Seasonal effects of rainfall on the shallow pyroclastic deposits of the Campania region (southern Italy)

The shallow deposits of unsaturated pyroclastic soils covering the slopes in the Campania region (southern Italy) are systematically affected by various rainfall-induced slope instabilities. The type and triggering of these instabilities depend on several factors, among which in situ soil suction—as an initial condition—and rainfall—as a boundary condition—play a fundamental role. Based on the available database—which includes a comprehensive catalogue of historical data, in situ soil suction measurements and soil laboratory tests along with the results of geomechanical analyses—this paper discusses the relationships among in situ soil suction and rainfall conditions and induced slope instability types. The goal is to reach a better understanding of past events and gain further insight into the analysis and forecasting of future events. In particular, the paper outlines how the season strongly affects the spatial distribution and the type of slope instability likely to develop. For example, erosion phenomena essentially occur at the end of the dry season and originate hyperconcentrated flows while first-time shallow slides prevail in the rainy season and later propagate as debris flows or as debris avalanches.