Analisi delle condizioni di stabilità del versante Settentrionale del Monte Pelmo (Dolomiti Orientali, Veneto) = Analysis of stability conditions of the northern side of Mount Pelmo (Dolomites, NE Italy)

Debris flows and rockfalls are hazardous phenomena that occur on Alpine slopes. In this study two of these types of landslides are analyzed. The study area is the northern side of Pelmo Mount in Dolomites area (Eastern Alps). The upper portion of this slope is a steep 900-m high cliff made by late Triassic to early Jurassic dolostones and limestones; on August 31st, 2011 a huge block of rock detached from the cliff, below the top of the mountain and fell killing 2 members of the mountain rescue. At the base of the slope, a thick scree slope is present, up to 20 m thick. This deposit is composed even by glacial till of a moraine ridge of Little Ice Age (LIA). Under this deposit an ice mass, up to 30 m thick, is settled, inherited from a local cirque glacier. On September 14th, 1994 a large debris flow triggered by a storm with a rainfall pick of 10 mm in 15 min, involved the western part of LIA moraine and mobilized 200,000 m3 of loose materials that reached the SP251 road. The aim of this work is to identify the causes of these events through traditional and innovative remote sensing methods. The use of terrestrial laser scanner (TLS) on the rockwall has allowed to work safely and to realize in a brief time, a detailed 3D elevation model (DEM) from the point cloud. Joint bearing and some geotechnical parameters of the rock mass, such as fracture spacing and joint roughness have been estimated. These data have been validated by scanline measurements. The volume of some instable blocks isolated by tectonic fractures, has been estimated by measures taken from the point cloud. It has resulted that the rockfall of 31st August, 2011 was triggered by freeze-thaw cycles on open, weathered tectonic fracture isolating a parallelepiped-shape block of 6600 m3 that crushed down the rockwall. The morphology of the debris flow has been investigated by aerial photogrammetry conducted with the use of a model aircraft equipped with a camera and differential GPS at ground. The DEM has been compared through GIS-software with another that has been extracted from a detailed map (1:5000) realized by ARPAV in October 1995 by topographic survey. From the DEM of Difference (DoD) it has been possible to asses, in the last 17 years, a loss of material at initiation area of 170,000 m3 and an accumulation of debris at deposition area of 145,000 m3. Erosion concentrated on the western side of moraine whereas sedimentation moved toward the north-eastern side of Pelmetto Mount. An analysis of temperature and rainfall data series collected from 1986 to 2011 at some stations managed by ARPAV, in the study area, shows an increase of annual mean maximum and minimum temperature. In this period the maximum summer temperature has increased of 1 °C. The distribution of seasonal rainfall has changed: the total amount in autumn has increased. Field surveys and observation of aerial photographs allowed the realization of a geomorphologic map at the scale 1:10,000 which shows the evolution of the northern side of Pelmo Mount in the last 50 years. It has been resulted that the channel of debris flow, the avalanche cones and the moraine ridge have changed in this period. The presence of a stepped surface formed in the last 10 years on the moraine surface seems to suggest a progressive melting of the buried ice mass in response to climate change. This fact can reduce the stability of boulders on the outer side of the moraine and promote erosion. Finally at about 350 rockwall events were considered from 3 database: 2 of these came from IFFI Project and another from the PERMAdataROC European Project. The comparison evidences the fact that data concerning the date of the event and the volume of the deposit are often missing. In IFFI database a very few landslides has occurred over 2000 m of altitude. This can probably been explained because the phenomena has been affected sparsely populated areas or has not caused damages to facilities or injuries. This fact highlights a problem about the knowledge of hazardous mass movements in Alpine environment. Landslides can affect: tracks frequented by hikers, rockwall by climbers and communication routes, especially during summer. It is important to solve this lack of information because the climate change in the future would favour landslide activity. This study would stress the importance of monitoring unstable slopes as shown on the northern side of Pelmo Mount.