Abstract
In this work, we describe soil moisture profiles related to typical colluvial slopes that were involved in rainfall-induced shallow failures occurring in alpine and pre-alpine areas of the Friuli Venezia Giulia Region (NE Italy). The trend of the volumetric water content (θw) showed a general increase from the ground surface to the bottom soil layer, with two or three marked moisture peaks. The saturation degree (S) varied from 65–70% (topsoil horizon) to nearly saturated basal colluvium (S = 95–100%). Soil moisture data demonstrates that, for a very humid climate, colluvial covers are often close to the saturation condition for most of the year. The calculated suction profiles indicated that maximum values ranging from 40 to 55 kPa often occur in the slope surficial soil (depth < 0.2–0.5 m). This negative pore-water pressure greatly decreases after a heavy rainfall event because of the infiltration process. Complete saturation of colluvial cover in the alpine and pre-alpine regions generally requires rainfall exceeding 150–200 mm for a 24-h storm duration. This results in a recurrence time of Tr ≅ 5–10 years for critical rainfall episodes involving colluvial slopes in the Friuli Venezia Giulia Region. The case histories analyzed demonstrate the importance of performing a detailed lithostratigraphic analysis of the colluvial deposit in order to properly define the suction measurement points, which there should be more of than the three-point determinations usually reported in the literature (for example, z = 0.5, 1.0 and 1.5 m).
Highlights
Colluvial slopes in mountain areas of the European Alps are often affected by shallow failures triggered by storm events
Infiltration and stability analyses of unsaturated colluvial slopes require an in-depth knowledge of the initial saturation condition of the slope
The water content is a well-known physical property that influences both the permeability and the shear strength of the soil, moisture profiles at depth are rarely reported in the literature
Summary
Colluvial slopes in mountain areas of the European Alps are often affected by shallow failures triggered by storm events. The assumption of a proper degree of saturation (S) of the colluvial cover is of key importance in order to set up an infiltration modeling of the slope, for both approximated and rigorous approaches These hydrologic models, which are of various complexity, can provide useful information as to whether the investigated colluvial slope can reach the saturation condition as a result of heavy rainfall, favoring the occurrence of an ephemeral subsurface groundwater table [8]. Sudden or even unexpected moisture variations in the slope profile can occur at greater depth as a consequence of marked lithostratigraphical changes (stratified colluvial deposits) due to distinct deposits mobilized by previous landslides or earth flow processes This is a typical situation for channelized colluvial slopes when previously failed landslide deposits are lodged within a “landslide gully” and are periodically remobilized during specific instability episodes caused by heavy or prolonged rainfall.
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