from an ensemble of recently developed convection-permitting models (CPM) and a threshold-based precipitation model for debris-flows triggering. We are considering CPM runs over historical (1996-2005), near future (2041-2050) and far future (2090-2099) decade-long periods. Given the biases affecting the CPM simulations and the desire to avoid bias-correction procedures, which may introduce distortions into the precipitation simulations, we propose a methodology to map the debris-flow threshold into the simulated climates. This is obtained by evaluating the return levels of the threshold precipitation rates at different durations, and mapping these in the climate simulations using the same return levels. The Simplified Metastatistical Extreme Value (SMEV) methodology is exploited for the precipitation statistical analysis. The suitability of the proposed framework is tested on the Moscardo catchment, a small study basin located in the eastern Italian Alps, where the debris flow activity is mainly transport-limited. This case study is particularly remarkable due to the high frequency of debris flows and a monitoring system working since 1990, which has permitted establishing reliable rainfall . The debris-flow triggering precipitation events are assessed by considering changes in their frequency, depth and seasonality. The promising preliminary results support the use of this approach to assess debris flow hazards in a changing climate.