Distribution of landslides in the Upper Tiber River basin, central Italy

A comprehensive analysis of the distribution of landslides was completed in the Upper Tiber River basin, central Italy. Landslide occurrence was ascertained through a large-scale inventory obtained through air-photo-interpretation (API) and local field surveys. Lithological and bedding information was obtained through photo-geological and field mapping, aided by the review of geological maps and reports. The probability density of landslide areas was found comparable to statistics obtained by investigators in other areas of the world. Adoption of empirical relationships to link landslide area and volume allowed estimating the total volume of landslide material in the 4098 km2 catchment in excess of 5.9 × 109 m3. Statistics of terrain elevation and slope in stable areas were found different from those in landslide areas. In particular, modal terrain slope in landslide areas was less steep (11.9°) than that in areas where landslides were not mapped (13.3°). Analysis of the landslide and slope areas revealed that a relationship exists between the size of the slope and the size of the most common landslide. Analysis of the spatial relationship between landslides and the geological setting revealed that slope failures are most abundant where soft and weak rocks crop out, and where bedding is chaotic or disorganized. Where rocks are regularly bedded, landslide abundance is largest in dip-slopes. These new findings are important to understand the long-term evolution of the Upper Tiber River basin, and may prove useful in the assessment of landslide hazard and risk.