Debris-flow Indicator for an early warning system in the Aosta valley region
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Debris‑flow Indicator for an early warning system in the Aosta valley region Michel Ponziani1 · Paolo Pogliotti2 · Hervé Stevenin1 · Sara Maria Ratto1 Received: 25 September 2019 / Accepted: 21 August 2020 © Springer Nature B.V. 2020
Abstract 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. Keywords Early warning system · Alert system · Debris-flow detection · Weather forecast · Alpine region
* Michel Ponziani [email protected] Paolo Pogliotti [email protected] 1
Functional Centre of Aosta Valley, Department of Civil Protection, Aosta, Italy
2
Environmental Protection Agency of Aosta Valley, ARPA Valle d’Aosta, Saint‑Christophe, Italy
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Natural Hazards
1 Introduction Debris flows are rapid mass movements in a steep channel, characterized by high velocities and large impact forces, with a sediment concentration greater than 47% by volume (Wilford et al. 2004). They occur periodically on established paths (Varnes 1978; Hungr et al. 2014), triggered by long-lasting or short-duration rainfall events. These events are typically associated with convective processes (Berti et al. 1999; Stoffel et al. 2011; Prenner et al. 2018), and they cause an increase in pore-water pressure (Rebetez et al. 1997). Other sources of water, such as snow melt and the local soil water content, may also
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