*2.1. The Study Area*

The study area is the hilly sector of Oltrepò Pavese (265 km<sup>2</sup> wide, Figure 1) that corresponds to the northern termination of the Italian Apennines. It is characterized by a complex geological and geomorphological setting [38–40] (Figure 1c). The northern part of the area presents a bedrock geology composed by sandstones and conglomerates overlying marls and evaporitic deposits. In this sector, superficial soils, derived from bedrock weathering, are mostly clayey or clayey–sandy silts. Their thickness, measured in micro-boreholes and in trenches, have a thickness ranging between a few tens of centimeters and 2 m. Hillslopes are steep, with an average slope angle between 15◦ and 20◦ and maximum values up to 35◦. Instead, the central and southern parts of the study area are characterized by calcareous and marlyflyshes, alternated with sandstones, marls, and mélanges with a peculiar block-in-matrix at the outcrop scale. Due to the different lithology of the bedrock, superficial soils have a clayey or a silty clayey texture. Their thickness is generally in the order of more than 1 m, mostly ranging between 1.5 and 2 m from ground level, as measured in micro-boreholes and in trenches. Hillslopes have a medium steepness, with a typical slope angle of 8◦–15◦.

The slope elevation ranges between 60 and 500 m a.s.l. According to Koppen's classification, the climatic regime of the Oltrepò Pavese area is temperate/mesothermal, with a mean yearly temperature of 12 ◦C and an average yearly rainfall amount between 700 and 1000 mm, increasing from western to eastern sectors and from northern to southern sectors.

The area is significantly prone to shallow landsliding [24,39]. Several triggering events have occurred in Oltrepò Pavese since 1970s [38–40]. In the last 10 years, more than 2500 shallow landslides (Figure 1b) occurred in this area as a consequence of several rainfall triggering events during the winter and spring months. Most of the shallow landslides are classified as complex phenomena, starting as roto-translational slides and evolving into flows [41]. They are generally 10–70 m wide and 10–500 m long. Sliding surfaces are generally located at 1 m in depth [24]. Rainfall-induced shallow landslides affect medium–steep and steep slopes, with a slope angle of at least 8◦–10◦.

**Figure 1.** The study area, Oltrepò Pavese hilly zone: (**a**) location of the study area; (**b**) main geomorphological features and distribution of shallow landslides occurred in a 2000–2018 time span; (**c**) bedrock lithological features [36–38].

An integrated hydrometeorological monitoring station was installed on 27 March 2012 in a test-site slope located near the village of Montuè (red circle in Figure 1), in the northeastern part of Oltrepò Pavese. This testsite (Figure 2) is representative of the typical geological and geomorphological settings of Oltrepò Pavese areas most prone to shallow landslides for the following reasons: (i) the presence of triggering zones of past shallow landslides; (ii) its position in areas with medium–high susceptibility to shallow landslides according to previous studies [1,24]; and (iii) the typical geomorphological (hillslopes with medium–high thickness) and lithological features (clayey and silty soils) of the sectors most prone to shallow landsliding in the study area. In this station, rainfall amounts are measured through a rain gauge with an accuracy of 0.1 mm. Soil water content is measured by means of Time Domain Reflectometer (TDR) probes, with an accuracy of 0.01–0.02 m3/m3. Soil pore-water pressure

is measured through a combination of tensiometers, with an accuracy of 1.5–2.0 kPa and measuring values higher than −100 kPa, and heat dissipation (HD) sensors, with an accuracy of 1.5–2.0 kPa and range of measure till –105 kPa.

**Figure 2.** Representative test-site slope of Montuè: (**a**) morphology and shallow landslides distribution; (**b**) typical soil profile.
