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A physically based mathematical model of the slope of Cervinara (southern Italy), which is characterized by a shallow pyroclastic soil cover laying upon a limestone fractured bedrock, has been developed. Previous and current ongoing monitoring suggested that leakage through the soil-bedrock interface occurred, with leaking water temporarily stored in a perched aquifer located in the upper part of the fractured limestone (epikarst). This aquifer supplied several springs, and recharge to the deeper groundwater circulation occurred. Hence, in the proposed model, the unsaturated water flow taking place within the soil cover is coupled with the saturated water flow in the perched aquifer. The application of the model to the simulation of the slope hydrologic behavior over a period of 11 years, between 2006-2017, provides realistic results in terms of soil storage, epikarst storage, spring discharge, and groundwater recharge. The different response times of soil and epikarst aquifer to precipitation input allow distinguishing the hydrological predisposing causes of potential landsliding (i.e., a few months of persistent rainfall that is capable of filling the epikarst aquifer) from the triggers, which are represented by single intense rainfall events. The application of the model offers a key of interpretation of the hydrological processes leading to the landslide that occurred on 16 December 1999.Water 2018, 10, 948 2 of 17 e.g., Glade et al., Chleborad et al. [11,12], which are in any case much longer than the triggering event duration. This confirms the role played by large-scale (in space and time) hydrological processes as the predisposing causes of landslides, with the triggering rainfall being only the last push for the slope to fail [13].In Campania (southern Italy), steep slopes covered with a few meters of pyroclastic loose granular deposits, which are mainly ashes (sand to loamy sand) with some layers of pumices (sandy gravel) laying upon fractured limestone bedrock, are sometimes subjected to fast destructive flowslides e.g., Picarelli et al. [14]. Although it is commonly recognized that slope failure occurs when rainfall infiltration leads to the vanishing of the contribution to shear strength offered by soil suction in unsaturated conditions e.g., Lu and Likos, Greco and Gargano [15,16], the process leading to the establishment of such a condition is still debated. Many studies indicate that a prominent role is played by the hydraulic behavior of the soil-bedrock interface. In some cases, the interface is considered as poorly pervious, owing to the presence of a layer of fine-textured altered soil (silty sand) with low hydraulic conductivity [17,18]. In other studies, the interface is modeled as a capillary barrier due to the coarse dimension of limestone fractures, under the hypothesis that they contain air at atmospheric pressure [19]. Conversely, other authors consider the soil-bedrock interface pervious, and focus on the formation of perched aquifers within the fractured limestone, which affect the wa...
Many mountainous areas in Campania, Southern Italy, are characterized by steep slopes covered by loose unsaturated pyroclastic deposits laying upon fractured limestone bedrock. The soil covers are mainly constituted by layers of ashes and pumices. Large and intense rainfall events trigger shallow landslides, often turning into debris flows that cause huge damage and casualties. The slope of Cervinara, around 40 km Northeast of Naples, was involved in a catastrophic flowslide on 16 December 1999, triggered by a rainstorm of 325 mm in 48 h. To capture the main effects of precipitation on the slope stability, hydro-meteorological monitoring activities have been carried out at the slope to assess the water balance for three years (2017–2020). The field monitoring data allowed the identification of the complex hydrological processes involving the unsaturated pyroclastic soil and the shallow groundwater system developing in the limestone bedrock, which control the conditions that potentially predispose the slope to landslide triggering. Specifically, late autumn has been identified as the potentially most critical period, when slope drainage processes are not yet effective, and soil covers already receive large amounts of precipitation.
Slopes covered with unsaturated shallow pyroclastic deposits, lying upon fractured limestone bedrock, are widespread in the mountains around Naples (southern Italy). Rainfall infiltration, reducing soil suction, eventually triggers shallow landslides. While drastic reduction of suction is unanimously recognized as the triggering mechanism, there is still debate about the hydrological processes controlling slope drainage and causing the establishment of landslide predisposing conditions. Field observations at the slope of Cervinara suggested that temporary storage of water in a perched aquifer, in the upper part of the fractured bedrock, may affect the leakage through the soil-bedrock interface. Hence, a physically based model, coupling flows in the unsaturated soil cover and in the perched aquifer, has been applied to three large rainfall events which occurred in December 1999 (when a landslide was triggered), January 2009, and November 2012. The results highlight that the different responses of soil and aquifer to precipitations, related not only to rainfall event characteristics (i.e., duration and mean intensity) but also to the initial conditions of the slope, determined by antecedent precipitations, can play a prominent role in the triggering of landslides. In fact, further simulations with synthetic rainfall events and different initial conditions provide a possible interpretation of the triggering of the landslide of December 1999, indicating that a soil profile with dry conditions at the base and a low level in the perched aquifer, typical of late autumn, can impede the drainage of infiltrating water through the soil-bedrock interface, thus favoring the buildup of pore pressure within the soil cover.
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