An inventory-based approach to landslide susceptibility assessment and its application to Virginio river basin, Italy

An inventory-based method for the assessment of landslide susceptibility is presented in this article. The method has been tested in the Virginio River Basin, a tributary of the Arno River whose confluence is located about 20 km downstream from Florence (Italy). The scope of this study includes setting up a procedure for landslide hazard zoning to be applied by those urban planners typically working on small areas at large scale. The proposed method deals with traditional and well-known landslide hazard analyses, based on geomorphological tools, and its most original contribution is represented by the attempt to carry out and apply a technique for landslide hazard assessment that takes into account two different scales of analysis. The basis of a detailed landslide inventory and the first phase of this research was an in-depth geomorphological investigation at basin scale (1: 25,000-1:10,000). This was aimed at indicating the most important factors influencing the landslide processes within the area, which also turned out to be the most generally accepted factors: a) slope gradient in which the landslide originated, b) lithology, and c) land cover. Once those factors were defined as thematic vector data, they were expressed using GIS overlay mapping, allowing the identification, for the entire Virginio River basin, of first-order homogeneous domains (Unique Condition Units, UCUs) that contain, for each landslide type, unique combinations (domains) of the selected hillslope stability factors. The domains are the basic Terrain Units for the subsequent landslide susceptibility assessment and mapping, which was carried out at slope scale (1: 10,000-1:2,000). Landslide factors not identified in the first phase of analysis, but considered to have played an important role in contributing to the activation of the mass movements, so-called second-order landslide preparatory factors, have been taken into account in the second phase of the analysis. Once mapped and spatially referenced, these factors were overlain by vector-based GIS techniques to define second-order UCUs which, in turn, constituted the basis of a landslide susceptibility function. Essentially, this is a logic function based on the presence/absence of preparatory factors and slope instability indicators within previously selected Unique Condition Terrain Units. The final mapping of the areas characterized by different landslide susceptibility levels was performed by vector- and raster-based GIS techniques on the basis of the number and rank of the preparatory factors. The final landslide susceptibility classes, defined by a logical and easily replicable procedure, are considered to be useful in the decision-making procedures associated with territorial planning.