Landslide susceptibility mapping at national scale: the Italian case study

Landslide risk in Italy is particularly high due to a densely populated and urbanized territory. Landslide susceptibility maps are key tools for land use planning, management and risk mitigation. The aim of the work is to present the methodology adopted by ISPRA with the University of Florence, University of Milano-Bicocca and University of Rome "La Sapienza" for the development of a Landslide susceptibility map of Italy at 1:1,000,000 scale. This research is carried out in the framework of the European Thematic Strategy for Soil Protection and in particular in the activities of the JRC European Landslide Expert Group dealing with the definition of guidelines for delineating landslide prone areas in Europe and the development of preliminary models for landslide susceptibility assessment at European and national scale. The Italian Landslide Inventory - Progetto IFFI and a minimum set of conditioning factors have been used for the realization of the Landslide susceptibility map of Italy: a) slope angle map derived from digital terrain model with cell size 20x20m; b) lithological map obtained from the geological map of Italy 1:500,000; c) land use map (Corine Land Cover). The Italian landslide inventory, which contains 485,004 mass movements covering an area of 20,721 km2 (6.9% of the Italian territory), has been subjected to a quality analysis with the aim of assessing the completeness, homogeneity and reliability of data, and identifying representative areas which may be used as training and test areas for the implementation of landslide susceptibility models. Similarly, this analysis has allowed the identification of critical areas in which data are incomplete or not very accurate. An analysis of the DEM 20x20m has been also carried out in order to assess the accuracy and reliability of the morphometric parameters, through a comparison with other DEM: LiDAR (1m resolution) and SRTM (80m resolution). In order to implement the susceptibility models, plain areas of the Italian territory have been excluded applying slope angle and elevation thresholds to the DTM and physiographic domains of homogeneous geology and geomorphology have been identified. Landslides of the IFFI database have been divided into three main classes in order to take into account specific sets of conditioning factors for each landslide type: a) rock falls; b) slow mass movements, c) rapid debris flows. Bivariate statistical analyses have been performed to assess the frequency distribution of contributing factors on the landslide area. After the definition of the mapping units, the susceptibility models have been calibrated and validated over the whole Italian territory. Landslide susceptibility map of Italy at 1:1,000,000 scale can be an important support for the implementation of pan-European landslide susceptibility map and a useful tools for the EU policies and measures finalized to the landslide risk reduction and mitigation.