Long-Term Hydrological Modelling of Pyroclastic Soil Mantled Slopes for Assessing Rainfall Thresholds Triggering Debris Flows: The Case of the Sarno Mountains (Campania—Southern Italy)

Air-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 step further toward the deterministic assessment of rainfall thresholds and landslide hazard. In this research, the hydrological modelling of a pyroclastic soil mantled slope of the Sarno Mountains is proposed to assess the role of antecedent hydrological conditions on rainfall triggering debris flows. The approach is based on the finite difference 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 tensiometer monitoring station. Among the main results, the pressure head distribution into the pyroclastic deposits showed a dominant unsaturated condition and a relevant seasonal variation extending below the root zone and down to the bedrock interface, about 4 m deep. This hydrological regime is attributable both to the distinctive water retention properties of pyroclastic soils and to the existence of a deciduous forest, which concentrates water losses due to evapotranspiration during the dry season. This behavior highlights the remarkable role of antecedent hydrological conditions as a not negligible predisposing factor to instability during short and heavy rainstorms.