Ground motion polarization induced by topography and buried morphologies is investigated using visco-elastic three-dimensional finite element simulations. The model features a topographic crest and a buried softer zone, oriented along perpendicular directions to each other. A circular acceleration pattern is imposed at the model base, with a sinusoidal and a cosinusoidal signal applied as the x- and y-components, respectively, to isolate polarization effects caused solely by site geometry. Results are presented using three complementary parameters: the principal direction orientation angle (identifying the predominant direction of shaking), the Directivity index (quantifying motion polarization degree), and the ellipticity parameter (describing particle orbit shape). Maps of the amplification factors of peak ground acceleration along the x- and y- components reveal distinct patterns along different directions. The combined analysis of directional parameters and amplification factors demonstrates that site geometry induces marked polarization effects. The predominant motion direction aligns normal to both the topographic and buried structures. The integrated approach is particularly valuable for engineering applications involving also elongated infrastructure such as tunnels and lifelines in seismically active regions.

Destructive rainfall-induced debris flows and debris avalanches occurring in air-fall volcanic ashes represent an impending risk in some densely populated areas of the Campania Region. Due to the magnitude of these events, inaccessibility of some sites, design/construction timelines, and high cost of interventions, the construction of structural protection works is not always the best solution for risk mitigation. In some cases, adopting early warning systems based on slope monitoring might be an effective alternative thanks to the good knowledge of the behavior and properties of these soils, their homogeneity across the territory, and the relatively low cost of monitoring systems. This paper describes an early-warning procedure based on the analysis of slope response to incoming precipitation, derived from measurements of initial soil conditions, weather forecasts, and predictions of pore-water pressure changes in the following 12–48 h. The results of some blinded analyses conducted at a monitored site are encouraging.

Flow-like landslides represent a major global hazard, typically affecting partially saturated sloping soils and triggered by intense rainfall events that reduce matric suction and soil shear strength. Within this framework, the PROMISE project (Integrated apPROach for MItigation of flowSlidE risk: full-scale test and advanced numerical modelling) aims to improve understanding of landslide triggering mechanisms through the design and execution of a full-scale experiment involving the application of controlled artificial rainfall on an instrumented natural slope. The study focuses on a high-risk area in the Lattari Mountains (Campania Region, Italy). The experimental site is located in a limestone and dolostone quarry, within the municipality of Salerno. This paper presents the strategy adopted for the design of the full-scale experiment. Specifically, on site investigations aimed at identifying the predisposing and preparatory conditions for failure, as well as the critical rainfall scenarios to be applied during testing, are discussed.

Flow-like landslides are a significant global hazard, threatening human life and causing extensive damage to structures and infrastructure. These events often occur on slopes composed of partially saturated soils and are typically triggered by intense rainfall, which reduces matric suction and, consequently, soil shear strength. Despite advancements in identifying key predisposing factors and preparatory factors, interpreting the mechanisms that trigger flow-like landslides remains challenging. The PROMISE project, ‘Integrated appPROach for MItigation of flowSlidE risk: full-scale test and advanced numerical modelling’ aims to address this gap. Its objective is to design and implement a full-scale test involving the application of an artificially induced critical rainfall event on an instrumented soil slope, with the aim of analyzing the hydro-mechanical response under failure conditions and validate a numerical predictive model. The study focuses on an area within the Lattari Mountains (Campania Region, Italy), historically affected by high-risk flow-like landslides. Specifically, the investigation area is located in a limestone quarry owned by ITALSUD Srl, situated in the municipality of Salerno, to the east of the Lattari Mountains. This paper presents preliminary results from the geological and geotechnical investigation of the area, along with the initial field monitoring data collected under natural weather conditions.

Flash floods are frequent natural hazard events in many parts of the world. Generally, they occur in small catchments drained by torrential streams that feed alluvial fans or fan deltas. In the Mediterranean region, these phenomena are particularly common during the spring and autumn seasons, often causing significant damage to buildings, infrastructures, agriculture, and sometimes resulting in fatalities and injuries. To better understand and manage the potential consequences of these events on physical systems, probabilistic damage quantification is essential. Fragility functions, which describe the probability of reaching or exceeding a specific damage state based on an intensity measure, are valuable tools for assessing damage conditioned on the intensity of a natural hazard. While such curves are widely reported and extensively applied, there is a notable lack of interdisciplinary methodologies for their development and integration into broader risk management frameworks. This gap often leaves initiatives such as flood insurance premium planning, probabilistic loss estimation, and flood risk management reliant on uninformed or generic tools. This study proposes an interdisciplinary approach to developing flood fragility functions using post-event flash flood damage data. The event that occurred on 14-15 October 2015 in Solopaca - Paupisi area (Benevento, Italy) is adopted as the case study. The reactivation of alluvial fan lobes is analyzed together with the recorded rainfalls. It is based on the processing of post-event field data acquired with classical and remote sensing technologies such as UAV imagery. Impact mapping is then conducted to depict the spatial extent of the flash flood. The event is then characterized in terms of inundation depth and thickness of mobilized material and grain size distribution. The area of the event and the thickness of the deposits are considered to estimate the transported solid volumes. Finally, the damage incurred to buildings and respective inundation depth is assembled to construct flash flood fragility functions. The outcomes of this study can be used in numerical flow model calibration and validation as well as flash flood risk assessment and management initiatives. The fragility functions developed in this study can serve as a tool for loss assessment, resilient construction prioritization, and insurance premium planning. The interdisciplinary approach developed and implemented in this study will be insightful to many other regions across the world in terms of flash flood mitigation planning.

Within the framework of the RETURN (multi-Risk sciEnce for resilienT commUnities undeR a changiNg climate) project funded by the Italian National Plan for Recovery and Resilience (PNRR), the Spoke2 “Ground Instabilities” aims at restituting temporal and space distribution of effects induced by ground instabilities (scenarios), i.e. landslides, sinkholes, subsidence and liquefaction. Preliminary results obtained for sinkholes are here reported, as these processes represent the most diffused geological hazard in karst areas. These are widespread in Italy in all areas characterized by outcrops of soluble rocks. Sinkholes are being analyzed in the project in terms of the predisposing factors as well as of preparatory and triggering processes. The distinction among them is made on a temporal basis: predisposing factors are considered invariable at the observation scale, while the preparatory factors show trends or periodic variations in the same time interval. As a consequence, a trigger is considered as a process which acts over a very short and well-defined time. Sinkholes are distinguished also on the basis of their kinematics, following the main internationally accepted sinkhole classification. The activities carried out in the first year of RETURN project brought us to collect a number of learning cases dealing with sinkholes in Italy, and to deduce rationales of process to be considered as tools for operative chains in view of the implementation of a scenario producer to be designed through an IT-technology. The outputs expected in the simulated scenarios are quantitative values attributed to ground instability effects (among which the sinkholes) in a multi-hazard perspective, with the final goal to offer multi-risk mitigation strategies to national stakeholders.

On the 30th of November 2022, a major rockfall event occurred in the Triassic dolostones of Castrocucco cliff (Maratea, Southern Italy), mobilising a volume of about 8000 m3 (Minervino Amodio et al. 2024) and destroying the underlying SS18 national road with no fatalities. The SS18 has critical importance in an area of high tourist, landscape, and historical interests, and determined the planning of a bypass tunnel to avoid the cliff, which has been affected by recurring instability events in the last decades (Pellicani et al. 2016). However, before the tunnel could be completed, the safe reopening of the road was critical for the region. For this reason, a high-resolution monitoring system was developed, enabling the timely road closure to the traffic in case of new failure (Santo and Massaro 2024).In this study, we describe the geo-structural investigation and reconstruction of the rockfall kinematics and triggering factors, as well as the susceptibility analysis carried out to develop the monitoring system that allowed the road to reopen. Such a system consisted of a network of sensors placed in the areas and on the rock blocks that showed high levels of susceptibility to rockfalls. The data collection was performed through field and digital surveys. The latter was carried out on Virtual Outcrop Models (VOM) following drone photo acquisition. Successively, the rock block trajectories were simulated under static and seismically induced conditions with different block volume scenarios. These results, integrated with the real-time deformation data recorded by the sensors, will enhance the mitigation plan further. Moreover, the developed methodological approach and workflow could be applied to similar situations where critical road infrastructures lie in areas of high susceptibility to rockfall. Minervino Amodio A, Corrado G, Gallo IG, Gioia D, Schiattarella M, Vitale V and Robustelli G (2024) Three-dimensional rockslide analysis using unmanned aerial vehicle and lidar: The Castrocucco case study, Southern Italy. Remote Sensing, 16 (12), 2235. doi: 10.3390/rs16122235Pellicani R, Spilotro G and Van Westen CJ (2016) Rockfall trajectory modeling combined with heuristic analysis for assessing the rockfall hazard along the Maratea SS18 coastal road (Basilicata, Southern Italy). Landslides, 13: 985-1003. doi: 10.1007/s10346-015-0665-3Santo A and Massaro L (2024) Landslide monitoring and maintenance plan along infrastructure: The example of the Maratea major rockfall (Southern Italy). Landslides. doi: 10.1007/s10346-024-02409-3

Flowlike landslides (debris flows, debris avalanches, and flowslides) often initiate as shallow slip and, due to their propagation downslope, pose significant threats to infrastructure and human lives worldwide. Such landslides are typically triggered by heavy rainfall infiltrating granular and partially saturated slopes. Field monitoring provides valuable insights into slope responses to hydraulic loads and strongly improves the forecasting of slip initiation. However, current strategies mostly rely on rainfall thresholds derived from empirical and statistical approaches, and the integration of field monitoring into prediction procedures remains a challenge. This paper focuses on the development of physically based rainfall thresholds for predicting shallow slip occurrence. The proposed approach relies on field monitoring designed to capture the short-term response of unsaturated slopes to individual rainstorms on an hourly scale. First, data from automatic field monitoring collected over two years on a shallow pyroclastic slope are reported. The test site is located on Mt. Faito in the Lattari Mountains/Sorrento Peninsula (Campania, Southern Italy), an area historically prone to flowlike landslides. Field monitoring was designed to detect the short-term response of the pyroclastic slope to rainstorms by collecting automatic readings of meteorological data, matric suction, and volumetric water content at hourly time intervals. These observations prove that the measured parameters can be effectively regarded as precursors of shallow slip occurrence. Then, the analyzed field data were used to validate a numerical model predicting the slope's hydrological response to rainstorms and to develop physically based rainfall thresholds for shallow slip occurrence. These thresholds are defined in terms of mean rainfall intensity, rainfall duration, and subsoil suction preceding the rainstorm and validated against homologous data on historical landslides within the reference geological context.

Pollen analysis was carried out on the infilling succession of the Fossa San Vito sinkhole, at the NE foothills of the Sarno Plain (Italy). Four 14C dates and six tephra layers constrain the pollen sequence between ca. 6000 and 500 cal yr BP. A forested environment, with a few signs of human activities, characterizes the pre-protohistoric period (ca. 6000-2750 yr BP). Stability of the arboreal pollen grains to non-arboreal pollen grains (AP/NAP) curve is due to climate-related opposite oscillations of deciduous and evergreen forest. In this period, the pyroclastic products from Neapolitan volcanoes that reached the upper Sarno Plain seem to have affected neither vegetation nor human activities. In the archaic and classic periods (ca. 2750-1500 yr BP), intensive deforestation and increase in anthropogenic indicators indicate the occurrence of grazing and crop activities managed by the main urban centers located in the plain: Pompeii, Stabiae, and Nuceria. After the Pompeii eruption in CE 79, a rapid re-afforestation and decline in all anthropogenic indicators testify to the temporary abandonment of the area, linked to the disastrous demise of the main economic centers. The upper plain was repopulated and exploited in the Late Ancient and Middle Ages (ca. 1700-500 yr BP), as indicated by the increase in all crop and grazing indicators.

Ischia (Southern Italy) is a volcanic island of the Phlegrean Volcanic District that was historically affected by multiple geological hazards, including floodings, landslides, rockfalls, and earthquakes. In this study, rockfall stability is analysed with an integrated approach aimed at investigating the rockfall source, the propagation, and the deposition areas. The case study is represented by two outcrops over a 400-m-wide cliff made of Green Tuff and located on the western area of Mt. Epomeo. They are respectively located at 280 and 420 m a.s.l., just uphill the village of Frassitelli, Forio d’Ischia, which is an area of high residential, tourist, and agricultural importance. We analysed the fracture systems of the tuff cliff to compute the kinematic analysis of the potential failure mechanisms and to perform numerical simulations of rockfall scenarios. Successively, numerical simulations of rockfall scenarios were computed based on the acquired structural information. This allowed us to identify the most hazardous scenarios based on the rock trajectories and the percentage of rock blocks affecting the urban area. The influence of the rock shape and volume on the rockfall trajectories was analysed. In the most likely scenarios, we observed that 15–25% of the rock blocks bypass the geomorphological barriers and reach the urban area, with kinetic energy values spanning between 10 2 and 10 4 kJ. Such detailed rockfall hazard analysis allowed the definition of the mitigation interventions necessary for the protection of the nearby residential area.

Landslides are one of the most hazardous secondary effects of earthquakes due to the potential for large-scale damage and long-term alterations to landscapes. During the 2016-2017 seismic sequence in Central Italy, many earthquake-triggered landslides (EQTLs) affected the road network and mountain trails. In this study, a methodological approach for analysing EQTLs, based on data derived from Unmanned Aerial Vehicle (UAV) surveys, is shown. The approach is applied to investigate the geometric, structural, geomechanical, and kinematic features of the Foce rockslide, which is introduced in the back analysis. The investigation involved three main steps: (i) set up of UAV-based Virtual Outcrop Models (VOMs) of the slope, (ii) a geomechanical characterisation of the rock mass through the VOM interpretation and conventional field data, and (iii) 3D Limit Equilibrium (LE) slope stability analyses. This study highlights the potential of UAV surveys for providing valuable data for stability analyses, especially in emergency conditions such as in the aftermath of seismic events.

This article presents a case study of the early warning monitoring system made to prevent or reduce risks in case of rock mass deformation and consequent rockfalls. A sever rockfall in November 2022 occurred and destroyed part of the national road n. 18 "Tirrena Inferiore" in Southern Italy, whom surveillance and maintenance are operated by ANAS SpA (Gruppo Fs Italiane), the Italian leading Concessionaire of national road and motorway network. ANAS SpA, in cooperation with other national and local authorities, accomplished to rebuild the road body and to mitigate the hazard. To enhance traffic safety and ensure the functionality of the protection structures, ANAS activated the onsite surveillance and implemented a remote real-time monitoring activity of the slope movement integrated with automatic real time early warning systems using a software that identifies settlements and/or displacements exceeding the threshold limits and sends an e-mail alert.

On the 30th of November 2022, a major rockfall event occurred in the Triassic dolostones of Castrocucco cliff (Maratea, Southern Italy), destroying the underlying SS18 national road with no fatalities or injuries. Successively, engineering works were applied to allow the restoration of the road, although the construction of a bypass tunnel to avoid the cliff was designed as the ideal solution. Before the tunnel can be completed, the critical importance of the road required its reopening with a high-resolution monitoring system that enables the timely road closure to the traffic in case of new failure. In this study, we describe the data collection and the reconstruction of the rockfall kinematics, as well as the susceptibility analysis carried out for the development of the monitoring system that allowed the reopening of the road. Such methodological approach and workflow could be applied to similar situations where critical road infrastructures lie in areas of high susceptibility to rockfall.

In this paper, we provide chronological constraints to reconstruct the Late Middle Pleistocene to Holocene geomorphological, stratigraphical and tectonic evolution of the inner Sarno plain, along the Tyrrhenian flank of Southern Italy. These constraints derive from tephrostratigraphical analysis and Ar-40/Ar-39 dating of volcanic deposits, chronologically constrained between ca. 40 ka and 184 ka, recovered from a borehole located in the pediment of the Sarno Mts. The lack of marine to transitional deposits in the borehole testifies that the entire area has been above sea level during this period. Conversely, analysis of previously drilled borehole data indicates the presence of an at least 200 m thick sequence of marine and transitional deposits beneath the Sarno Plain. The top of the marine to transitional deposits in the Sarno basin has been correlated with those in the San Valentino Torio High to the SW, dated to the MIS 5. Consequently, it may be envisaged that the upper portion of the marine deposits in the Sarno Plain also correlates with the MIS 5. This allows to estimate a mean subsidence rate in the last 125 ka, which is 0.32 mm/a. On the other hand, the uninterrupted 200 m-thick marine succession means that subsidence in the Sarno basin was >= 0.7 mm/a during the preceding interval between similar to 285 ka and the last interglacial (LIG; MIS 9-MIS 5). The subsidence was likely driven by the activity of the Sarno Fault, whose slip rate in the last 40 ka was significantly lower than the subsidence rate since the LIG and the MIS 9-LIG interval. This implies that the fault slip rate has decreased during the last 285 ka. This paper provides a valuable contribution to the study of the complex interplay between glacio-eustatic oscillations and tectonics to assess reliable paleo-sea level indicators.

The evaluation of sediment budgets of flash-flood prone torrential catchments is important for the assessment of flood susceptibility and hazard in piedmont areas. Here, we developed and tested a predictive, quantitative geomorphological technique for estimating the magnitude of debris flows related to flash floods triggered by extreme rainfall events. We applied our method to steep torrential catchments that allowed testing of our technique through quantification of the effects of recently-occurred flash floods. The three study catchments are located in southern Italy and experienced flash flood-related debris flows triggered by a rainstorm with a recurrence interval > 200 yrs. The study basins are small size, steep, narrow and the stream network has a low order (maximum 3rd order). We estimated the volumes of loose sediment stored in the bottoms of the mainstem valleys of the investigated mountainous catchments prior to the flash flood and compared our results with the sediment volumes entrained by the debris flows. The latter volumes were estimated by post-event data collected by means of field surveys. The results were mutually consistent so supporting our approach. The approach is fit to the analysis of torrential catchments with geomorphological features comparable to those of our study basins, provided (i) the stream is confined by hard rock slopes not affected by channel lateral erosion, (ii) all stored sediment is eroded and transported downstream during the flood, and (iii) no landslide occurs along the hillslopes. For these types of ungauged catchments, our method can be applied extensively, even in the absence of field data and measurements, for flash flood hazard assessment and planning of mitigation strategies.

Campi Flegrei is a densely populated volcanic area in Italy. Its inner caldera periodically experiences uplift and subsidence, known as bradyseism, also accompanied by seismic activity. In the last decade, with uplift rates up to 2 cm/month, about nine-thousand earthquakes were recorded. Upon request of the local administration, the most updated data were collected and analyzed to evaluate the risk management strategy consisting of structural retrofitting according to the building code. Here it is shown that the reference moment magnitude is in the range 4.4, 5.1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left({{\mathrm{4.4,5.1}}}\right)$$\end{document}, based on fault mapping, geomorphological inference, earthquake relocation, stress-drop analysis, and ground motion modelling. Earthquake forecasting enabled computing the exceedance probabilities of these magnitudes. Earthquake engineering showed that the minimum magnitudes expected to cause exceedance of design ground motion, are larger than the reference magnitudes. Finally, the risk reduction implied by the safety levels of new constructions was assessed for reinforced concrete buildings.

The assessment of soil thickness plays a primary role in different areas of the geosciences, including landslide hazards and soil erosion assessment. Nevertheless, the thickness of the cover deposits is difficult to measure over large areas while the prediction with numerical models is still challenging. In this study, some topographic at-tributes, namely slope angle and profile curvature, are investigated as proxies for the estimate of soil thickness of pyroclastic cover through empirical relationships. The study area is the North-facing slope of the Lattari Mts. (Campania, Southern Apennines), which were historically affected by several flow-like landslides and flash floods. The dataset was collected with both direct and indirect measurements from field surveys and in-vestigations in two test sites of the study area.The topographic parameters were derived from DEMs at three different cell sizes, respectively at 1 x 1 m, 5 x 5 m, and 10 x 10 m. The slope angle at 5 x 5 m resulted in the most fitting parameter with the soil thickness, whereas the curvature showed a low correlation at all the investigated scales. The relationship for the Pyroclastic Thickness Estimation (PTE) is defined by an exponential law, which was validated by an internal and external dataset and checked against other predictive models used in the literature for similar study areas.

<p>Ischia (Southern Italy) is a volcanic island of the Phlegrean Volcanic District that was recently affected by multiple geological hazards, including floodings, landslides, rockfall and earthquakes.</p> <p>In this study, rockfall stability is analysed, assuming as a case study a 400m-wide cliff made of Green Tuff and located on the western area of Mt. Epomeo. The two outcrops studied are located at 280 and 420 m a.s.l., above the site of Frassitelli, Forio d’Ischia, which is an area of high residential, touristic and agricultural importance. The former is a high-angle outcrop affected by tens of meters-long faults, whereas the latter is characterised by high-dip pinnacles.</p> <p>We analysed the fracture systems affecting the examined formation to compute the kinematic analysis of the potential rupture mechanisms and to perform numerical simulations of potential rockfall scenarios. The data acquisition was carried out by means of classical geological field surveys and structural analysis on Virtual Outcrop Models (VOM) obtained from images acquired by drones. The VOMs were analysed with ‘CloudCompare v2.10.2’ and ‘OpenPlot’ software. The former allowed the automatic digitalisation of the exposed discontinuities by applying the ‘Facets’ plugin, based on a least-square fitting algorithm (Fernández, 2005). ‘OpenPlot’ enabled the extraction of the geostructural information from the VOM, by computing the best-fit planes of the polylines manually drawn along the interference between the geological surface and the outcrop topography (Tavani et al., 2011).</p> <p>The measured and the extracted features were classified following their attitude. Three main sets were defined, striking N-S, NW-SE and NE-SW. The fracture dataset was used to perform a kinematic analysis with ‘DIPS’ software on the surface discontinuities extracted from ‘Facets’ plugin. The 'wedge sliding' resulted the most critical potential rupture mechanism to occur on the analysed outcrops. Successively, numerical simulations of rockfall scenarios were computed based on the acquired structural information. The latter permitted us to identify the maximum run out of the potential blocks and draw some consideration on the rockfall hazard of the area.</p>

Flow-like landslides, which occur mainly in shallow granular deposits resting on steep bedrock, represent a major natural hazard worldwide. The pore water pressure distribution and the soil water content directly affect the soil shear strength, thus controlling the triggering of these landslides. Criticalgeomorphological and topographical settings, together with peculiar stratigraphic and hydrogeological features, are commonly recognized as predisposing factors for flow-like landslides occurrence. Hence, investigating the spatial and temporal variability of hydraulic slope conditions is a fundamental activity that consists of identifying local geological factors and seasonal monitoring of the subsurface water regime. The present work proposes an integrated geological, geophysical and geotechnical approach to identify the spatial variability of the local stratigraphic setting and hydrogeological conditions in a partially saturated slope, in order to set up a procedure able to provide a prediction of the flow-like landslides occurrence atslope scale. The multidisciplinary study has been applied to a test site on Mt. Faito, in the Lattari Mts. (Southern Italy), where extensive geophysical, geological and geotechnical soil characterization and in situmonitoring data collected over two years are available.

The triggering of shallow landslides in granular deposits is highly controlled by the groundwater regime, namely, the pore water pressure distribution, which directly affects soil shear strength. The slope hydraulic state is usually variable over time and space due to the high variability of atmospheric loads at the ground surface and local geologic conditions, such as stratigraphic irregularities and preferential groundwater flow paths. The unfavourable combination among critical geomorphological and topographical settings with stratigraphic and hydrogeological features are commonly recognized as predisposing factors of flowslides and debris flow occurrences. This paper proposes a multidisciplinary approach that combines geological, geophysical and geotechnical investigations to identify the role of local geological and geotechnical factors on the groundwater regime in slopes prone to flow-like landslides. The study is based on seasonally repeated electrical resistivity tomography measurements integrated with geotechnical numerical modelling of hydraulic phenomena affecting the soil cover. The latter is used to analyse the effects of the stratigraphic variability in terms of the geometry, continuity, and thickness of the soil horizons on the groundwater regime over time. The proposed approach has been applied to a test site located on the northern slope of Faito Mt. in the Lattari/Sorrento Peninsula mountain chain (southern Italy), an area historically affected by many rapid instability phenomena, such as flow-like landslides and flash floods. Both geophysical and geotechnical models obtained for the test site were cross-checked and validated, providing significant insights into the hydraulic response of the soil cover to rainfall and its hydraulic interaction with the underlying bedrock. Specifically, the integrated approach proved that i) the buried paleo-morphology of the bedrock severely affects the pore water distribution in the soil cover and ii) ashy soil fills the upper karst portion of the bedrock, providing a hydraulic connection of the water flow infiltrating from the topsoil downwards.