Control of Hydrological Seasonal Variability of Ash-fall Pyroclastic Deposits on Rainfall Triggering Debris Flows in Campania (Southern Italy)

Ash-fall pyroclastic deposits covering Campanian mountain slopes (southern Italy) are very prone to instability under heavy and prolonged rainfall. In such a geo-hazard framework, to understand hydrological dynamics of pyroclastic mantle is a further step toward the assessment of rainfall thresholds and landslide hazard. In this research, the hydrological and stability modelling of representative ash-fall pyroclastic soil mantled slopes of the Sarno Mountains is proposed to assess the role of seasonal hydrological variability of the pyroclastic cover on rainfall triggering debris flows. The approach is based on the numerical modelling, from seasonal to inter-annual timescales, of unsaturated/ saturated flows occurring into the pyroclastic mantle upslope of a source area of a debris flow. Modelling results were calibrated by means of field measurements achieved by a monitoring station equipped with tensiometers and Watermark sensors. Among the main results, the recorded pressure head time series showed a dominant unsaturated condition and a very relevant decrease in the summer season, whose effects are not limited to the root zone but extended down to the bedrock interface, about 4 meter deep. This seasonal hydrological regime of the ash-fall pyroclastic mantle was correlated both to the distinctive water retention properties of pyroclastic soils and to the existence of a deciduous forest, which concentrates the evapotranspiration demand during the dry season. Hydrological and stability modeling of the representative slopes allowed the reconstruction of deterministic rainfall thresholds for both dry and wet seasons and the assessment of the significant predisposing role of antecedent hydrological conditions to slope instability during short and heavy rainstorms.