Abstract

Rainfall thresholds define the conditions leading to the triggering of shallow landslides over wide areas. They can be empirical, which exploit past rainfall data and landslide inventories, or physicallybased, which integrate slope physical–hydrological modeling and stability analyses. In this work, a comparison between these two types of thresholds was performed, using data acquired in Oltrepò Pavese (Northern Italian Apennines), to evaluate their reliability. Empirical thresholds were reconstructed based on rainfalls and landslides triggering events collected from 2000 to 2018. The same rainfall events were implemented in a physicallybased model of a representative testsite, considering different antecedent pore-water pressures, chosen according to the analysis of hydrological monitoring data. Thresholds validation was performed, using an external dataset (August 1992–August 1997). Soil hydrological conditions have a primary role on predisposing or preventing slope failures. In Oltrepò Pavese area, cold and wet months are the most susceptible periods, due to the permanence of saturated or close-to-saturation soil conditions. The lower the pore-water pressure is at the beginning of an event, the higher the amount of rain required to trigger shallow failures is. physicallybased thresholds provide a better reliability in discriminating the events which could or could not trigger slope failures than empirical thresholds. The latter provide a significant number of false positives, due to neglecting the antecedent soil hydrological conditions. These results represent a fundamental basis for the choice of the best thresholds to be implemented in a reliable earlywarning system.

Highlights

  • Shallow landslides are slope instabilities of a mass of soil and/or debris, which could involve the most superficial colluvial layers till around 2.0 m from ground level

  • Water 2019, 11, 2653 means of rainfall thresholds, defined for different geological, geomorphological, and environmental settings [4]. These thresholds represent the main tool to estimate the daily or hourly level of hazard across a territory prone to shallow landslides or to implement earlywarning systems [5], representing the lower bound of rainfall conditions that caused the triggering of shallow landslides [3,6]. These thresholds are expressed as curves which separate the rainfall conditions leading to shallow slope failures from the ones where stability is maintained, sometimes with associated different probabilities of occurrence with uncertainties related to the possible incompleteness of the input data required to define the same thresholds [5,7]

  • This paper aims to reconstruct and compare empirically and physicallybased rainfall thresholds for the occurrence of shallow landslides in a susceptible area of the Northern Italian Apennines (Figure 1)

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Summary

Introduction

Shallow landslides are slope instabilities of a mass of soil and/or debris, which could involve the most superficial colluvial layers till around 2.0 m from ground level. Water 2019, 11, 2653 means of rainfall thresholds, defined for different geological, geomorphological, and environmental settings [4] These thresholds represent the main tool to estimate the daily or hourly level of hazard across a territory prone to shallow landslides or to implement earlywarning systems [5], representing the lower bound of rainfall conditions that caused the triggering of shallow landslides [3,6]. These thresholds are expressed as curves which separate the rainfall conditions leading to shallow slope failures from the ones where stability is maintained, sometimes with associated different probabilities of occurrence with uncertainties related to the possible incompleteness of the input data required to define the same thresholds [5,7]

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