Critical rainfall thresholds for debris flow initiation in a small catchment of Western Italian Alps

The present study aims at giving a contribution to the current state of knowledge concerning rainfall conditions able to activate debris flows in high-altitude mountain environments. To the purpose, the paper presents some findings deriving from the monitoring activity in the Marderello Torrent experimental basin. Marderello Torrent is a left tributary of Cenischia stream (NW Italian Alps), with a catchment area of 6.6 km2. Since the early nineties, the Marderello catchment was chosen for study, chiefly because of its aptitude to generate muddy and debris flows with a relatively high frequency: 31 activations are known to be occurred during the last one hundred years and, according to the chronicles of the last three centuries, events with significant volumes are on the average liable to take place every 3-4 years; furthermore, minor events may occur even twice per year. Since 1994 the Marderello catchment was equipped with three meteorological stations, located at different elevations (3150, 2150 and 830 m a.s.l.), to collect rainfall and other meteorological data (air moisture, temperature, atmospheric pressure and wind). In most recent years, the monitoring network has been further improved and extended on the alluvial fan, with the installation of one ultrasonic device, two video-cameras and four geophones, in order to detect debris flow wave-fronts depth and time-to-arrival (Turconi et al., 2014). This equipment was installed in Spring 2013 and on the 17th July it already allowed to record a mud flow of Marderello Torrent (Coviello et al., 2014). High-altitude experimental basins, equipped for debris flows monitoring and providing long time series of data, are not frequent in European Alps. Indeed, instrumentation is subjected to severe technical challenges in such environments, mainly related to the extreme weather conditions (e.g. temperature peaks down to -20ºC, wind strokes, lightening). In such a context, the Marderello basin, with a twenty-year sequence of observations, represents a rare exception and enables to analyse and to compare different triggering conditions within Mt. Rocciamelone area. The monitoring activity in the Marderello basin is devoted to investigate the soil erosion and flood discharge contribution to the sediment transport process along the hydrographic network. Based on the available observations, intense rainfalls, often corresponding with summer storm events, are the most frequent cause in debris flows activation. Hence it was necessary to assess the critical rainfall conditions able to trigger muddy-debris flows. To identify a kind of "critical rainfall threshold", in the present study an empirical approach has been used: empirical methods rely upon the analysis of past rainfall conditions responsible for landslides activation (Palladino et al., 2014). Preliminary analysis of collected data highlights intense rainfall, with durations between 30 and 45 minutes, are able to activate both mud flows and debris flows processes in the Marderello catchment. Rainfall data deriving from rain gauges at different elevations within the basin, suggest the lead time of the Marderello catchment to a overhead cloudburst is about 45-50 minutes. In order to identify rainfall conditions (duration-intensity combinations) proving to be critical for mass movements' activation, rainfall time series were aggregated based on different time intervals, namely: 5, 10, 15, 20, 25 and 30 minutes. Data analysis allowed the identification of a minimum critical threshold, corresponding to the rainfall event occurred on August 2nd 2005: rainfall intensities recorded during this event (1.6 mm/5 min÷2 mm/20 min) mark the lower limit of rainfall conditions able to induce landslides activation in Marderello catchment. Rainfall data