Recurrent rainfall-induced landslides on the slopes with pyroclastic cover of Partenio Mountains (Campania, Italy): Comparison of 1999 and 2019 events

Rainfall-induced shallow landslides occurring along slopes of mountains around the city of Naples (southern Italy), characterized by pyroclastic covers laying upon fractured limestone bedrock, pose serious threat to settlements and towns located at their foot. Although the triggering mechanism, related to suction reduction in the initially unsaturated soil cover is quite clear, there is still debate about the hydrological processes which predispose the slopes to failure. In this paper, the predisposing factors are analyzed with reference to two intense storms, both with about 300 mm rainfall, occurred in the same period of the year in 1999 and 2019. The two storms triggered several shallow landslides in an area of few km(2) of the northeast-facing slopes of Partenio Mountains. The two events are compared analyzing both the characteristics of antecedent and event precipitations, as well as simulating the response of the slopes to rainfall with the help of a mathematical model. Specifically, two slopes are analyzed: one, near the town of Cervinara, where landslides were triggered only in 1999; another, near the town of San Martino Valle Caudina, where landslides occurred during both the events. The results of the model simulations indicate that rainstorm duration (shorter in 1999) and antecedent precipitation during the weeks before the event (smaller in 1999) are the main reasons of the different responses of the slopes to the two rainfall events. In fact, both drier initial conditions at the base of the soil cover and shorter storm duration limit the possibility of drainage of infiltrating water out of the soil cover through the soil-bedrock interface, thus favoring the suction reduction required for slope failure. The obtained results provide a key for the interpretation of landslides, often occurring in late autumn-early winter in the area.