Debris flow monitoring at Mardarello catchment, Italian Western Alps

In this work we report latest outcomes of 18 years-long dataset of monitoring in a very active debris flow-producing catchment, left tributary of the Cenischia valley, NW Italian Alps. The Mardarello catchment (6.61 km²), due to very unfavourable geological and geomorphologic conditions, can be considered a perennial source of debris (the bulk can be estimated at 2.6 106 m3) containing even large-sized blocks. It is incised in carbonatic, massive Mesozoic rocks ("Calcescisti con Pietre Verdi"), interbedded with clayey-arenitic schist, very steeply dipping downslope and widely overlied by deep-seated slope collapse deposits and partly by detrital talus. A north-south oriented fault system together with minor failures has resulted in a complex network of rock joints and cracks. The vegetal cover is developed down 2400 m elevation with pasture herbs and sparse shrubs. Around 2000 m a.s.l. Swiss mountain pine and Larix decidua partly colonize unstable slopes, below such altitude other kinds of Conifers are widely spread all over the catchment, still below chestnuts are also present; such biotopes are in part inherited from re-forestation works carried on for decades by the National Forestry Corps to hinder the soil degradation. The high drop between maximum basin altitude (Rocciamelone Mt., 3538 m a.s.l.) and the fan apex (900 m a.s.l.) is quickly attained over a stretch of only 4 km, with an average slope gradient of 60% and maximum of 80%. The average annual precipitation equals to 820 mm, monthly rainfall depth is highest in Autumn and Springtime, accounting as a whole for 62% of the yearly total. A 1-3 m thick snow mantle usually caps the slopes above 2500 m a.s.l. from half October to the end of June. In spite of reforestation and hydraulic works realized in recent years, debris flows (2-years return period between 1991-2011) still pose significant hazard to human settlements located downstream. In the last one hundred years, thanks to historical documents andon-site monitoring conducted by the CNR-IRPI 31 debris flow events which caused significant damages have been reported. Generally, the mass transport is dominated by channelized debris flow occurring during the summer season, without damages for settlements; nevertheless, major events in the past impacted on the national road at the valley bottom. Since July 1994 the Marderello catchment had been instrumented for a best knowledge of the triggering factors, debris flow dynamics and related effects, in such a severe mountain environment, also in the sight to suggest possible structural interventions for preventing or reducing future damage in the underlying Novalesa village and ensure safety to hundreds of resident and floating population (Tropeano et al., 1996). The rainfall monitoring network consists of four raingauges placed at different elevations, between 800 and 2854 m a.s.l.; others meteorological data (air moisture and temperature, atmospheric pressure, wind speed and direction) are provided by three MICROS® radio-transmitting stations located at 3150, 2150 and 830 m a.s.l.. In 2013 the monitoring system has been further improved and extended on the alluvial fan with the installation of one ultrasonic device and four geophones (at a distance of around 50 m reciprocally) in order to detect debris flow wave-fronts depth and time-to-arrival. To reduce the amount of recorded data, the original ground velocity signal measured by the geophones is transformed into a mean value of amplitude of the velocity signal second per second (Arattano, 1999). The research presently focuses on: (i) the investigation of rainfall characteristics, as recorded to different catchment elevations, in the sight to detect rainfall triggering values for debris flow initiation (Turconi et al., 2008); (ii) the collection of debris-flow seismic data for the future development of a warning system for the lowest part of the alluvial fan. References. Arattano, M. (1999). On the use of seismic detectors as monitoring and warning systems for debris flows. Nat. Hazards, 20, 197-213. Tropeano D., Casagrande A., Luino F., Cescon F. (1996). Processi di mud-debris flow in Val Cenischia (Alpi Graie). Osservazioni nel bacino del T. Marderello. Quaderno di studi e di documentazione n°20 - Suppl. a GEAM Anno XXXIII, n. 2-3. Turconi L., Kumar De S., Tropeano D., Savio G. (2010). Slope failure and related processes in the Mt. Rocciamelone area (Cenischia Valley, Western Italian Alps), Geomorphology, Volume 114, Issue 3, 115-128.