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

L. TURCONI, V. COVIELLO, M.R. PALLADINO, M. ARATTANO, G. SAVIO, D. TROPEANO, 2014, Critical rainfall thresholds for debris flow initiation in a small catchment of Western Italian Alps, 15th Biennial Conference Euromediterranean Network of Experimental and Representative Basins, Coimbra, Portugal, 9-13 September, 2014,

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 analysis also draws attention on the peculiarity of some events_ 1) On September 3rd 2011 a rainfall event activated concomitant multi-surge debris flows along all tributaries of the Cenischia Valley (Gioglio, Claretto, Bar and Marderello torrents); based on available historical information, only two cases with analogous concomitant activations are documented, in 1947 and 1868 (Turconi and Tropeano, 2008). 2) In the course of 2011 two activations (August 18 and September 3) were detected with associated rainfall intensities greater than 9 mm/5 min, never recorded before in the available time series. 3) On the other side, very important rainfall events, accounting for 100 mm in one day, have been recorded without debris flow occurrence. Such situations put into evidence the complexity and variability of triggering mechanisms liable to occur in the same basin, do to the overlapping of different factors in the control of the activations. This suggests that triggering mechanisms need for further explanations. One of the most interesting aspects to take into account is the catchment capability as a source of debris, able to feed the mixture during the rainfall events. Currently, the "catchment-response" of the Marderello basin is surveyed through a number of devises, able to gain possible changes in its geo-structural set up, which is particularly developed at the catchment head (Turconi et al., 2010). Nevertheless, a bound sediment-source area has not been identified yet, and the whole catchment, together with the channel banks, are supposed to be the major contributors to sediment production.

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