Debris-flow Indicator for an early warning system in the Aosta valley region

Aosta Valley, an Alpine region in north-western Italy, has an early warning system (EWS) that issues hydrogeological alerts based on hydrological modelling and rainfall thresholds that identify the possibility of shallow landslides being triggered in different areas of the region. The high headwater catchments are characterized by the presence of permafrost and glacial sediments, and they are frequently prone to debris flows. The summer debris flows are initiated by short-duration, high-intensity rainstorms, which are associated with high meteorological uncertainty; therefore, they are not always detected by the early warning system of shallow landslides. In this study, the hydro-meteorological and permafrost conditions related to the debris-flow events from 2013 to 2018 are investigated in order to determine the variables affecting the triggering of debris flows. Debris-Flow Indicator (DFI), an early warning system specific for debris flows, was developed using recorded air temperatures, thunderstorm alerts and forecast rainfall. Two alert levels of the DFI were defined by two thresholds (S1 and S2) of the freezing level determined from performance metrics. The performance of the DFI was then studied with a back-analysis from 2013 to 2019, using observed air temperatures and forecast rainfalls. This analysis showed that the experimental implementation of the DFI in the EWS of the Aosta Valley region resulted in detecting most of the events with some false alerts (for the lower threshold, S1) or detecting only major events, but without generating false alerts (for the higher threshold, S2). Consequently, the DFI can be applied for issuing debris-flow alerts for large areas in mountain regions based only on meteorological data and forecast.