Geomorphological studies of slope instabilities in permafrost regions of the Aosta Valley (NW-Italy): a statistic and cartographic approach to the analysis of landslide susceptibility

Recent changes in global climate are showing their evident effects both in polar and alpine environments. The observed effects are various, with significant implications on natural hazard but also on the socio-economic context, with evident risk implications: reduced snow cover duration, glaciers retreat, permafrost degradation as well as increased slope instability in high mountain areas. Geomorphological studies have been performed at a regional scale in the Aosta Valley Region (a small alpine region located in NW Italy) to better understand landslide spatial and temporal frequency and to investigate the main controlling factors of instability in high mountain areas of possible permafrost presence and subsequent degradation following climatic changes. A comparison between the high mountain areas and the rest of the territory has been also performed. A great amount of data has been collected and analyzed into a GIS system: different landslides inventories (IFFI Project National Database, Regional “Catasto Dissesti”), permafrost distribution maps and several geothematic maps produced within the research for the analysis of controlling factors of instability (i.e. slope and aspect maps derived from the Digital Elevation Model, for topographic factors, and some geological maps for litho-structural ones). Then, spatial analysis and statistical studies have been applied to 255 identified landslides within the areas with permafrost: mainly rock falls, corresponding to a landslide density equal to 0.48 events/km2 and a landslide index of 11%. Data considerably lower than the analogous values calculated for the whole region (respectively 1.18 events/km2 and 19%). Furthermore, in these areas it has been observed that, in terms of landslides mean areal extent, the obtained values are higher than those of the rest of the region for almost all types of movement (i.e. rock falls increase in the mean areal extent is more than 40%). Only in the case of Deep-seated Gravitational Slope Deformations the values are considerably inferior, because of their lower evolution degree in high mountain areas. The landslide index calculated for all the classes of the controlling factors considered within the research highlighted how, almost in all cases, areas with permafrost show lower values than the rest of the territory. This result could have a double interpretation: a frequency underestimation of the surveyed phenomena in these areas, bias that is typical of high mountain areas, and a lower landslide susceptibility related to fair rock mass geomechanical properties, which usually result in a higher degree of stability due to the presence of permafrost.