Geological Setting
The area is characterised by a turbiditic (flysch) sequence composed of alternating pelites/hemipelagites and arenites with a thickness ranging between 50 and 900 m. This sequence belongs to the early middle Miocene Corniolo member of the Marnoso-Arenacea formation, the latter cropping out extensively in this sector of the Apennine belt.
In the Poggio Baldi area, two thrust surfaces (denoted as Th1 and Th2) were recognised, which were dissected by NNE-SSW and WNW-ESE-oriented strike-slip faults (F1–F8). These last can be associated with the western termination of the Bidente Line, a main strike-slip, left-lateral fault zone which has been active since the Late Miocene and shows neotectonic evidence far from the study area. Landslides on the Verghereto, Corniolo, and Capanne slopes were better identified as wide areas affected by shallow and complex movements involving both the bedrock and either colluvial or debris deposits. (From Esposito et al., 2021).
Legend: 1) recent, complex landslide deposits; 2) ancient, complex landslide deposits; 3) debris fan deposits; 4) slope-debris and colluvial deposits; 5) channel gravel and sand deposits; 6) overbank gravel and sand deposits; 7) active and relict alluvial fan deposits); 8), 9), 10) pelitic marly horizons; 11) Calanca (Ca) layer; 12) arenaceous-marly sequence; 13) thrust; 14) transpressive fault; 15) leftlateral, strike-slip fault; 16) strike-slip fault; 17) low-angle normal fault. Labels: F1–F9 = fault codes.
Slope Evolution
The 3D CD analyses were implemented to conduct an accurate study of the geomorphological evolution of the main scarp of the Poggio Baldi landslide. The analyses enabled us to estimate volume changes in terms of fallen and accumulated debris and materials. The general loss of volume is estimated to be in the range of 2.0 to 2.8 × 10^3 m^3 per year.
A persistent retrogressive activity of the detachment area, due to frequent rockfall events, was recorded. Additionally, a group of areas, also affected by a loss of volume, was identified near the left and upper-left portions of the main scarp. Here, rockfalls occur with daily frequency, increasing the material accumulated along the underlying areas.
A wide and elongated area lying at the toe of the vertical rock cliff, at an altitude of approximately 760–630 m above sea level, was recognised. As can be seen, similar zones affected by a volume increase and covering a similar area width were identified in every gravitational instability scenario. Indeed, the main central sector of this area has been undergoing continuous processes of accumulation of fallen debris and blocks.
Then, a short-term geological/geomorphological evolutionary model based on the following two stages (STAGE I and STAGE II), was developed:
- STAGE I: Slope instabilities are primarily represented by rockfalls and toppling. Predisposing factors are geological, geomorphological, and structural. The detached debris and blocks, following their trajectories, find deposition surfaces over the cantilever arenaceous strata.
- STAGE II: Over time, the volume of fallen material increases on the bulging arenaceous strata surfaces. At the same time, continuous volume loss causes a constant retrogressive activity of the above vertical rock cliff in some specific sectors. These events, repeated over time, affect the main scarp until the accumulated debris on the bulging strata surfaces, reaching non-equilibrium conditions, collapse, causing small debris avalanches.
This almost constantly increasing loading could act as one of the main predisposing factors for the long-term reactivation of the Poggio Baldi main landslide. If the above phenomenon were combined with an increase of pore water pressure induced by rainfall events and/or rapid snow melting (as occurred in 2010), it could cause the reactivation of the Poggio Baldi landslide. (From Mazzanti et al., 2021)
Controlling Factors
The Poggio Baldi detachment area is characterised by a low-grade tectonic deformation with few but outstanding faults. A main ENE-WSW-oriented and N-dipping (counter-slope) low-angle normal fault (LANF) is observed at a middle-slope elevation cutting, with a very low displacement and a ramp-flat-ramp geometry, over the entire turbiditic sequence. In the same outcrop high-angle, NNE-SSW-oriented faults are observed extending for the entire 100-m high escarpment, showing a left-lateral, transpressive kinematic also revealed by mesoscale positive flower structures.
The reconstruction proposed is rather similar and outlined by the geometric attitude of the pml0–2 layers. East of the wide backlimb zone, the fault-propagation fold geometry was again presented, showing SW-dipping kink bands (II–I), a sub-horizontal dip-panel (III), an overturned forelimb (IV), and a forelimb syncline (V). GSI values were projected from outcrops found along the Alpicella and Ritorno erosive valleys on the western and eastern sides of the Corniolo slope, respectively. Geomorphological features ascribable to an active DSGSD are confined within the hangingwall block of the high-angle breakthrough thrust. This is probably mainly due in particular to the geometric setting of the fold and the overturned forelimb.
The results of the automated structural analysis on UAS-derived point clouds of the Poggio Baldi detachment area are shown below. Five different joint sets and their spacing values were identified along the slope. The bedding strata (J2), were south-dipping, with dip angles varying from 33 to 46 degrees.
Overhangs have been identified as a potential source of rockfalls (Matasci et al. 2018; Dunham et al. 2017; Noël et al. 2018). The geometrical attributes of the existing overhanging rocks were measured on the model. Overhang stability is subject to the size and weight of the overhanging rock mass being the detachment driver. Preferential erosion of argillitic layers takes place resulting in more or less extended overhangs, which eventually fall. Mavrouli et al., 2024, investigated the relationship between the overhang area and the number of rockfall events for seven slopes.
Rockfall Detection and Mapping
Multi-temporal inventories of rockfalls are essential for understanding the frequencies of the events respect to their magnitude. To investigate the spatiotemporal distribution of events, an accurate mapping procedure was conducted. Each record is represented on the 3D multi-temporal models and as a point on the orthomosaic of the rock slope with the corresponding referencing information. For each identified rockfall, a specific point was assigned, and the following parameters were assigned: date of occurrence, number of pixels, Ground Sampling Distance (GSD), areal size in m^2, the lithology, and the specific sedimentary interval of the Corniolo Member to which it belongs.
The development of a comprehensive multi-temporal inventory of rockfalls for each 10 × 10 m grid cell facilitated the generation of a discretised map containing information on the probability of rockfall occurrences (Pf). The figure below reports the five largest rockfalls detected during the monitoring period together with the spatial variation of Pf along the slope. As a result, grid cells with higher failure probability (1–2.25%) are distributed in the upper central part of the rock slope but do not necessarily correspond with the location of the larger events.
(From Mastrantoni et al., 2024).
Hazard Assessment
The relation between the magnitude and the rockfall release frequency can be modelled by a mathematical power law, allowing for hazard assessment (i.e., failure frequency estimates by rockfall magnitude) beyond the observed data range and within its validity domain. The rockfall frequency magnitude relation of the full inventory is shown below. The corresponding failure probability (Pf), power law fit (λst ), and the R2 coefficient are reported. The failure probability of the overall rock slope stands at 0.35 rockfall per day. According to the extrapolated power law, the Poggio Baldi rock slope has a unitary activity Ast = 2.66 and a uniformity coefficient B = 0.78. Hence, 2.66 rockfalls with an area ≥ 1 m^2 are expected to occur from 1 hm^2 of the cliff in one year. A rollover effect is denoted for rockfalls smaller than 3 × 10^2 m^2 (Mastrantoni et al., 2024).
The volumes, trajectories, and velocities of rock fragments represent the main parameters that directly determine rockfall hazard. The rockfall model was implemented using Rockyfor3D. The software uses a three-dimensional rigid-body impact model that allows the calculation of the trajectories of single individually falling rocks with discrete geometry. Below is reported the spatial distribution of the intensity of deposited block, i.e., the number of blocks deposited. The NBD layer (number of blocks deposited) is compared against the target areas, where the warm colours represent the highest number of individual blocks that stopped in the pixel (Robiati et al., 2023).
