Debris flow hazard assessment in small catchments with diffuse pyroclastic soil deposits: a case study in Cervinara (southern Italy)

The study presents an example of application of the guidelines for the assessment of hydraulic hazard and risk assessment in small catchments, developed within the research project RETURN-PB (https://www.fondazionereturn.it/en/portfolio/nuovi-approcci-per-la-valutazione-della-pericolosita-idraulica-nei-piccoli-bacini-montani-return-pb/). The case study refers to the limestone reliefs of Campania (Italy), characterized by diffuse presence of loose pyroclastic soil deposits, originated by air-fall deposition from several eruptions of the volcanic complexes of the area (Somma-Vesuvius and Phlegrean Fields).

The soil deposits are a few meters thick and consist of ashes (loamy sands) and pumices (gravels with sand), characterized by very high porosity (up to 75% in the ashes) and saturated hydraulic conductivity (in the order of 10^-4 m/s). These characteristics make the infiltration and retention capacity of the soil deposit so high that, even during the most intense rainfall, overland runoff is quite small, with runoff coefficient rarely exceeding 2%. The deposits are fairly cohesionless and present effective friction angles in the range of 36° to 38°. Nonethless, unsaturated soil deposits with thickness of 1 to 2 meters rest also on slopes with inclination higher than the friction angle, thanks to the apparent cohesive contribution given by soil suction. After intense and prolonged rainfall, the increase of soil moisture and the consequent reduction of suction can lead to the instability of the soil deposit and the triggering of shallow landslides featured as debris avalanches. Owing to the unstable loose soil fabric and the coarse particles, the deposits undergo volumetric contraction under shear deformation, which can lead to the establishment of positive pore water pressure, favoring soil liquefaction. This behavior is responsible for the frequent evolution of landslides in the form of debris flows. Thanks to the steepness of the slopes, the flows reach speed as high as tens of m/s, running out long distance from the original landslide scarp, often channelized through streams that reach nearby towns and villages, with huge damage.

As an example of how debris flow hazard can be assessed in the studied context, the debris flow occurred on 16 December 1999 in Cervinara is modelled. The debris flow was triggered after a rainfall with more than 300 mm in 48 hours, as recorded by a rain gauge less than 2 km from the failed slope. The failure involved a volume of around 30000 m³ of soil, that flew in the form of a liquefied mud hitting the village of Ioffredo, a hamlet of Cervinara, where several buildings were destroyed and five people were killed.

A modelling chain consisting of rainwater infiltration modelling, slope stability analysis, debris flow propagation and impact is applied. The effects of the uncertainty of slope and soil properties, as well as of debris flow behavior are discussed, with an ensemble modelling approach. Specifically, the propagation of the debris flow is simulated with different modelling approaches under different hypotheses (i.e., fixed or erodible bed; single- or double-phase fluid; various rheological formulations with dilution-dependent parameters). The results highlight how the application of ensemble modelling allows introducing the effects of uncertainty in the assessment of hydraulic hazard and risk.