Ferdinando Manna

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.

The Instability of Colluvial Mantle in Turbidite Flysch Series of the Cilento Region (Campania–Southern Italy): the November 26, 2010, Ostigliano Translational Slide

On November 26, 2010, heavy rainfall triggered a shallow landslide which threatened the historical center of Ostigliano (Cilento and Vallo di Diano National Park, southern Italy). Faint but diffused damages on ancient buildings led to the evacuation of 60 families and to the assessment of the reactivation hazard by a monitoring campaign. The landslide caused a very low morphological impact due to: displacement of the depleted mass, limited to less than 1 m, shallowness of the surface of rupture and translational kinematics. Geological surveys, boreholes, inclinometric measurements, dynamic penetration and geotechnical laboratory tests were carried out to characterize and monitor the mass movement. The analysis of these data allowed to reconstruct an engineering geological model of the landslide, and to characterize it as a shallow phenomenon involving the colluvial deposits formed over a turbidite flysch bedrock. Based on peak and residual shear strength tests, infinite slope stability analyses of surficial deposits were carried out, simulating different thicknesses and pore pressure conditions. Among principal results, the landslide extension was observed constrained to zones where the thickness of colluvial deposits is greater than 2 m. Moreover, instability conditions were found for residual value of shear strength only. Results of this study are proposed as a first contribute to depict this type of shallow mass movements affecting the Cilento region, whose most part is characterized by turbitide flysch series and colluvial mantled slopes.

Recharge in karst aquifers: from regional to local and annual to episodic scale

The assessment of groundwater recharge for karst aquifers of southern Italy is a major scientific task due to the relevant socio-economic and environmental role of the related groundwater resources. In this paper, the results of two methods, applied at different spatial and time scales, are reported.At regional and mean annual scales, through a multidisciplinary approach, the mean Annual Groudwater Recharge Coefficient (AGRC) was estimated for four sample karst aquifers, with available long-lasting spring discharge time series. Such estimations were extended to other karst aquifers of southern Italy by means of an empirical law that was found linking the AGRC to percentages of outcropping lithologies and endorheic/summit plateau areas.At local and episodic scales, the groundwater recharge of a test perched karst aquifer, belonging to the Mount Terminio hydrogeological structure (Campania region, southern Italy) was estimated. For such a purpose, an improvement of the Water Table Fluctuation (WTF) method, known as Episodic Master Recession (EMR), was applied to estimate the Recharge to Precipitation Ratio (RPR) coefficient, which represents the amount of precipitation recharging groundwater.Results obtained through the first approach furnished AGRC values varying between 50% and 79% and well matching with estimations of infiltration coefficients known in literature for other karst aquifers of Europe. Moreover, the mean value of RPR determined for the local karst aquifer (73%) resulted well matching with the AGRC estimated for the whole Mount Terminio karst aquifer (79%).By the comparison of these outcomes, at the regional and mean annual scales, the groundwater recharge of karst aquifers was found as mainly controlled by both the extension of outcropping lithologies and endorheic/summit plateau zones. While at the local and episodic scales, the groundwater recharge was recognized as chiefly influenced by the rainfall intensity and soil hydrological condition.