Interaction between Perched Epikarst Aquifer and Unsaturated Soil Cover in the Initiation of Shallow Landslides in Pyroclastic Soils

Abstract

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...

Soil moisture information can improve shallow landslide forecasting using the hydrometeorological threshold approach

“…However, the soil depth involved in a shallow landslide (the most prominent landslide type globally) is typically 1-2 m below the surface (Fiorillo et al 2001;Greco et al 2018), which is much thicker than the soil depth currently directly measured by remote sensing techniques, and in many cases overlaps with the root penetration depth. Such zone is influenced by antecedent precipitation, soil texture, vegetation, and therefore a clear relationship with nearsurface soil moisture may be difficult to find.…”

“…in the order of 10 −5 -10 −4 m s −1 for the ashes, and even higher for the pumices). The soil-bedrock interface is pervious, and the leakage supplies both shallow and deep groundwater circulation (Allocca et al 2014;Greco et al 2018). Hence, the soil cover has high water retention capacity and allows for rapid infiltration and drainage processes.…”

Deformation and Stability Characteristics of Layered Rock Slope Affected by Rainfall Based on Anisotropy of Strength and Hydraulic Conductivity

“…Rainfall is one of the main factors that trigger landslides [1]. In the past two decades, research on rainfall-induced soil landslides has been gaining popularity [2][3][4][5][6], but research on rock landslides is less reported [1,[7][8][9][10][11][12]. According to available statistical data, more than 90% of rock slope failures are related to fluid hydraulic conduction [13].…”

Overview of Landslide Hydrology

“…This dynamic nature of subsurface hydrology depends on the complex interactions among precipitation inputs, physical properties and heterogeneity of soils and bedrock, local geomorphology, and vegetation and associated biomass. These factors influence the timing of landslides with respect to precipitation inputs and antecedent soil moisture [14][15][16][17], the mass and mode of failure [18], and the extent of runout or transformation of landslides into debris flows [19].Both the infiltration of rainwater and snowmelt and bedrock exfiltration provide the local trigger of these landslides, while drainage and evapotranspiration tend to stabilize hillslopes by rerouting and removing subsurface water. Subsurface hydrology is strongly affected by preferential flow within the soil, substrate topography, and exfiltration from fractures in bedrock [2,13,18,[20][21][22]; the overall regolith moisture regime and recharge rates are influenced by evapotranspiration, soil development processes, soil water-groundwater interactions, and landform aspect and shape [14,16,[23][24][25].…”

“…This dynamic nature of subsurface hydrology depends on the complex interactions among precipitation inputs, physical properties and heterogeneity of soils and bedrock, local geomorphology, and vegetation and associated biomass. These factors influence the timing of landslides with respect to precipitation inputs and antecedent soil moisture [14][15][16][17], the mass and mode of failure [18], and the extent of runout or transformation of landslides into debris flows [19].…”