Sarno Mountains Research Articles

Triggering conditions and propagation of the December 2019 Palma Campania landslide: Implications for residual hazard estimation at recurrent landslide sites

Flow-like landslides triggered by rainfall are a major concern in the peri-Vesuvian region of southern Italy. On December 19th, 2019, a landslide occurred along the Crocelle slope (Sarno Mountains), which had previously experienced a similar event in February 1986. This study was conducted to assess the characteristics of this event in terms of triggering and propagation conditions and, with reference to the 1986 landslide, to evaluate the evolution of residual landslide hazards at recurring landslide sites. Data collection included field surveys, visual analysis of a Unmanned Aerial Vehicle (UAV)-based photogrammetric model, geotechnical laboratory testing, rainfall and slope stability analysis, and propagation modeling. Results suggest that the landslide i) originated below a trail, ii) involved an initial volume of 450 m3, which did not significantly grow due to the limited entrainment of slope sediments, iii) was initiated after 13 h of rainfall (return period ∼4 years), and iv) propagated along a channel with an estimated velocity of up to 10 m/s. Comparison of the 2019 event with the 1986 event revealed a significant difference in terms of landslide magnitude (i.e., volume), potentially due to the occurrence of multiple events, which may have led to the temporal depletion of available materials along a slope repeatedly affected by landslides, and possibly to the different behavior of primary layered deposits (involved in the 1986 event) compared to reworked deposits (involved in the 2019 event). This suggests a reduction in the residual landslide hazard associated with a decrease in the expected magnitude and frequency of prospective events.

Clues of wildfire-induced geotechnical changes in volcanic soils affected by post-fire slope instabilities

Wildfires can significantly affect mountain hillslopes through the combustion of trees and shrubs and changes in soil properties. The type and magnitude of the associated post-fire effects depend on several factors, including fire severity and soil physical–mechanical-hydraulic features that, coupled with climate and topographic conditions, may cause increased runoff, erosion, and slope instability as consequence of intense rainfall. The post-fire response of slopes is highly site-specific. Therefore, in situ surveys and laboratory tests are needed to quantify changes in key soil parameters. The present study documents the post-fire physical and hydromechanical properties of pyroclastic topsoil collected from three test sites that suffered wildfires and rainfall-induced post-fire events in 2019 and 2020 in the Sarno Mountains (Campania Region, southern Italy). The tested pyroclastic soils in burned conditions show (i) no significant changes in grain size distribution, soil organic matter, and specific gravity; (ii) a deterioration in shear strength in terms of decreased soil cohesion caused by the fire-induced weakening of root systems; and (iii) a decrease in hydraulic conductivity. Accordingly, it can be argued that the documented post-fire erosion responses were mainly caused by the reduced cohesion and hydraulic conductivity of the burned topsoil layer, as well as by the loss of vegetation cover and the deposition of fire residues. Although deserving further deepening, this study can represent the necessary background for understanding the initiation mechanism of post-fire erosion processes in the analyzed area and on several natural slopes under similar conditions.

Hydrological regimes in different slope environments and implications on rainfall thresholds triggering shallow landslides

Assessing hazard of rainfall-induced shallow landslides represents a challenge for the risk management of urbanized areas for which the setting up of early warning systems, based on the reconstruction of reliable rainfall thresholds and rainfall monitoring, is a solution more practicable than the delocalization of settlements and infrastructures. Consequently, the reduction in uncertainties affecting the estimation of rainfall thresholds conditions, leading to the triggering of slope instabilities, is a fundament task to be tackled. In such a view, coupled soil hydrological monitoring and physics-based modeling approaches are presented for estimating rainfall thresholds in two different geomorphological environments prone to shallow landsliding. Based on the comparison of results achieved for silty–clayey soils characterizing Oltrepò Pavese area (northern Italy) and ash-fall pyroclastic soils mantling slopes of Sarno Mountains ridge (southern Italy), this research advances the understanding of the slope hydrological response in triggering shallow landslides. Among the principal results is the comprehension that, mainly depending on geological and geomorphological settings, geotechnical and hydrological properties of soil coverings have a fundamental control on the timing and intensity of hydrological processes leading to landslide initiation. Moreover, results obtained show how the characteristics of the soil coverings control the slope hydrological response at different time scales, making the antecedent soil hydrological conditions a not negligible factor for estimating landslide rainfall thresholds. The approaches proposed can be conceived as an adaptable tool to assess hazard to initiation of shallow rainfall-induced landslides and to implement early-warning systems from site-specific to distributed (catchment or larger) scales.

Hydrological control of soil thickness spatial variability on the initiation of rainfall-induced shallow landslides using a three-dimensional model

Thickness and stratigraphic settings of soils covering slopes potentially control susceptibility to initiation of rainfall-induced shallow landslides due to their local effect on slope hydrological response. Notwithstanding the relevance of the assessment of hazard to shallow landsliding at a distributed scale by approaches based on a coupled modelling of slope hydrological response and slope stability, the spatial variability of soil thickness and stratigraphic settings are factors poorly considered in the literature. Under these premises, this paper advances the well-known case study of rainfall-induced shallow landslides involving ash-fall pyroclastic soils covering the peri-Vesuvian mountains (Campania, southern Italy). In such a unique geomorphological setting, the soil covering is formed by alternating loose ash-fall pyroclastic deposits and paleosols, with high contrasts in hydraulic conductivity and total thickness decreasing as the slope angle increases, thus leading to the establishment of lateral flow and an increase of pore water pressure in localised sectors of the slope where soil horizon thickness is less. In particular, we investigate the effects, on hillslope hydrological regime and slope stability, of irregular bedrock topography, spatial variability of soil thickness and vertical hydraulic heterogeneity of soil horizons, by using a coupled three-dimensional hydrological and a probabilistic infinite slope stability model. The modelling is applied on a sample mountain catchment, located on Sarno Mountains (Campania, southern Italy), and calibrated using physics-based rainfall thresholds derived from the literature. The results obtained under five simulated constant rainfall intensities (2.5, 5, 10, 20 and 40 mm h−1) show an increase of soil pressure head and major failure probability corresponding to stratigraphic and morphological discontinuities, where a soil thickness reduction occurs. The outcomes obtained from modelling match the hypothesis of the formation of lateral throughflow due to the effect of intense rainfall, which leads to the increase of soil water pressure head and water content, up to values of near-saturation, in narrow zones of the slope, such as those of downslope reduction of total soil thickness and pinching out of soil horizons. The approach proposed can be conceived as a further advance in the comprehension of slope hydrological processes at a detailed scale and their effects on slope stability under given rainfall and antecedent soil hydrological conditions, therefore in predicting the most susceptible areas to initiation of rainfall-induced shallow landslides and the related I-D rainfall thresholds. Results obtained demonstrate the occurrence of a slope hydrological response depending on the spatial variability of soil thickness and leading to focus slope instability in specific slope sectors. The approach proposed is conceived to be potentially exportable to other slope environments for which a spatial modelling of soil thickness would be possible.

Incorporating the Effects of Complex Soil Layering and Thickness Local Variability into Distributed Landslide Susceptibility Assessments

Incorporating the influence of soil layering and local variability into the parameterizations of physics-based numerical models for distributed landslide susceptibility assessments remains a challenge. Typical applications employ substantial simplifications including homogeneous soil units and soil-hydraulic properties assigned based only on average textural classifications; the potential impact of these assumptions is usually disregarded. We present a multi-scale approach for parameterizing the distributed Transient Rainfall Infiltration and Grid-Based Regional Slope-Stability (TRIGRS) model that accounts for site-specific spatial variations in both soil thickness and complex layering properties by defining homogeneous soil properties that vary spatially for each model grid cell. These effective properties allow TRIGRS to accurately simulate the timing and distribution of slope failures without any modification of the model structure. We implemented this approach for the carbonate ridge of Sarno Mountains (southern Italy) whose slopes are mantled by complex layered soils of pyroclastic origin. The urbanized foot slopes enveloping these mountains are among the most landslide-prone areas of Italy and have been subjected to repeated occurrences of damaging and deadly rainfall-induced flow-type shallow landslides. At this scope, a primary local-scale application of TRIGRS was calibrated on physics-based rainfall thresholds, previously determined by a coupled VS2D (version 1.3) hydrological modeling and slope stability analysis. Subsequently, by taking into account the spatial distribution of soil thickness and vertical heterogeneity of soil hydrological and mechanical properties, a distributed assessment of landslide hazard was carried out by means of TRIGRS. The combination of these approaches led to the spatial assessment of landslide hazard under different hypothetical rainfall intensities and antecedent hydrological conditions. This approach to parameterizing TRIGRS can be adapted to other spatially variable soil layering and thickness to improve hazard assessments.

Seasonal and Event-Based Hydrological and Slope Stability Modeling of Pyroclastic Fall Deposits Covering Slopes in Campania (Southern Italy)

The pyroclastic fall deposits mantling mountain slopes in the Campania region (Southern Italy) represent one of the most studied geomorphological frameworks of the world regarding rainfall-induced debris flows threating urban areas. The proposed study focused on advancing knowledge about the hydrological response of pyroclastic fall coverings from the seasonal to event-based time scales, leading to the initiation of slope instability. The study was based on two consequential tasks. The first was the analysis of a six-year monitoring of soil pressure head carried out in a sample area of the Sarno Mountains, located above a debris flow initiation zone. The second was based on coupled hydrological and slope stability modeling performed on the physical models of slopes, which were reconstructed by empirical correlations between the slope angle, total thickness, and stratigraphic settings of pyroclastic fall deposits mantling slopes. The results obtained were: (a) The understanding of a soil pressure head regime of the volcaniclastic soil mantle, always ranging in unsaturated conditions and characterized by a strong seasonal variability depending on precipitation patterns and the life cycle of deciduous chestnut forest; and (b) the reconstruction through a deterministic approach of seasonal intensity–duration rainfall thresholds related to different morphological conditions.

Hydro-geomorphological modelling of ash-fall pyroclastic soils for debris flow initiation and groundwater recharge in Campania (southern Italy)

Carbonate mountain ranges surrounding volcanic centers in the Campania region of southern Italy are covered by discontinuous ash-fall pyroclastic deposits of variable thicknesses. The cover thickness and stratigraphy are relevant to hydrological controls on both rainfall-induced landslides and groundwater recharge. To improve understanding of the hydrologic regimes within the pyroclastic soil mantle, a hydrological monitoring station was installed upslope of a debris flow source area in the Sarno Mountains. Monitoring results demonstrate consistently unsaturated conditions, strong seasonal and inter-annual variations in pressure head, and delayed and damped dynamics at different depths related to rainfall and evapotranspiration patterns. Frequencies of recorded pressure head time series were analyzed to quantify the seasonal hydrological regime of the cover as a whole, as well as variations within individual soil horizons. For the whole ash-fall pyroclastic soil cover, variable seasonal frequencies of pressure head were recognized exceeding landslide alert and groundwater recharge threshold values. Analysis of frequencies for individual soil horizons showed a strongly delayed timing determining in winter and summer an opposite hydrological behavior between the shallowest and deepest soil horizons. A model that accounts for topographic variations in cover thickness and these hydrological regimes is proposed to quantify hydro-geomorphological controls on debris flows triggering and groundwater recharge. The model is based on the estimation of ash-fall pyroclastic soil thickness along slopes by the total thickness fallen in a given area and an inverse relationship with slope angle, allowing the assessment at the distributed scale over peri-volcanic mountainous areas. Moreover, it links the spatially variable thicknesses of ash-fall pyroclastic soils to the amount of soil water storage allowing the assessment of frequency of hydrological conditions leading to debris flow initiation and groundwater recharge.

Post-fire erosion response in a watershed mantled by volcaniclastic deposits, Sarno Mountains, Southern Italy

In this study we document a post-fire erosion response to a short-lived, intense rainstorm occurred on 6 September 2012 in the Sant'Angelo creek watershed, Sarno Mountains, Southern Italy. The rainstorm occurred one month after a wildfire that burned about 11ha of the steep watershed (55ha), almost entirely mantled by volcaniclastic deposits. The research was based on fieldwork and laboratory analysis addressed to the understanding of the geomorphic effects of the wildfire and their impact on erosional and depositional processes triggered by subsequent rainstorms. Field evidence indicates that a series of overland flows caused significant runoff and sediment yields along the hillslope and accumulation of hyperconcentrated flow deposits in a concrete channel occluded by a sealed culvert at the outlet of the watershed. The results of geomorphological and sedimentological analysis suggest that the occurrence of volcaniclastic covers mantling the slopes likely favored accelerated soil erosion, especially where vegetation and litter had been removed by the fire. Chemical analysis on sediment samples, revealed the occurrence of iron oxides that enhanced soil water repellency conditions over wide areas of the burned watershed compared to the unburned areas.Quantitative analysis of sediment budgets showed that the rainfall-induced erosion response at Sant'Angelo creek watershed resulted in a soil loss of 19.8–33.1tonsha−1 over burned areas. Post-fire erosion response following severe rainstorms needs to be considered in the spectrum of natural hazards associated with the geomorphological evolution of mountainous landscapes mantled by volcaniclastic deposits.

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.

Hydrological behavior of ash-fall pyroclastic soil mantled slopes of the Sarno Mountains (Campania - southern Italy)

The instability of ash-fall pyroclastic deposits that mantle mountain ranges around the Mount Somma-Vesuvius (Campania, southern Italy) are well known to represent a main geological hazard due to their frequent involvement in shallow flow-like landslides, triggered by high-intensity and prolonged rainfall. In such a geomorphological framework, which is similar to other mountain areas that surround volcanic centers of the world, the comprehension of hydrological dynamics occurring into pyroclastic deposits is a key factor for assessing and modelling the landslide hazard. Despite the general relevance of hydrological analysis in studying shallow landslides, this type of approach is particularly significant for pyroclastic soils due to their special hydrological properties and complex stratigraphic settings. Along with this goal, in a test area of the Sarno Mountains, upslope of a debris flow source area, a monitoring station was installed to assess hydrological processes that predispose and lead to landslide triggering. Monitoring data, covering about three hydrological years (Dec. 2010 - Dec. 2013), showed a regime of pore pressures always ranging in unsaturated conditions. Marked seasonal and interannual variations of pore pressure were observed at different depths with respectively delayed and damped dynamics. These observations were correlated to rainfall patterns and the evapotranspiration regime, which is concentrated during the activity period of the existing deciduous forest. Results of this approach allow understanding hydrological processes at slope scale, to set and calibrate numerical models for estimating rainfall thresholds by a deterministic approach as well as to assess landslide hazard.

A combined use of Archie and van Genuchten models for predicting hydraulic conductivity of unsaturated pyroclastic soils

Hydrogeophysics has been developing in recent years to improve characterization and monitoring of saturated/unsaturated aquifers, through the combination of geophysical and hydrogeological methods. To exploit the potential benefits of this integration, definition of petrophysical relationships that allow the translation of geophysical data into hydrogeological parameters (and vice versa) is essential. In this paper, Archie's and van Genuchten's models, which relate electrical resistivity and hydraulic conductivity to the degree of saturation, are combined to obtain a closed-form link between hydraulic conductivity and electrical resistivity. Such an expression is used to characterize pyroclastic deposits covering Sarno Mountains (southern Italy), which are often affected by landslide phenomena. As expected, an inverse relationship is found between hydraulic conductivity and electrical resistivity due to predominance of the electrolytic component over the interfacial component of the electrical conductivity. The variability of the hydraulic conductivity marks out the texture of the investigated pyroclastic horizons. Finally, a hydraulic conductivity–matric suction relationship is retrieved by using an empirical correlation between electrical resistivity and matric suction.

A study of the stability analysis of pyroclastic covers based on electrical resistivity measurements

Usually, the degree of stability of a slope is quantified by the Factor of Safety whose values depend on physical and mechanical soil properties analyzed on samples of much reduced sizes or referring to very small soil volumes around porous probes. To overcome the limit of punctual information, we propose a semi-empirical approach based on the use of geophysical methods and the employment of a geophysical Factor of Safety recently introduced by the authors in terms of local resistivities and slope angles. In this paper, we show an application of our proposal on a test area of about 2000 m2 on Sarno Mountains (Campania Region - Southern Italy), where shallow landslides involving pyroclastic soils periodically occur triggered by critical rainfall events. Starting from two resistivity tomography surveys performed on the test area in autumn and spring, we obtained maps of the geophysical Factor of Safety at different depths for the two seasons. We also estimated the values of the Factor of Safety by using the infinite slope model in the dry and saturated scenario. A comparison between the values of the geophysical and geotechnical Factor of Safety shows advantages and disadvantages of our approach.

A study of the correlation between electrical resistivity and matric suction for unsaturated ash-fall pyroclastic soils in the Campania region (southern Italy)

In the territory of the Campania region (southern Italy), critical rainfall events periodically trigger dangerous fast slope movements involving ashy and pyroclastic soils originated by the explosive phases of the Mt. Somma-Vesuvius volcano and deposited along the surrounding mountain ranges. In this paper, an integration of engineering-geological and geophysical measurements is presented to characterize unsaturated pyroclastic samples collected in a test area on the Sarno Mountains (Salerno and Avellino provinces, Campania region). The laboratory analyses were aimed at defining both soil water retention and electrical resistivity curves versus water content. From the matching of the experimental data, a direct relationship between electrical resistivity and matric suction is retrieved for the investigated soil horizons typical of an ash-fall pyroclastic succession. The obtained relation turns out to be helpful in characterizing soils up to close saturation, which is a critical condition for the trigger of slope failure. In such a regime, the water content and the matric suction have small variations, while electrical resistivity variations can be appreciated in a larger range of values. For this reason, besides suction measurements on very small soil volumes through classical tensiometers, our analyses suggest the direct monitoring of in situ electrical resistivity values as an effective tool to recognise the hydrological state of larger and more representative soil volumes and to improve early warning of dangerous slope movements.

Water storage mapping of pyroclastic covers through electrical resistivity measurements

The knowledge of the geological setting of pyroclastic covers and their water content distribution represents crucial information for stability analyses of slopes potentially subject to debris-flow phenomena. The study we here present would provide a contribution to this issue by means of an approach based on electrical resistivity measurements. Specifically, we describe the results of high-resolution 2D resistivity surveys carried out in a test area on Sarno Mountains (Campania Region – Southern Italy), where shallow landslides involving pyroclastic soils periodically occur triggered by critical rainfall events. We discuss the results in relation to the geology of the area in order to locate characteristic horizons of pyroclastic soils below the ground surface. Then, on the basis of a laboratory characterization of pyroclastic samples collected from the same test area at representative depths, we provide an estimation of the soil water content distribution in the field. Finally, we analyze temporal variations of the soil water content distribution by comparing the data of two surveys carried out in the autumnal and spring seasons , respectively. ► Water is the main driver of pyroclastic debris-flow phenomena. ► We examine changes of soil water content in the field using electrical resistivity tomography. ► Empirical relationships between soil resistivity and effective saturation are found. ► Geoelectric monitoring can identify the thickness of pyroclastic layers in saturated condition.

Geophysical surveying of slopes affected by debris flows: the case of S. Felicea Cancello (Caserta, Southern Italy)

This paper contains the results of a series of geophysical investigations carried out on the largest debris flow to have taken place in Tavernole, S. Felice a Cancello (Caserta, Southern Italy). The landslide occurred in concurrence with other catastrophic events in the Sarno Mountains in May 1998. This research project is part of a series of geological, geomorphological and geotechnical studies whose purpose is to improve the knowledge of this type of phenomenon. The project also tested and compared various survey methods in the sample area of S. Felice a Cancello. Geophysical surveying allowed us to collect information regarding the physical features and thickness of the materials affected by landslide phenomena and to verify the applicability and effectiveness of the various indirect surveying methods adopted. The preliminary results of the study enabled us to generate a series of suggestions which could prove useful in formulating the correct approach to this type of problem to be adopted in ordinary professional practice. These indications concerned the type of geophysical surveying to be conducted and, where applicable, the means of implementation. In general, seismic refraction was found to be the best technique for collecting information on the area studied.

Influences of hydrological and hydrogeological conditions on debris flows in peri-vesuvian hillslopes

Abstract. The paper illustrates some results of research carried out to assess factors triggering debris flows which involve the pyroclastic overburdens covering carbonate mountains around Vesuvius. The aims of the research were to reconstruct a relationship between rainfall and debris flow occurrence and to highlight empirical hydrological thresholds through rainfall pattern analysis. The research was also aimed at investigating hydrogeological features of a pyroclastic cover-carbonate bedrock system to analyse factors inducing temporary hydraulic flow, critical for pyroclastic soil stability. The results of research are the following: i) rainfall pattern highlights empirical hydrological thresholds that differentiate the Lattari and Salerno Mountains from the Sarno Mountains; ii) in some sample areas of the Sarno Mountains close to the trigger zones of the landslides of May 1998 strong variation in hydraulic conductivity has been found in the first few meters below the surface; iii) these permeability variations would seem to justify temporary perched water tables that might affect the stability of the pyroclastic mantle.