Natural Hazard Assessment in the Southeastern Margin of the Ría de Arosa (Pontevedra, Spain) Using GIS Techniques

Abstract

The characterization of natural hazards in coastal environments is of great necessity, especially in the current context of global climate change and increasing population concentrations. This research focuses on a multi-hazard analysis of the main geotechnical, geomorphological, hydrological, and lithological risks in the southeastern margin of the Ría de Arosa using Geographic Information System techniques. The integration of geotechnical characterization maps and natural hazard maps has allowed for the identification of areas with a high susceptibility to natural disasters, which is crucial for territorial planning and management in the context of growing urban pressure and global climate change. The results indicate that poorly consolidated surface formations, especially in transitional areas such as dunes and marshes, are particularly vulnerable. Additionally, areas with higher lithological competence have been identified, where slope changes contribute to ground instability. This analysis provides valuable tools for decision-making and the implementation of risk management policies, promoting sustainable development, the protection of coastal ecosystems, and the prevention of risks from urban planning and civil engineering activities in the Ría de Arosa.

1. Introduction

The characteristics of coastal areas, from an ecological and economic standpoint, make these territories suitable for the development of human activities [1,2,3,4]. These areas currently host 11% of the human population, amounting to approximately 896 million inhabitants [5]. Economic activity, based on urban and industrial development, along with the rise in tourism and recreational activities, makes coastal ecosystems among the most affected and altered ecosystems [1,6,7]. Additionally, these natural environments are threatened by natural processes that annually cause significant socioeconomic damage and, in the most extreme cases, a loss of human lives [8,9]. The current global climate change context leads to a scenario in which extreme events are occurring more frequently [5].

Natural disasters constitute one of the environmental problems that generate significant ecosystem alterations and affect urban areas with high population densities [10,11]. For this reason, there is a need to understand the geological processes associated with these events and delineate the areas that are more vulnerable and have a lower response capacity [12,13,14]. These coastal environments, where geological hazards play an important role, need to be the subject of effective land-use planning strategies [15,16] and of passive or active measures to mitigate these hazards and ensure sustainable development [17,18]. Furthermore, for the development of urban and civil engineering projects, geotechnical knowledge is crucial to detect potential hazards while planning [19,20].

This vulnerability scenario is clearly observable in the southeastern margin of the Ría de Arosa (ría: an estuary formed by an ancient coastal fluvial valley flooded during the Quaternary), which is an environment of great ecological importance due to the extensive protected natural areas (PNAs) it encompasses (Ons–O Grove and Umia–O Grove complexes). These environments include intertidal zones (wetlands) with a wide variety of protected fauna (birds, insects) and flora [21,22]. The degradation of wetlands directly affects the regenerative capacity of a coastal area, which in turn directly influences the vulnerability of that territory to the occurrence of a catastrophic event [23]. Additionally, the Ría de Arosa is an area with significant tourism activity, driving the development of infrastructure (hospitality, recreational) that directly increases the vulnerability of the area [6,24].

Therefore, a proper characterization of natural hazards through a multi-risk analysis is needed to evaluate the interactions between individual natural hazards [25,26,27]. A multi-risk analysis is essential in this context due to the diversity of natural processes studied (coastal flooding, erosion, etc.) that affect the Ría de Arosa [28,29]. The combination of GIS tools, remote sensing, and qualitative or quantitative analysis allows for a comprehensive understanding of these natural risks [30,31]. Numerous studies follow different multi-risk approaches, with the analytical hierarchy process (AHP) and the equal weights method (EWM) standing out [31,32,33,34,35,36,37].

In this study, we aim to carry out land-use planning through the development of a natural hazard mapping using ArcGIS 10.8 © (with a 1 m × 1 m resolution), which will serve as a basic tool for decision-making regarding construction processes in the territory [30,31]. This mapping allows for an understanding of the different risks from lithological, geomorphological, hydrogeological, and geotechnical perspectives through the combination of qualitative (geotechnical zoning) [38] and quantitative analyses (the stages of coastal flooding) [28]. A review and an increase in the level of detail of the geotechnical characterization mapping (1:50,000 scale) will be carried out, and the available coastal flooding information for the area will be used [39]. With this detailed mapping, the goal is not only to identify the inherent risks in the area but also to establish a solid foundation for future mitigation and adaptation strategies in the context of vulnerable coastal environments such as the southeastern margin of the Ría de Arosa.

Study Area

This research was conducted in the southeastern margin of the Ría de Arosa, in the province of Pontevedra (Galicia, Spain) (Figure 1). The area has approximately 57,358 inhabitants spread over 9600 hectares. Compared to the national average population density, which is 96 inhabitants per km², this area has a high population density (598 inhabitants per km²) (data from the National Institute of Statistics (INE), https://www.ine.es/, accessed on 30 September 2024). These values exclude the population increase that occurs during the summer season (June, July, and August), because of high tourism activity. The most important municipalities are located along the coastal margin, including Sanjenjo, Portonovo, Cambados, Villanueva de Arosa, El Grove, and La Isla de Arosa. Climatically, the area is considered temperate, with a dry summer compared to the rest of the year. It falls within the Csb type according to the Köppen classification system [40]. Due to the ocean’s moderating effect, its average temperatures are mild, ranging from 19.3 °C in summer to 9 °C in winter. The average annual precipitation is 1455 mm, with the rainiest months being October, November, and December. All autumn months experience precipitation above 200 mm (information provided by the Spanish Meteorological Agency (AEMET), https://www.aemet.es/es/serviciosclimaticos/datosclimatologicos, accessed on 30 September 2024).

The geology and current relief of the area are the result of a long and complex geological history [41]. The area belongs to the northwestern sector of the Iberian Massif, corresponding to the Galicia–Tràs–Os–Montes Zone. Lithologically and structurally, it represents an internal zone of the Variscan Orogen, where allochthonous domains can be recognized in superposition with autochthonous Paleozoic metasediments (d’Home–La Lanzada Complex) and plutonic igneous rocks [42]. The granites and granodiorites exhibit early (syn-kinematic) and late (post-kinematic) characteristics [43,44]. The area is represented on four geological sheets at 1:50,000 scale from the National Geological Map (MAGNA) (Spanish Geological and Mining Institute (IGME), http://info.igme.es/cartografiadigital/geologica/Magna50.aspx?language=en, accessed on 30 September 2024) [45,46,47,48] (Figure 2A).

The area’s geomorphological characteristics define a highly distinctive relief resulting from an evolutionary process of weathering and differential erosion. Slopes are gentle throughout most of the territory, except in areas dominated by rocky outcrops that reveal residual lithostructural reliefs [49]. The higher granitic areas form peaks, ridges, and hills, often represented by dome-shaped structures, where the evolution is more pronounced; tors; and other minor landforms [50]. The flat areas correspond to the peneplain, which is the result of the differential erosion of granite (mainly the Caldas de Reyes Batholith) and Paleozoic metasediments. Extensive erosion surfaces, known as “rasas” or marine terraces, have developed. In many cases, they are covered by poorly consolidated sandy or conglomerate deposits, resulting from the alteration of the rocky substrate. The strongly transitional marine environment of the area facilitates the formation of coastal landforms through the accumulation of fine deposits by tidal action (marshes and wetlands) and wind action (beaches and dune systems) [49]. In connection with this, two protected natural areas have been designated by the Natura 2000 Network: The Umia–O Grove Intertidal Complex and the Ons–O Grove Complex. The fluvial system is represented by fluvial valley deposits and transitional systems that include active alluvial fans and cones. In more stable settings, extensive floodplain surfaces, such as glacis and pediments, can be identified (Figure 2B).

2. Materials and Methods

The foundation of the work described here is based on fieldwork, satellite image analysis, and data modeling using computer software. First, the study area was initially recorded through sampling, data collection, and the photointerpretation of both current and historical images. All fieldwork and the collection of bibliographic and digital information during this and other previous studies of the area allow for effective planning that addresses the issue in a more detailed and comprehensive way. The selection of strategic sites allows for subsequent validation of the data obtained during digital modeling, which lends greater rigor to the entire study. All digital modeling, both results generated for geotechnical characterization mapping and for natural hazard mapping, was carried out on the Geographic Information System “ArcGIS 10.8 ©”. Figure 3 represents the methodological flow followed in this research, from the initial data collection to the generation of the natural hazard mapping. Each stage is described in detail, including the geoprocessing (reclassification, union, or export) used in the layers to obtain the data on the hazard factors (geomorphological, hydrological, lithological, and geotechnical) that make up the natural hazard mapping, which is useful in territorial planning and in preventing associated risks.

2.1. Geotechnical Characterization Mapping

The creation of the geotechnical characterization mapping relies on three thematic maps, each requiring a preliminary review and synthesis of information to achieve a more representative and easy-to-interpret result. Geotechnical characterization mapping is essential, as it generates a basic information document that must be consulted prior to any decision-making in territorial planning for a specific area, especially for construction projects. It aims, therefore, to prevent any risky situation and generate socioeconomic benefits for the affected civilian population. The zoning of the territory is based on different geological criteria—lithological, hydrogeological, and geomorphological—which overlap and delineate different “geotechnical zones.” These various criteria, collectively, aim to show the vulnerability to potential natural or anthropogenic risks of the study area:

• Lithological Mapping: This map starts with a geological map of the area [45,46,47,48], which is simplified by grouping lithologies based on compositional, textural, or geomechanical similarities. Each group defines the homogeneous lithological units that will make up the geotechnical characterization mapping. In this case, lithological groups from Quaternary surface formations are differentiated and reclassified into various lithological domains.
• Hydrogeological Mapping: Creating the hydrogeological map requires combining data sources and criteria from fieldwork with filtered information from the creation of the lithological map. Hydrogeological data primarily come from official cartography from the Spanish Geological and Mining Institute (IGME) and digital information from the Galician Health Service (https://www.sergas.es/Saude-publica/GIS-Litoloxia-xeoloxia, accessed on 31 October 2024). These data include the hydraulic properties of the different lithological formations. After characterizing lithological units with similar hydraulic properties, hydrogeological units are described. These hydrogeological units are defined based on their permeability degree (from impermeable to very permeable) and porosity type (intergranular, fissure, tectonic, or alteration). These characteristics are intrinsic to each type of rock or substrate. Crystalline rocks, which compose the Variscan basement (igneous and metamorphic), generally exhibit low or no permeability, a characteristic influenced by the low or null effective porosity of these lithologies, allowing water to pass through fractures, joints, or tectonic plates (schistosity). The igneous rocks (granitoids, granodiorites) and metamorphic rocks (slates and schists) found in the study area exhibit these hydrogeological behaviors [45,46,47,48]. The substrate belonging to Quaternary surface formations, which shows little or no consolidation, has very different hydrogeological characteristics. Its high effective porosity, which is of an intergranular type, leads to very high permeability values. However, there are no areas with significant thicknesses that would prevent the formation of aquifers with high water-capturing potential in the territory [45,46,47,48]. Each hydrogeological unit is assigned a color based on its permeability level for proper differentiation. In this case, the different units are represented in blue tones, with intensity varying according to their permeability level. The darkest color represents very high and impermeable values, which become lighter as permeability increases. In addition, each porosity type is differentiated by a characteristic transparent pattern. Both layers, formed by the reclassification of lithological domains, are combined to create the hydrogeological map.
• Geotechnical Zone Mapping: The vector polygons from the lithological and hydrogeological maps are merged, and, through a reclassification of the resulting polygons, different geotechnical zones are determined. In this case, the geotechnical zones are numbered with Roman numerals (I, II, III, …). Each represents a lithological type with specific hydraulic properties (porosity). This leads to the creation of the geotechnical zone mapping.
• Geomorphological Domain Mapping: This is derived from the detailed geomorphological map, with a resolution of 1:50,000 for the study area [49]. This type of mapping encompasses the fundamental features of the reliefs, synthesizing them. This simplification requires an in-depth relief analysis, relying on the slope map, DEM, specific fieldwork, and photointerpretation. Its purpose is to facilitate an understanding of the terrain and highlight its physiographic characteristics, which are grouped into morphogenetically coherent domains independent of the substrate type, though the substrate type often influences the morphological appearance of the relief.
• Geotechnical Characterization Mapping: The combined polygons generated from the geotechnical zone mapping and the geomorphological domain mapping create a 1:50,000 geotechnical characterization map for the southeastern margin of the Ría de Arosa. This map presents both the geotechnical properties of the substrate and the surface formations on it, enabling the identification of areas with limitations or negative triggers for civil engineering, urban, or industrial activities.

2.2. Natural Hazard Mapping

The sectoral analysis of external geodynamic processes, which are responsible for generating potential natural risks, along with their recognition and spatial delimitation, characterizes the natural hazard mapping of a territory. In this area, lithological, geomorphological, geotechnical, and hydrological issues are identified, which can often trigger natural or anthropogenic risks. Geotechnical characterization mapping will serve as a basis for defining and recognizing the various types of issues present:

• Lithological hazards: These relate to the textural, structural, and compositional characteristics of each lithology. Processes such as karstification and the resulting consequences for carbonate rocks, the planes of weakness seen in some lithologies, or degrees of consolidation represent risks associated with this type of hazard. In our case, the main lithological issues are associated with poorly cemented conglomerate deposits found on terrace or marine “rasas” formations. These polygons are derived from the geomorphological domain map, exported, and reclassified as lithological hazards.
• Geomorphological hazards: Each geomorphological feature impacts the terrain’s construction conditions differently. In these cases, natural or predominantly anthropogenic action can induce instability in certain surface formations. Generally, areas with steeper slopes will have a less favorable response in terms of stability, potentially triggering active gravitational processes like landslides, rockfalls, or soil creep. In the study area, ridges, hills, and summits, with their steep slopes and occasional presence of granite boulders, which may cause instability during construction, are classified as geomorphological hazards. Alluvial fans and gentler slopes also pose risks to stability during construction phases and are classified similarly.
• Hydrological hazards: The identification of high-risk flood areas is based on integrating coastal flood risk data with a geomorphological analysis to delimit and describe valley floors. Coastal flood risk maps for the southeastern margin of the Ría de Arosa were created using the Flood Hazard Index (FHI) method, which has been successfully applied to areas on the Atlantic margin of the Iberian Peninsula [28]. The parameters analyzed include significant wave height (Fw), annual sea level rise (Fsl), and extreme tidal range (Ftr), which were obtained from public data on PORTUS (Spanish Ports) (https://portus.puertos.es/#/, accessed on 31 October 2024). The rate of sea level rise (mm/year) is considered a variable factor, with values based on extreme scenarios presented by the Intergovernmental Panel on Climate Change (IPCC), where greater increase rates are expected [5]. A raster layer is created from the digital elevation model (DEM), with pixels below the maximum water sheet level for each scenario selected. The higher-risk scenarios Xa and Xb, at 25 and 100 years, respectively, are extracted in order to be included in the flood risk mapping, a part of the natural hazard mapping. Geomorphological data of the area were used to identify alluvial deposits in the valley floors, regarded as flood-prone zones. These deposits, characterized by a low altitude and proximity to fluvial systems, were highlighted as hydrological vulnerability areas. Both maps were combined and used to reclassify hydrological hazards.
• Geotechnical hazards: The geotechnical characterization map results allow for the identification of favorable, acceptable, or unfavorable areas for active construction processes. Each polygon is evaluated using a construction conditions matrix that crosses geotechnical zones (described in the geotechnical zone mapping) with geomorphological domains. As a result, polygons with unfavorable values are exported and reclassified as geotechnical hazards.

The hazard map enables the combined identification of the different issues previously described. The process of developing this map is based on a Multicriteria Decision-Making (MCDM) methodology, which is used to combine the hazard parameters described and obtained in the previous steps without assigning a hierarchy. Here, the layers are combined through a qualitative overlay of multiple layers (based on union and reclassification) to integrate the analyzed parameters into the resulting polygons. The qualitative overlay allows for the integration and recognition of areas where the different identified risks act individually or jointly. The vector polygon layers of each risk are geoprocessed using the union procedure. In this way, new polygons are created that describe the individual or combined occurrence of a risk. This method aims to provide a descriptive, qualitative approach. No weighting process is applied to the different risks, as this would require a detailed study of the impacts of each risk. This would then provide a solid foundation for the quantitative prioritization of impacts. By avoiding this, subjectivity is prevented in this research, making the natural hazard mapping a preliminary tool for identifying hazard-prone areas in relation to construction processes and their monitoring. The interpretation of the results is based on identifying areas with higher or lower hazard levels according to the number (greater, lesser, or none) of processes that have the potential to pose a hazard in each area.

3. Results and Discussion

3.1. Geotechnical Characterization

The qualitative superposition of the lithological (Figure 4A), hydrogeological (permeability and porosity) (Figure 4B–D), and geomorphological (Figure 4E) maps allows for the differentiation of the various geotechnical areas which are represented in the geotechnical zone and geotechnical characterization mapping (Figure 5A,B).

3.1.1. Lithological Mapping

This mapping synthesizes the lithologies into different domains with lithological, textural, or compositional affinities based on what is reflected in the geological map (Figure 2A). In this case, the superficial formations and the Paleozoic metamorphic rocks remain unchanged. Synthesis occurs within the igneous rocks based on their early or late character (syn- or post-kinematic). The granites and granodiorites with internal deformation (early) are grouped together, while the late ones (Caldas de Reyes batholith) are classified separately (Figure 4A).

3.1.2. Hydrogeological Mapping

Hydrogeologically, a precise differentiation of the various lithological domains present can be made (Figure 4D). The superficial formations exhibit the best hydrogeological characteristics due to their low or nonexistent consolidation. They demonstrate a high degree of permeability and well-connected intergranular porosity. However, the limited thickness of these formations prevents the development of significant aquifers [45,46,47,48]. The metamorphic rocks (schists and gneisses) present rather poor hydrogeological characteristics. They show a very low degree of permeability, and in many cases can be considered practically impermeable (<1%) [47,48]. Areas where tectonic weakness planes (schistosity, fractures, jointing) are concentrated and areas with slight surface alterations show an increase in permeability, but in general, this increase is not significant. The two groups of igneous rocks (early and late) show slight variability from a hydrogeological standpoint. Early granites and granodiorites have very low, almost nonexistent permeability (very low–impermeable, Figure 4B,D), which increases as the degree of fracturing and surface weathering rises. The Caldas de Reyes batholith exhibits low permeability, induced by fracturing, and slightly higher permeability where its weathering is better developed [46,47,48]. The localized development of weathering–fracturing-type aquifers is possible, especially where granitic clay is found [46,47,48].

3.1.3. Geomorphological Domain Mapping

Geomorphological domain mapping is the grouping of different Quaternary superficial formations found in the study area in a morphogenetically coherent manner (Figure 4E). It is presented according to the morphogenetic systems present and, in turn, based on additional parameters such as slopes or the degree of consolidation of the deposit. Additionally, the main areas of anthropogenic infrastructures (population centers or industrial areas) are represented, which interact with the different domains or are included within them. This map is crucial for understanding the spatial distribution of landforms and their relationship with the evolution of the terrain and with the geotechnical characterization mapping (Figure 5B).

3.1.4. Geotechnical Zones and Geotechnical Characterization Mapping

• Zone I₁^L: This zone exhibits a varied distribution and is prominently present in the El Grove Peninsula and the NE sector of the ría coastline. It occupies almost the entirety of the islands of Arosa and La Toja. Its presence around Punta Lanzada, on the NW margin of the Castrove Peninsula, is also notable. Lithologically, it corresponds to Caldas de Reyes granodiorite, which shows a slightly higher permeability than earlier facies (Figure 5A). This is particularly evident in tectonized areas, where fractures and weathering promote percolation processes and the development of minor aquifers. This area is well-defined by its residual litostructural relief, characterized by systems of summits, hills, and crests. In specific locations where the slopes are steeper, such as in the interior of the El Grove Peninsula or the northern sector of Arosa Island, dome-like morphologies or more mature rocky outcrops are visible (Figure 6A,B). Colluvium, along with pediments and glacis, is well represented in areas with more pronounced slope changes. Coastal environments make up the “marine rasas” (terraces) that show a smoothing of the relief, with almost nonexistent slopes (Figure 5B and Figure 6C).
• Zone I₂^L: Located on the S–SE margin of the study area, this zone is entirely represented by early granite facies that are affected internally by tectonic deformation processes. This area shows very low permeability conditions that increase slightly where weakness planes (fracturing or jointing) are concentrated or where there is a high degree of alteration (Figure 5A). The relief structures distinguished here are part of the regional modeling of granite, creating a residual relief where, in some cases, domed structures stand out (Figure 5B).
• Zone II^VL–I: This zone is found in the inner sector and SW margin of the Castrove Peninsula, where significant cliff areas are highlighted (Punta Fagilda, Punta de Cabicastro) (Figure 6D). Lithologically composed of metamorphic rocks (schists, gneisses) belonging to the “d’Home–La Lanzada Complex”, these rocks show very low, almost impermeable, permeability values, with exceptions in sectors with a greater development of tectonic plates (schistosity) and higher concentrations of fractures and joint systems (Figure 5A). Geomorphologically, the inner areas where slopes are slightly steeper represent residual litostructural reliefs, primarily defined by hills and summits. In the southern zone, more gentle alluvial fans and cones have developed. The area closest to the coast contains some rasas (Figure 5B and Figure 6D).
• Zone III^H: This zone has a heterogeneous distribution throughout the study area. The places where it is best recognized are in the Tombolo of La Lanzada and the N zone of the Castrove Peninsula, where the most significant alluvial fan systems are located. Composed of a substrate with a low or nonexistent consolidation of superficial formations, it has high permeability due to its high porosity (Figure 5A). Important coastal deposits such as dunes, beaches, or marsh areas are within this zone, as in the case of the Umia River mouth (Figure 6E,F). The poorly evolved drainage network of the territory also demonstrates the presence of alluvial bottoms. Significant rasas are identified in areas near the coast with very gentle slopes (Figure 5B).

3.2. Natural Hazards

By recognizing the active processes in the study area, potential risks of a lithological, geomorphological, geotechnical (Figure 7A), or hydrological (Figure 7B) nature can be identified. By overlapping these, we arrive at the natural hazard mapping (Figure 8).

3.2.1. Geotechnical Hazard Map

Based on the delineation of different geotechnical areas and supported by the construction conditions matrix (

Table 1. Construction condition matrix.

Geomorphological Domains	Zone I1L	Zone I2L	Zone IIVL–I	Zone IIIH
Alluvial (valley bottoms)	Unfavorable	Unfavorable	Unfavorable	Unfavorable
Alluvial fans and slopes	Unfavorable	Unfavorable	Unfavorable	Unfavorable
Glacis and pediments	Favorable	Favorable	Favorable	Acceptable
Dunes, beaches, and marshes	Unfavorable	Unfavorable	Unfavorable	Unfavorable
Marine terraces	Acceptable	Acceptable	Acceptable	Unfavorable
Ridges, summits, and hills	Unfavorable	Unfavorable	Acceptable	Acceptable

), the geotechnical hazard map is constructed (Figure 7A).

It is evident that most of the territory presents unfavorable construction conditions from a geotechnical point of view. Among all of these, the territories with poorly consolidated substrates, which exhibit low load-bearing capacity and are prone to potential settlements, stand out [39,45,46,47,48]. Critical areas include the dune systems of La Lanzada, the coastal environment influenced by tidal activity, and the alluvial deposits in valley bottoms. Other areas where the substrate is somewhat more consolidated and which display low slope gradients are represented by alluvial fan and cone systems. However, their active nature may favor the development of slope processes, such as landslides, in response to anthropogenic activities [39,49]. Despite being competent lithologies with high load-bearing capacities [39], the granite areas are generally steep regions with higher elevations, which complicate their stability in terms of potential construction, giving them low or no settlement potential. The instability of their slopes is often exacerbated by the presence of boulder fields, where rocks of various sizes appear. Metamorphic lithologies also exhibit high competence, granting them high load-bearing capacities and making settlement development impossible [39]. The areas with more pronounced reliefs, coinciding with the lithostructural system, offer acceptable construction conditions. These areas, unlike granite ones, are more stable due to their slightly gentler slopes and the absence of “boulder-like” morphologies. Additionally, acceptable construction conditions are found in coastal areas, while considering the retreat of cliffs due to marine and fluvial erosion processes. Across the entire metamorphic zone, the orientation and inclination of schistosity planes must be considered, as they may occasionally pose a problem when carrying out a project. This also prevents these outcrops from being classified as acceptable. Marine terraces or “rasas” are considered acceptable from a construction perspective, except for those with a significant thickness of poorly consolidated coarse-grained substrates (unfavorable). The pediments and glacis, which develop on highly competent rocks (granites and metamorphic rocks), have very gentle slopes and exhibit greater stability, making them favorable areas from a construction perspective.

3.2.2. Hydrological Hazard Map

The coastal flood risk mapping proposed for this area was utilized (Figure 7B) [28]. In this case, areas at potential hydrological risk are those with the possibility of flooding within a return period of 25 to 100 years. Additionally, valley bottoms are added to this layer due to their potential risk of flooding. It is observed that the areas of greatest hazard are found in coastal environments with gentle slopes, which the water sheet can penetrate more easily, particularly in the area adjacent to the mouth of the Umia River.

3.2.3. Natural Hazards Mapping

The natural hazard mapping highlights the areas of this territory that are susceptible to various issues that could be triggered during the active phase of a construction project (Figure 7B). As such, it serves as an essential tool for the decision-making stage when selecting a project site. Consulting this map is recommended to enable effective risk management and thereby reduce the negative impacts that may arise in subsequent project phases. This, in turn, will directly reduce the economic resources needed to mitigate impacts from geomorphological, hydrological, geotechnical, and lithological issues, as well as contribute to sustainable land use. The main issues identified during the phases of a construction project are related to the rise in the water table and ground movement or settlement [51].

The areas where hydrological issues are concentrated are those closest to the coastline or situated on alluvial valley floors. Under specific conditions, other natural issues may arise that increase the risk in these areas. Surface formations made up of weakly consolidated substrates are prone to problems related to rising water tables and waterlogging following heavy rainfall or flooding events. This is the case of the tombolo area of La Lanzada, where highly unconsolidated substrates (coastal formations such as beaches, dunes, or marshes) exhibit a low load-bearing capacity, high permeability, and susceptibility to flooding. These areas, which have combined hydrological and geotechnical issues, may also present geomorphological or lithological problems. Geomorphological issues are identified as being when active formations, such as alluvial fans, lose stability, potentially triggering mass movement processes. The northern flank of the Castrove Peninsula is an example of where these three issues converge. The poorly cemented conglomerate deposits found on some marine terraces cause lithological issues. They overlap with hydrological and geotechnical problems in many coastal areas around the mouth of the Umia River, north of the municipality of Cambados, or in the southern area near Villanueva de Arosa. In most cases, major urban or industrial projects are not recommended in these areas due to their combined issues.

Inland areas, beyond the valley floors, generally pose fewer hydrological issues. However, periodic monitoring of the water table is recommended in areas with high-permeability surface formations.

The most abundant zones in the study area are those with combined geomorphological and geotechnical issues. Generally, these are areas with a more pronounced relief where granite morphologies have been identified. There are no issues related to rising water tables or potential settlement due to load-bearing capacity loss thanks to the high competency and low permeability of the granitoids present here. The main issues that may arise in these locations are influenced by the terrain slopes, which can trigger mass movement processes. In many cases, the presence of granite boulders could lead to falls if stability is lost during the active phase of a project. The interior of the El Grove Peninsula (Mount Siradella) and the western coastal margin contain extensive areas with these characteristics. Similarly, the eastern margin of the Castrove Peninsula exhibits these same characteristics. Where surface formations associated with alluvial fan and cone systems are present, issues arise due to the mobile nature of these formations when stability is lost, potentially triggering mass movements, and, where the substrate thickness is greater, minor settlements. All alluvial fan and cone systems on the Castrove Peninsula present these issues.

Geomorphological issues appear in isolation in the rocky outcrops of the metamorphic facies found in inland areas and at various headlands along the western coast of the Castrove Peninsula. In these cases, the main issue stems from the steep slopes of the residual relief morphologies. In the peninsula’s inland areas, mass movement processes may occur when instability is reached, which is sometimes promoted by tectonic planes of weakness in the metamorphic rocks. In coastal areas, where cliffs have significant escarpments, rockfall problems may arise. Due to the competence of metamorphic rocks, these locations are more suitable for civil works projects, provided that mass movement processes are considered. The continuous monitoring of stability would be advisable, with reinforcement in areas showing instability if necessary. Clifftop lands are not recommended for any projects, with the suggestion being to seek suitable sites further inland.

In areas with lithological issues, where poorly cemented conglomerates situated over marine terraces are found, geotechnical issues may be detected. It is recommended that this layer, which generally has a thin thickness, be removed in advance [39]. The removal of problematic lithology and the level nature of these erosion surfaces make these locations feasible for civil works projects.

4. Conclusions

Research in the southeast margin of the Ría de Arosa has allowed for a detailed characterization of the main lithological, geomorphological, hydrological, and geotechnical risks in the area. The integration of geotechnical characterization maps and natural hazard maps has facilitated the identification of areas with a greater susceptibility to one or several types of risks, a crucial aspect for planning and managing this territory in the context of increasing urban pressure and global climate change.

The results, based primarily on the geological and geomorphological categorization of the territory, allow for the identification of areas susceptible to natural hazards. Ground instability and susceptibility to flooding, rising water tables, or settlements are the main issues that may be triggered in this area. Zone III^H of our geotechnical characterization mapping, where poorly consolidated substrate surface formations are found, is especially vulnerable to geotechnical, geomorphological, and hydrological issues. The transitional areas containing dunes and marshes around La Lanzada, as well as the alluvial deposits in the northern part of the Peninsula of Castrove, stand out for their high environmental quality. Therefore, there arises a need to adopt sustainable strategies to protect wetland areas and enhance the environment’s capacity to respond to natural and anthropogenic hazards, which is largely influenced by the large impact of tourism and infrastructure development.

On the other hand, Zone I₁^L, Zone I₂^L, and Zone II^VL–I, defined by a more competent lithic substrate, are primarily affected by geotechnical and geomorphological problems. This is due to their pronounced changes in slope, which increase ground instability for construction activities. In these areas, it would be advisable to prioritize locations with gentler slopes, a lower density of fractures or joints, and less alteration.

The preparation of these maps has created a valuable tool for decision-making in the pre-design phases of urban planning, facilitating the identification of critical areas that require attention and mitigation measures. This work not only contributes to our understanding of the interaction between natural processes and human activity but also lays the groundwork for future research and the implementation of risk management policies that protect both the population and coastal ecosystems. Additionally, it opens the door to a detailed analysis of high-problem areas, allowing for an examination of the issues causing the greatest challenges. This approach enables future risk analyses that can establish a hierarchy, revealing the relative importance of each issue in relation to others.