Rockfall susceptibility analysis through 3D simulations in marine protected areas of the Portofino coastline: case studies of San Fruttuoso and Paraggi bays

The paper focuses on the risk assessment of ongoing and potential rockfall hazard affecting the Regional Natural Park of Portofino (Roccati et al. 2021). Located at less than 20 km east of Genoa (Liguria region of Italy), the park covers a natural and cultural area of more than 18 km² whose 13 km is of coastal strip. Along the whole area, it is possible to walk in a unique natural and cultural heritage, visiting small historical village and sites (e.g., Portofino, Camogli, San Fruttuoso, and Paraggi bays) and, at the same time, in a stunning landscape shaped for thousand years (e.g., track trails along the ancient terraces of vineyards and olive grove) (Brandolini et al. 2006; Faccini et al. 2018; Coratza et al. 2019).

Main geo-hazards that affect the site are rockfalls and landslides, both closely related to the increasing meteo-climatic extreme events, such as windstorms or extreme precipitation, driven by climate changes, and affected by marine erosion due to sea storms (Kabisch et al. 2016; Ruangpan et al. 2019; Roccati et al. 2020; Turconi et al. 2020). As a contribution to the short- and long-term sustainable conservation policies of sites, a research team was established in the framework of a collaboration among the Istituto Superiore per la Protezione e la Ricerca Ambientale (ISPRA), Geographical Information System International Group (GISIG), Università di Genova (UNIGE) and Universidad del País Vasco (UPV/EHU), under the RECONECT Project (www.reconect.eu). In this sense, a specific survey was carried out both in Paraggi and San Fruttuoso bays with the aim to test and calibrate the 3D simulation of rockfalls in the whole Park. The structural setting of the rock mass, related to the stratigraphical setting and to the geomorphological evolution of the slopes, was identified, collected, and classified. Additional information on local rock mass conditions, potentially triggering rockfall that may affect both the heritage itself and the visitors, were collected. Some focus areas and catchments were tested in order to assess the preliminary exposure and vulnerability level: the San Fruttuoso Abbey and the Paraggi Bay. Morphometric and geo-mechanical parameters, as input for the modeling, were calibrated along the most representative park trails to check the model’s reliability. The activities are characterized through a multidisciplinary approach (Perrone et al. 2021) including expertise in geomorphology, engineering geology, rock mechanics, nature and earth science, landslide risk assessment and management, as well as conservation, protection, and sustainable mitigation measures (Crosta et al. 2017; Calista et al. 2019; Pazzi et al. 2019). Reliable and true advanced modeling results are fundamental for proper conservation and mitigation intervention on the Natural Park heritage. The use of nature-based solutions (NBSs) (Naumann et al. 2014; Kumar et al. 2020; Villegas-Palacio et al. 2020) in areas of high cultural, natural, and landscape value is strongly recommended, also because of the high reduction of cost and impact (Debele et al. 2019; Kumar et al. 2020). The above-mentioned collaborative activities, between the different research teams, are aimed at the conservation and protection of cultural landscape sites (Jongman 2002), with the ultimate target of making the area accessible to the public in a complete state of safety from rockfalls and slides (Margottini and Spizzichino 2021).

Laser Imaging Detection and Ranging (LiDAR) technologies developed in this research are now widely used in geological risk management, including rockfall hazard assessment (Kenner et al. 2014; Rieg et al. 2014). LiDAR techniques can provide high-resolution and spatially accurate point clouds making them indispensable tools for accurately capturing dense information to facilitate detailed topographic analysis. With this base information, the point clouds modified using cloud processing tools such as CloudCompare allow the comparison of different LiDARs with specific algorithms, to determine the regional evolution of the terrain. In this sense, adjusted and georeferenced models can be developed, which combined with three-dimensional simulation software for rockfalls and georeferenced aerial orthophotographies in the same common reference system, let realistic modeling of each studied area.

In our research, we pursue making progress in the recognition of instability processes in the Portofino Park, which preserve a natural environment including San Fruttuoso and Paraggi bays, with the aim of proposing nature-based solution strategies. At this point, this paper focuses mainly on rockfalls, which are a highly recurrent process, and whose fast development makes it advisable to implement protection measures in agreement with the values of the environment. The main objective is to advance in the combined use of in situ and remote analysis techniques for the development of three-dimensional models that allow the establishment of specific management measures, adapted to each space, combining constructive friendly actions (Morales et al. 2021), where necessary, within the framework of a global approach based on efficient, environmentally, socially, and economically beneficial solutions, which contribute to increasing resilience (European Commission 2015).

Thanks to its wide recognized landscape, natural, and cultural values (Fig. 1), the Portofino Promontory has been protected since 1935 with the establishment of the Portofino Natural Park (Coratza et al. 2019). Since 1995, the protected area has been managed by the Regional Park Authority: current protected area is approximately 1056 ha wide, covering the territory of the municipalities of Camogli, Portofino, and Santa Margherita Ligure.

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The resident population of the Portofino Park is about 750 (Turconi et al. 2020), while the total population of the three municipalities is about 15,000. The number of tourists is very high throughout the year: along the coast, in Portofino town, there are more than one million tourists/year, while in San Fruttuoso, the boat connections from the Tigullio and Paradiso Gulfs provide around 400,000 tourists/year (Faccini et al. 2018). In addition to tourists, there are also hikers along the more than 80 km of paths (Brandolini et al. 2006): the section from “Portofino Vetta” to “Pietre strette” is travelled by more than 70,000 hikers/year.

In addition to the Portofino Park, there is the Portofino Marine Protected Area, established in 1999; finally, in 2017, the process to convert Portofino into a National Park was started, even if the boundaries are still to be well defined.

The entire promontory is also extraordinarily rich in cultural heritage, not only represented by the seaside towns of Camogli and Portofino, but above all linked to the ancient medieval religious trails that connected various monastic centers (Figs. 1 and 3): on the western side, there is the Church of San Nicolò di Capodimonte (Fig. 2a), dating back to the twelfth century; in San Fruttuoso, there is the Benedictine abbey (Fig. 2b) dating back to the tenth to eleventh century; and on the eastern side, there is the Cervara monastery complex (Fig. 2c), built in 1361 and maintained by Benedictine monks. Other important buildings of great cultural interest are the Hermitage of S. Antonio di Niasca and the Church of Divo Martino in Portofino (Figs. 1 and 3).

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Most of this cultural heritage is however threatened by geomorphological hazards: many of these centers were built on ancient and relict landslides (San Nicolò, Divo Martino, S. Antonio di Niasca, Abbazia della Cervara), but occasionally they are reactivated, especially by heavy, short-term rainfall triggered by the atmospheric low depression over the gulf of Genoa (the so-called Genoa Low) (Roccati et al. 2020). On September 25, 1915, an intense and concentrated rainfall triggered a debris-mud flow that channeled into the hydrographic network and partially destroyed the San Fruttuoso abbey and surrounding houses (Paliaga et al. 2022). In fact, it is located exactly at the mouth of the Fontanini valley, which is also subject to frequent rockfall phenomena (Faccini et al. 2008b, 2009). Noteworthy is the event that occurred in 2016, where a 3-m-diameter block impacted a house in San Fruttuoso, causing considerable material damage (Roccati et al. 2021). The Cervara complex was severely threatened by the effects of the 2018 Vaia sea storm surge (Turconi et al. 2020; Betti et al. 2021).

The geology of the whole area is dominated by two formations (Fig. 3): the “Mt. Antola Flysch” on the north (Elter and Pertusati 1973; Corsi et al. 2001), dated between 90 and 55 million years ago, and the “Portofino Conglomerate” at the south, dated about 30 million years ago (Terranova 1964). Conglomerates, which outcrop at the entire southern slope of the promontory, are made by of heterogeneous pebbles, mainly from marly limestone and secondarily from other lithotypes, in a sandy-limestone matrix (Faccini et al. 2008a). The formation shows a fragile tectonic deformation, with several fault and fracture systems oriented mainly NW–SE and NE-SW (Fig. 3) (Bonaria et al. 2016; Terrone et al. 2021). Regarding the quality indexes of the Portofino Conglomerate, its high resistance stands out (Table 1). At rock mass scale, the Geological Strength Index (GSI) (Marinos and Hoek 2000) ranges from 65 to 70, consequence of the good surface conditions together with a blocky structure (Faccini et al. 2008a). In a detailed scale, field tests using the Schmidt hammer, distinguishing between limestone, arenaceous clasts, and matrix (Cevasco et al. 2004), with a modal rate between 25 and 50 were calculated (Faccini et al. 2008a). Besides, laboratory tests have been developed using a point load test, carried out directly on rock samples, and providing compressive strength ranges between 50 and 100 MPa (Faccini et al. 2008a).

This characterization serves as a basis for the definition of different terrains in the modeling part, depending on the values obtained by the techniques used, in addition to the calibration and adjustment process. The tectonic structures, along with the high resistance of the conglomerate, control the development of instability processes along the entire coastline, which are especially evident in the south-facing conglomeratic outcrops, such as those identified in San Fruttuoso area (Fig. 3).

At present, the bays of San Fruttuoso and Paraggi, respectively, located to the south and east of the promontory, have different morphologies. San Fruttuoso has local outcrops of conglomerate, which are found among abundant vegetation of various types, where a deeper terrain is developed, which means that during heavy rainfall, several debris and mudflow processes have historically developed. Shallow landslides often occur in terraced areas with dry stone walls (Paliaga et al. 2016, 2020). In the case of the rocky outcrops, which are relatively isolated and with metric continuity of fractures in the rock mass, rockfalls with a largely spherical shape (Roccati et al. 2021) develop, not exceeding 1 m in diameter. These dimensions of spacing and block sizes were obtained during the days of fieldwork developed in the area, with a classical tape measure, in order to make more accurate measurements for the simulation. On the other side, in Paraggi Bay, the coastal zone draws practically a continuous line of rocky cliffs, where diverse processes of rockfalls with decimeters size take place.

In the whole investigated area, the present and active morphological processes should be framed within a management and conservation master plan having two different timeline references: emergency mitigation measures, to be defined and undertaken in the short-medium term, and preventive mitigation measures to be implemented in the medium-long term (Spizzichino et al. 2016).

Short-medium-term actions should be always preceded by an investigation phase including the following:

The long-term actions include the following:

In both cases, the mitigation options must be supported by scientific and technical analysis in a holistic framework. The proposed numerical 3D rockfall modeling is a fundamental tool to support the above-mentioned strategies in the case of rockfalls, the process that is the focus of this work. The incorporation of complementary studies more focused on landslide susceptibility analysis (e.g., Roccati et al. 2021), will complete the information for the development of comprehensive management strategies.

The management of coastal areas needs to advance in methodological approaches that allow detailed studies of the current dynamics, in order to develop specific management plans for each environment, based on the guidelines of the NBSs (Villegas-Palacio et al. 2020; Kumar et al. 2021; Vojinovic et al. 2021). In previous works, a large number of authors have assessed these strategies as a line of action for natural hazards in different environments (Pontee et al. 2016; Castelle et al. 2019; Van der Meulen, 2022), but rarely dedicated to geological hazards.

The proposed study in the Portofino Natural Park has allowed the development of a methodology that combines remote sensing techniques with local digital terrain models, as long as with data collection through fieldwork. This detailed topographic information is derived to the design of three-dimensional point clouds, which are modified and adjusted by means of specific software, which serve as a basis for the elaboration of investigations on slopes that present complex morphology (Abellán et al. 2010; Pham et al. 2016; Ansari et al. 2018), such as the coastline of Portofino, and especially the San Fruttuoso Bay, where a realistic representation of the relief is the only way to develop geo-hazard studies useful in future management strategies (Ratter 2013; Jia et al. 2016; Preti et al. 2021).

The advance of adding a third dimension to traditional 2D rockfall modeling involves taking into account the lateral deviation of block trajectories (Bourrier et al. 2012; Asteriou and Tsiambaos 2018; Ji et al. 2020), which was previously overlooked considering only terrain profiles, and which is essential to understand the processes in detail in order to mitigate their negative impacts (Volkwein et al. 2011). This is especially important in coastal environments, where sometimes the shape of the cliffs tends to develop trajectories with lateral evolutions almost in their entirety.

With respect to the detached blocks, the worst-case scenario is generally that of the largest rock which remains intact while traveling down a slope (Pfeiffer and Bowen 1989), attaining the highest energy (Akin et al. 2021; Morales et al. 2021). In addition, the shape of the blocks determines the mechanical responses on impact with the ground, which has a remarkable implication for rockfall hazard assessment (Bonneau et al. 2019; Caviezel et al. 2021). In the case of spherical blocks, which are the prevalent ones throughout the study areas in this research, the shape is especially relevant, because it yields a maximum volume for a given radius, which will tend towards the worst case (Pfeiffer and Bowen 1989).

The detailed analysis of the most important rockfall trajectories studied in the past (most of them of 1 m³) that reach high energies and volumes up to 3 m³, even destroying local structures, lead to study and recognize the potential new events in the selected environments, with which evaluate, adjust, and simulate detachments occurring in the near and far future. Thus, it will be possible to develop management strategies that adhere to the environment in a more respectful, environmentally friendly, controlled, and efficient way, improving in turn the safety against geo-hazards (Morales et al. 2021; Domínguez-Cuesta et al. 2022). These actions that combine constructive elements with warning campaigns and delimitation of access and use areas, will result in establishing a base methodology applicable to other environments with similar characteristics, where the population’s negative perception because of these interventions will be significantly reduced (Touili et al. 2014; Gray et al. 2017).

The development of analysis maps of different parameters that define rockfalls makes it possible to analyze the necessary height and the minimum kinetic energy that the hypothetical control structures to be installed in the area should withstand, or the response time and velocity against possible block falls (Sarro et al. 2018; Fanos and Pradhan 2019; Akin et al. 2021). This information provides a deeper understanding of the dynamics of instability processes in the environment, which in any case must constantly update its data and add new information, in order to progressively generate more realistic and accurate models of the environment.

The investigated area of Portofino Natural Park covers a wide natural and cultural area where it is possible to visit remarkable heritage sites walking in an incredible high-value landscape. The entire area is characterized by the presence of conglomerate rocks. The geological and geo-mechanical characteristics of this formation affect the potential instability of the natural and cultural heritages sites, especially during heavy rainfall and extreme events.

In the present paper, a comprehensive analysis of potential instability mechanisms (rockfall) and their spatial evolution (e.g., runout distance, trajectories and impact energy) is described for the selected area of San Fruttuoso and Paraggi bays. Such approach and methodology allow to carry out further studies and analysis supporting the adoption of the so-called nature-based solutions (NBSs), aiming to reduce the impact by the construction of mitigation measures and to increase resilience in high-value natural and cultural areas.

At the same time, the proposed advanced modeling constitutes a useful tool for the verification and calibration of the adopted design choices (e.g., size and correct location of the barriers, hypothesis of structural interventions for the protection of exposed elements such as paths, terraces, and cultural heritage), as well as a useful guidance for the implementation of medium-/long-term non-structural measures, such as in situ monitoring systems to be eventually transformed into early warning systems.

The detailed knowledge of the morphological dynamics of the area is in fact a fundamental cognitive element to develop participatory planning, directly involving stakeholders and providing the tools for a correct cost–benefit analysis to policy makers and funders. A co-creation process, in fact, runs across all the steps for the implementation of NBS interventions, from the initial co-design, to co-monitoring of their performance, to maintenance and finally decommissioning.

The advanced 3D modeling phase here proposed, in addition to being a real digital twin of the natural dynamics of the area, is expected to be integrated into this co-creation process for the sustainable management of high-value areas.