Susceptibility analysis of shallow landslides source areas using physically based models

Rainfall-induced shallow landslides of the flow-type involve different soils, and they often cause huge social and economical disasters, posing threat to life and livelihood all over the world. Due to the frequent large extension of the rainfall events, these landslides can be triggered over large areas (up to tens of square kilometres), and their source areas can be analysed with the aid of distributed, physically based models. Despite the high potential, such models show some limitations related to the adopted simplifying assumptions, the quantity and quality of required data, as well as the use of a quantitative interpretation of the results. A relevant example is provided in this paper referring to catastrophic phenomena involving volcaniclastic soils that frequently occur in southern Italy. Particularly, three physically based models (SHALSTAB, TRIGRS and TRIGRS-unsaturated) are used for the analysis of the source areas of huge rainfall-induced shallow landslides occurred in May 1998 inside an area of about 60 km². The application is based on an extensive data set of topographical, geomorphological and hydrogeological features of the affected area, as well as on both stratigraphical settings and mechanical properties of the involved soils. The results obtained from the three models are compared by introducing two indexes aimed at quantifying the “success” and the “error” provided by each model in simulating observed source areas. Advantages and limitations of the adopted models are then discussed for their use in forecasting the rainfall-induced source areas of shallow landslides over large areas.