Role of land use in landslide initiation on terraced slopes. Inferences from numerical modelling

Land use/land cover is widely considered as a relevant factor for shallow landslide occurrence; therefore, in this work we used a physically-based model, whose input parameters have been evaluated taking into account the land use setting. In detail, assuming that vegetation affects the surface runoff and infiltration rate, as well as increases the shear strength of soil due to the root reinforcement effect, we calibrated several specific model parameters (such as friction angle, cohesion and hydraulic conductivity) by varying their values within a reasonable range, on the basis of a detailed land use map specifically prepared for some small coastal basins located in the Cinque Terre area (eastern Liguria, Italy). Since this area is characterized by the widespread presence of agricultural terraces, mainly abandoned, their degree of abandonment has also been considered. Different numerical simulations have been performed with TRIGRS (transient rainfall infiltration and grid-based slope stability), a well-known regional, physically-based stability model, with the aim of reproducing the landslide event occurred in the study area on October 25th, 2011. The safety factor (FS) maps obtained by using TRIGRS have been compared with the inventory map of the landslides triggered during the event. Then, a ROC (receiver operating characteristic) curve analysis has been carried out in order to quantify the performance of the model. The obtained results emphasize the influence of land use in shallow landslide occurrence.