**1. Introduction**

Landslide hazard assessment (LHA) is a challenging task for the prevention and prediction of a territory for local land managemen<sup>t</sup> and security services. Proper managemen<sup>t</sup> of landslide hazard, as well as saving human lives, can minimize socioeconomical impact that in many developing countries may equal a large percentage of the gross national product [1,2]. LHA is usually based on the spatial and temporal probability of landslide occurrences and is performed following three main steps: (i) the creation of a phenomenon inventory, (ii) a landslide susceptibility analysis and (iii) a landslide hazard analysis [3–5].

In Italy, the National Plan for Hydrogeological Risk Assessment (PAI) [6] is the cognitive, regulatory and technical–operational tool through which actions, interventions and rules concerning the defense against hydrogeological risk of the territory are planned and scheduled. Although the term LHA (in its Italian translation) is often mentioned within the PAI, the significance is at times contradictory, and the products of the Plan (maps, inventory sheets, analyses, etc.) rarely come from the steps described above; on the contrary, they are often realized based on an empirical approach and basic available data. In particular, hazard, vulnerability and risk degree (the latter in terms of exposed value) is closely linked to the quality of the expertise, while only in a few cases have specific studies (numerical models, statistical/probabilistic approaches) been applied [7–11]. As a consequence, the representation of the landslide hazard that emerges in some areas of the Italian territory can be over/underestimated, and divergent opinions may arise among technicians and public administrators. This problem, among other aspects, has also been highlighted in other countries of the European Union such as France [3,4,12,13].

In a more general context, the methods currently in use in Italy for LHA include two main types of approach: the field-based qualitative approach and the data-driven quantitative one [14]. The former type includes the so-called "geomorphological" methods [3,5–8,15–17], while the latter includes the statistical analyses (i.e., bivariate and multi-

**Citation:** Materazzi, M.; Bufalini, M.; Gentilucci, M.; Pambianchi, G.; Aringoli, D.; Farabollini, P. Landslide Hazard Assessment in a Monoclinal Setting (Central Italy): Numerical vs. Geomorphological Approach. *Land* **2021**, *10*, 624. https://doi.org/ 10.3390/land10060624

Academic Editor: Massimiliano Alvioli

Received: 7 March 2021 Accepted: 7 June 2021 Published: 11 June 2021

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variate statistical techniques, [7,18–22]) and the deterministic methods that involve, among others, the analysis of specific sites or slopes based on numerical models [23–30].

The present work compares the results of a landslide analysis carried out in a sample area using both geomorphological and numerical approaches. The area chosen for the analysis is located in a high hilly sector of the Adriatic side of the Central Apennines (Italy), characterized by the presence of monoclinal reliefs and typical cuesta morphologies, formed by differential tectonic movements in a recent uplift area [31,32]. Despite the relative simplicity of the geological model, these contexts can generate complex mass movements, both for characteristics (type of movement) and size (extension and depth of the failure zone) and kinematics (velocity and return time) and consequently represent high hazard conditions in the presence of built-up areas and/or infrastructures. The LHA (following the significance given by Italian PAI) is typically conducted based on a geomorphological approach and an expert judgment as regards the attribution of the degree of vulnerability and the exposed value. For any reclassification of the degree of risk, site-specific analyses (i.e., instrumental monitoring, geognostic bore-holes, geophysical prospecting) or the use of numerical models (slope stability analyses) are usually required by Italian guidelines. Nevertheless, both types of investigations proposed have limitations. In the first case, the non-negligible costs of direct surveys and prospecting limit the representativeness of the surveys themselves; in the case of numerical models, on the other hand, reliability is mainly linked to a correct choice of input parameters, is sometimes not homogeneous in the area considered and is often deduced from bibliographic data.

In this study, using finite-difference software (ITASCA FLAC/Slope v.8.0 [33]) the factor of safety (FoS) was calculated on representative slope sections where detailed geomorphological surveys highlighted the presence of gravitational phenomena or stability conditions. This analysis, in particular, refers to medium-to-low depth landslides, while more complex phenomena (as specified in the following paragraphs) recognized in the area and associated with deep-seated gravitational slope deformations were not included in the study [34,35].

The objective of this study was to clarify the role, usefulness and limits of the different methods to be used in the LHA and also provide useful information for their correct use in any context of territorial planning where specific indications have not been provided. A further aim was to demonstrate a combined and intelligent use of the two methods, pending clearer and universally accepted regulatory indications on the methods to be used for the LHA, which seems at the moment the most suitable choice both in economic and safety terms.

#### **2. Geological and Geomorphological Setting of the Study Area**

The study area (around 13 km2) is located east of the Sibillini Mts. Massif (Central Apennines) (Figure 1a). This sector corresponds to a vast sedimentary basin where, starting from the early Pliocene, thick levels of sandstones and conglomerates alternated with peliticarenaceous levels are deposited in transgression over a Miocene (Messinian) turbidite bedrock, mainly consisting of alternating arenaceous-pelitic and pelitic-arenaceous levels (Laga formation) [36,37] (Figure 1b,c). The contact between post- and pre-transgressive sediments is marked by an erosion surface approximately parallel to the Miocene levels [32,38]. The area is particularly characterized by the presence of weak levels and ductile deformation zones, corresponding to the weathered levels of the pre-transgressive clayey basement. Such strong weathering is probably due to the long period of immersion of the Messinian sediments as well as the lithostatic charges and the constant presence of water in the arenaceous-calcarenitic aquifer.

The structuring of the pre-transgressive bedrock was essentially carried out in the early Pliocene when, after intense compressive tectonics, east-verging folds and thrusts (the latter emerging or buried) developed within the Messinian and pre-Messinian formations. Pliocene sediments, on the other hand, show a generalized monoclinal setting, linked

to the subsequent tectonic uplift that affected the whole area starting from the early Pleistocene [39–41]; the strata generally dip between 15◦ and 20◦ (Figure 1b).

**Figure 1.** (**a**) Schematic geological map of the study sector: 1—main continental deposits (Pleistocene– Holocene); 2—sands and conglomerates (Pliocene–Pleistocene); 3—clays and sands (Pliocene– Pleistocene); 4—arenaceous-marly-clayey turbidites (late Miocene); 5—limestones, marly limestones and marls (early Jurassic–Oligocene); 6—trace of cross-section shown in Figure 1b; 7—study area (see Figure 4). (**b**) Schematic geological cross-section from the Apennine chain to the Adriatic Sea, modified from [42].

The resulting landscape, characterized by alignments of strongly asymmetrical and NNW–SSE oriented reliefs, is typical of "cuestas", with the main element consisting of the Mount Falcone relief (Figure 2a,b). Selective erosion, due to the presence of tough and massive lithotypes (sandstones and conglomerates) overlying less resistant clayey formations, creates steep escarpments between 50 and 300 m on the southwestern flanks.

The monoclinal structure, as a whole, is displaced by direct faults, mainly oriented NNW–SSE and WSW–ENE, the displacement of which can exceed 10 m [36,42]. Micro- and meso-structural analyses carried out on middle Pliocene and upper Pleistocene formations highlighted intense fracturing according to two main joint systems, N70 ± 15, N150 ± 15, and N20 ± 15, N100 ± 10, both compatible with the abovementioned fault systems. In the arenaceous-conglomeratic body of Mount Falcone, a third system of joints, N–S and E–W oriented, has also been observed. The former, dipping W of 70–80◦, completely crosses the rigid plate with a spacing of the order of a few tens of meters; the latter, characterized

by less frequency and continuity, is found at the edges of the plate itself. The genesis of this third system is attributed to the expansion processes of the relief resulting from the Pleistocene tectonic uplift [43,44] to the passive action of discontinuities developed with the same direction within the pre-transgressive bedrock and, in general, to the high seismicity of the area.

**Figure 2.** (**a**) 3D digital elevation model of the study sector: 1—edge of cuesta; 2—river. (**b**) The arenaceous-conglomeratic body of Mount Falcone.

From a geomorphological point of view, the joint systems described above, particularly developed within the arenaceous-conglomeratic body of Mount Falcone (924 m a.s.l.), create strong instability, especially corresponding to the W–SW portion, where the high structural scarp (about 150 m) is affected by retreat processes due to past and ongoing falls and topples (Figure 3). The accumulations of these processes, mainly consisting of pebbles and blocks, constitute an extensive and continuous talus at the base of the slope itself but can also be found further downstream, through rolling and/or passive transport processes induced by slow deformations of debris material; isolated blocks of decametric dimensions were found within the Tenna and Aso riverbeds (north and south of the relief, respectively).

**Figure 3.** Open fractures and toppling phenomena affecting the arenaceous plate of Mount Falcone.

#### **3. Data and Methods**
