The Potential of Blue–Green Infrastructures (BGIs) to Boost Urban Resilience: Examples from Spain

Abstract

Urban areas globally are increasingly challenged by climate change, rapid urbanization, and environmental degradation, necessitating innovative solutions for sustainable development. Blue–Green Infrastructures (BGIs) have emerged as a promising approach, integrating water management systems with natural vegetative elements to create resilient urban landscapes. By fostering synergy between urban ecosystems and built environments, BGIs offer multifunctional benefits including flood mitigation, heat reduction, and biodiversity enhancement. This article examines the role of BGIs in boosting urban resilience, highlighting several exemplary projects in Spain in a qualitative and quantitative way that demonstrate its potential to transform urban areas into sustainable and adaptive spaces.

1. Introduction

Blue–Green Infrastructures (BGIs) are an innovative approach to urban planning that strategically integrates natural water systems (blue) with vegetative landscapes (green) to create sustainable and resilient urban environments. BGIs emphasize the use of natural processes and ecosystems to manage water, reduce urban heat, enhance biodiversity, and improve the quality of urban life [1]. This approach is rooted in the idea that combining blue and green elements can provide synergistic benefits that exceed those of traditional gray infrastructures, such as concrete stormwater systems and conventional parks.

BGIs are an approach that combines natural and semi-natural systems with engineered solutions to address urban water management, climate adaptation, biodiversity, and human well-being. It emphasizes the integration of water bodies (rivers, lakes, wetlands) and green spaces (parks, gardens, green roofs) to create multifunctional urban landscapes. These infrastructures provide multiple ecosystem services such as flood mitigation, air quality improvement, urban cooling, and recreational spaces. Additionally, BGIs support the natural water cycle by enhancing infiltration, evapotranspiration, and groundwater recharge, which are often disrupted in heavily urbanized areas. By mimicking natural hydrological processes, BGIs help to restore balance in urban ecosystems and mitigate the negative impacts of urbanization.

The concept of Blue–Green Infrastructures (BGIs) has its origins in the late 20th century as a response to the growing awareness of the limitations of traditional gray infrastructure in addressing urban environmental challenges. The concept draws from earlier ideas in ecological design and sustainable water management, such as Sustainable Urban Drainage Systems (SuDS) and Low-Impact Development (LID). However, historical urban transformations have often been driven by water-related challenges [2,3,4,5]. In many cases, severe flooding has led to the development of new infrastructure, such as stormwater drainage systems, levees, and flood retention basins [6]. For example, the construction of the Thames Barrier in London was a response to repeated storm surges, while cities like Rotterdam have embraced water squares that temporarily store excess rainwater. Approaches to managing stormwater vary, with some prioritizing traditional engineered solutions like concrete channels, while others integrate natural processes, such as green roofs, permeable pavements, and wetlands, to mitigate flooding, reduce urban heat, and enhance biodiversity.

The origins of BGIs can be linked to the rise of environmental movements in the 1960s and 1970s, which advocated for more harmonious relationships between urban development and natural ecosystems [7,8]. During this period, growing concerns about the environmental impact of rapid urbanization and industrialization led to a push for more sustainable urban planning practices. These movements emphasized the importance of integrating natural ecosystems into urban environments to create more harmonious and sustainable cities [9,10]. Activists and urban planners began to advocate for the preservation of natural landscapes, the restoration of waterways, and the inclusion of green spaces in urban design. These early efforts laid the groundwork for BGIs, focusing on the need to balance built environments with ecological systems. By promoting the idea that natural processes should work in tandem with urban development, these movements set the stage for the development of BGIs as a key strategy for enhancing urban resilience and sustainability.

Multifunctional and interconnected blue–green systems have been recognized as a promising solution to bolster urban resilience. Also, BGIs encompass vegetated urban spaces designed to serve as decentralized stormwater management systems, mitigating flooding through runoff infiltration, while also offering recreational benefits for residents in areas such as parks, sports fields, and school grounds.

Several examples illustrate the early application and evolution of Blue–Green Infrastructure (BGI) concepts in urban planning, driven by the environmental movements of the 1960s and 1970s [11]. These examples reflect a growing recognition of the value of integrating natural ecosystems into urban spaces, such as Portland’s Green Streets Program or the High Line in New York City.

Portland’s Green Streets Program was initiated in the 1970s, and Portland became one of the first cities to adopt green streets as part of its stormwater management strategy. The program focuses on incorporating vegetation and permeable surfaces into streetscapes to reduce runoff, improve water quality, and enhance urban greenery. This initiative set a precedent for future BGI projects worldwide, and it still continues [12,13].

Regarding the High Line, it was conceived in the 1970s and realized in the 2000s; the High Line transformed an abandoned elevated railway into a public park featuring native vegetation and sustainable design elements. The project blends urban revitalization with ecological restoration, offering a green corridor that supports biodiversity and provides a unique recreational space [14,15]

In Europe, the first trends started in United Kingdom with the Royal Parks in London. While London’s Royal Parks date back centuries, the environmental movements of the 1960s and 1970s influenced their modern management practices. These parks are now managed with a focus on biodiversity, water management, and ecological balance, aligning with BGI principles and demonstrating the enduring impact of environmental advocacy on urban green spaces.

Since the 1990s, the concept of “Green Infrastructure” gained traction, focusing on preserving and connecting natural landscapes to provide ecosystem services. Concurrently, the “Blue Infrastructure” concept emphasized the management of water bodies and waterways as integral parts of urban planning.

One of the earliest examples during this decade was Hammarby Sjöstad in Stockholm, Sweden. Hammarby Sjöstad is an eco-district that incorporates comprehensive BGI strategies. The area includes green roofs, sustainable drainage systems, and water purification features. These elements were informed by the environmental ideals of earlier decades, aiming to create a sustainable urban model that integrates natural processes into urban living [15].

In the 2000s, the projects multiplied. The Cheonggyecheon Stream restoration project in Seoul, South Korea, transformed a covered urban stream into a vibrant public space with restored natural water flow and adjacent green areas, significantly improving urban resilience and quality of life [16].

In Europe, cities like Copenhagen and Rotterdam began adopting BGI principles in response to severe flooding events. Copenhagen’s Cloudburst Management Plan, developed after a devastating flood in 2011, integrates BGI elements such as green roofs, permeable pavements, and water retention parks to manage excess rainwater. Similarly, Rotterdam’s Climate Proof program focuses on creating a network of water plazas and green roofs to address rising sea levels and urban heat stress [17].

These early initiatives demonstrated the practical benefits of BGIs, encouraging more cities globally to incorporate these strategies into their urban planning frameworks. By the 2010s, BGI typologies multiplied [18] and had become a key component of sustainable urban development policies, supported by international organizations like the European Union, which promoted BGI through its Green Infrastructure Strategy aimed at enhancing Europe’s natural capital and addressing environmental challenges. Also, different studies to evaluate their role in urban resilience have been performed, like the index of resilience in Spanish cities in 2016 [19]. However, ultimately, the pressure remains on local institutions [20].

Despite numerous studies assessing urban resilience and significant research evaluating the benefits of BGI [21,22,23,24,25,26], the extent to which these systems contribute to urban resilience remains unclear (

Table 1. Criteria for resilience index based on [19].

Dimension	Criteria	Description
Hydrological Resilience	Flood Risk Reduction	Measures flood frequency, water absorption capacity, and stormwater management.
	Water Resource Management	Evaluates sustainable water supply, reservoirs, and efficiency of drainage systems.
Climate Resilience	Urban Heat Island Mitigation	Assesses tree canopy coverage, temperature regulation, and green space availability.
	Air Quality Improvement	Tracks pollution levels and green infrastructure contributions to air purification.
Urban Planning and Infrastructure Resilience	Sustainable Mobility	Considers pedestrian-friendly spaces, bike lanes, and public transportation expansion.
	Land Use and Development	Evaluates zoning regulations, mixed-use development, and disaster-proof urban design.
Social and Institutional Resilience	Equitable Green Space Access	Measures distribution of parks and nature-based solutions across socio-economic groups.
	Governance and Community Engagement	Assesses city policies, multi-sector collaboration, and citizen participation in resilience initiatives.
Economic Resilience	Investment in Sustainable Development	Evaluates funding for resilience projects and economic benefits of green infrastructure.
	Employment and Innovation	Measures job creation in sustainability sectors and technological advancements in resilience planning.

Source: Own elaboration.

). The criteria for the resilience index can be structured based on key dimensions that assess a city’s ability to adapt, recover, and thrive in the face of environmental, social, and economic challenges.

This article seeks to bridge this gap by examining the example of BGI in Spain and its impact on urban resilience.

2. The Evolution of BGIs in Spain: From Water Management to Climate Change Issues

The Spanish legal framework supporting Blue–Green Infrastructures (BGIs) is shaped by a combination of national policies, European Union directives, and regional regulations aimed at promoting sustainable urban development and environmental protection. Urban design in many countries today is influenced by 19th- and 20th-century efforts to address water management issues [27,28]. During this era, rapid urbanization and the need to manage water-related problems, such as disease outbreaks and waste removal, led to the adoption of new urban planning strategies [29,30,31,32]. For example, in the late 19th century, England became a leader in sanitation infrastructure, which shifted the focus from environmental management to public health.

In Spain, modernization of water management was slow compared to other European nations due to economic, political, and social challenges [33,34]. By the late 19th century, Spanish cities faced significant water infrastructure deficiencies. For instance, Madrid struggled with over 3000 cesspits, neighborhoods without sewer traps, and many homes lacking direct water supply [35]. Similarly, Barcelona undertook sanitation projects between 1885 and 1893 to improve its sewer system but faced challenges due to limited residential connections [36]. Other cities like Seville [37], Zaragoza, Valencia [6,37], Burgos [3], or Malaga [5] also faced severe water infrastructure issues, which led to hazardous conditions during heavy rainfalls. These infrastructure deficiencies contributed to high mortality rates and underscored the need for urgent reforms in urban planning and water management in Spain [38].

Spain’s regulatory framework evolved over time to address the specific needs of each era. Initial laws focused on epidemic prevention and public health crises. The General Law of Public Welfare (1855) and the Health Law (1885) were early examples of coordinated responses to public health emergencies.

Flood management gained attention with the 1866 Water Law, which addressed urban overflow through the construction of canals and dikes. In the 20th century, the Public Works Act of 1907 and the 1926 Water Law further emphasized flood control, with the establishment of Hydrographic Confederations.

Post-Civil War, the 1941 Civil Protection Law set a benchmark for state-organized disaster response. The mid-20th century saw expansions to cover both natural and human-induced emergencies. The Civil Protection Act of 1985 coordinated emergency responses across state, regional, and local authorities, and Spain’s EU membership brought new disaster management standards, including the 2002 European Civil Protection Mechanism. The 2015 National Civil Protection System Law modernized Spain’s framework, focusing on prevention, planning, and climate risk management.

However, in the 21st century, Spain faces new challenges from climate-induced phenomena like heatwaves and torrential rains [39]. These challenges necessitate adaptive policies and integrated disaster management strategies to ensure urban resilience and environmental sustainability. The evolution of Spain’s legal framework, combined with EU directives, positions the nation to address these modern challenges through BGIs and sustainable urban planning.

The European Union has implemented several directives that provide a robust framework for environmental protection and sustainable urban development, aligning with BGI principles, listed as follows:

• Water Framework Directive (WFD): This directive establishes a comprehensive framework for protecting all water bodies. It encourages natural water retention measures, integral to BGIs, by promoting practices that enhance water infiltration, storage, and purification through natural landscapes. This supports sustainable water resource management and reduces reliance on gray infrastructure, enhancing urban resilience against flooding and drought [19].
• Floods Directive: Aimed at mitigating flood risks, this directive integrates flood risk management plans with natural water management solutions. It emphasizes restoring floodplains and creating green spaces that absorb excess water during heavy rainfall, enhancing the ecological value of urban areas and improving adaptability to climate change [40]. The Intergovernmental Panel on Climate Change (IPCC) highlighted in its Sixth Assessment Report (AR6) that extreme precipitation events will become more common in Europe.
• Habitats Directive and Birds Directive: These directives focus on conserving Europe’s natural habitats and protecting wild fauna and flora (Council of the European Union, 1992 and Council of European Union, 2009). They promote the integration of green infrastructure into urban planning to ensure biodiversity is not compromised by city development. By safeguarding natural habitats and encouraging ecological corridors, these directives support urban environments that coexist with rich biodiversity, a core BGI principle [40,41].

These EU directives are legally binding for all member states. Once adopted, directives must be transposed into national law, meaning each country is required to implement them through its own legal and regulatory frameworks. However, the degree of enforceability varies. For example, in the case of the Water Framework Directive, it requires member states to achieve “good status” for all surface waters. Countries must develop and implement river basin management plans, and failure to comply can lead to legal action from the European Commission, including potential fines [42,43].

In the case of the Floods Directive, it mandates member states to assess flood risks, develop flood risk management plans, and implement measures to mitigate flooding [44,45,46,47]. While it allows for flexibility in how countries meet these requirements, failure to comply can lead to enforcement actions, as in the case of Lithuania [48]. For the Habitats and Birds Directives, these directives require member states to designate and protect conservation areas (Natura 2000 sites). They impose legal obligations on governments and developers to ensure that projects do not negatively impact protected habitats or species [49].

While these directives establish a framework rather than prescribing specific actions, member states must comply. The incentives for following them include, for example, legal enforcement. Non-compliance can result in EU infringement procedures and fines. They also provide funding opportunities. Compliance with these directives often qualifies cities and governments for EU funding under programs like the LIFE Programme or Horizon Europe. Together, these directives create a legislative environment that fosters BGI adoption, guiding member states in building resilient, sustainable, and biodiverse urban landscapes, such is the case of Spain.

3. The Role of BGIs in Urban Resilience

Resilience in urban contexts refers to the capacity of cities to absorb, recover, and adapt to various shocks and stresses, including those related to climate change and socio-economic challenges. In that sense, Zhang and Li [25] emphasize the importance of distinguishing urban resilience from urban sustainability, while Elmqvist [26] highlights the need to identify problems in cities in addressing and mitigating future social and environmental challenges.

However, each challenge could be unique. For that reason, diversity is a fundamental principle of resilience theory, and the variety of approaches to urban resilience highlighted above reflects the flexibility and adaptability of this growing field [50]. Due to the concept of resilience being quite malleable, it is sometimes broadly equated with reducing vulnerability or enhancing adaptive capacity [38]. To ensure that the term resilience remains meaningful, it is crucial to continually evaluate how it has been and is applied in urban contexts. As resilience becomes integrated into climate-focused development efforts, there must be ongoing scrutiny by researchers, policymakers, and private stakeholders to ensure that strategies are the most adequate.

The role of Blue–Green Infrastructure (BGI) in urban resilience has become increasingly recognized as a vital strategy for addressing the challenges faced by cities in Spain, especially in the context of climate change and rapid urbanization [51]. As urban areas expand, they often rely on traditional gray infrastructure such as concrete drainage systems, roads, and buildings, which, although functional, are not always equipped to cope with the increasing frequency of climate-related shocks. BGI, which integrates natural and semi-natural systems like green spaces, wetlands, and urban forests, offers a more sustainable approach to urban development. Unlike traditional gray infrastructure, which often requires high maintenance and replacement costs, BGI leverages ecosystem services to enhance stormwater management, reduce flood risks, mitigate the urban heat island effect, and improve air quality. Additionally, it promotes biodiversity by creating habitats for wildlife and contributes to public well-being through access to green spaces that support mental and physical health. To operationalize sustainability in the context of resilience, BGI can be measured through indicators such as runoff reduction, infiltration rates, biodiversity indices, temperature regulation, economic efficiency, and social benefits [52]. In Spain, cities like Barcelona and Madrid are beginning to incorporate BGIs to enhance urban resilience—as other places in the world have [53]—providing solutions that not only address environmental issues but also offer economic and social benefits.

One of the primary ways that BGI contributes to urban resilience is through flood mitigation. Urban flooding is a growing concern, especially in Spanish cities that are often prone to heavy rainfall and sudden storms. By enhancing water retention and infiltration, BGI helps to reduce surface runoff, alleviating the pressure on traditional drainage systems that may otherwise overflow. Natural landscapes like parks, wetlands, and permeable surfaces allow rainwater to be absorbed, preventing flash floods that could damage infrastructure and disrupt daily life. This method of stormwater management is particularly beneficial in cities like Valencia, where urban sprawl has led to increased flood risks [54,55,56].

BGI also plays a crucial role in reducing the urban heat island effect, a phenomenon where cities experience significantly higher temperatures than surrounding rural areas due to human activities and the prevalence of gray infrastructure [56,57,58]. In Spain, where heatwaves are becoming more frequent and intense due to climate change, green spaces and water bodies offer a reprieve by providing shade and cooling through evapotranspiration. In cities like Seville, known for its high temperatures, BGI elements such as green roofs, tree-lined streets, and urban parks can reduce ambient temperatures, making urban areas more comfortable and resilient to extreme heat events. This is vital for the health and well-being of urban populations, especially vulnerable groups such as the elderly and children.

Finally, BGI offers multiple social and health benefits, which further strengthen urban resilience. Urban green spaces not only promote biodiversity by providing habitats for various species but also offer recreational opportunities that enhance physical and mental well-being [59,60]. Biodiversity strengthens resilience by maintaining ecosystem stability, adaptability, and the ability to recover from disturbances. Diverse ecosystems are better equipped to withstand environmental shocks, such as extreme weather events, as different species perform overlapping ecological functions, which help ensure the system’s continuity even if some species are affected [52,61]. In cities like Bilbao and Malaga, where parks and community gardens are increasingly being integrated into urban planning, BGI helps foster social cohesion by offering spaces for interaction and community-building. These spaces promote healthier lifestyles, reduce stress, and offer an opportunity for people to reconnect with nature, which in turn enhances the overall quality of life. The holistic benefits of BGI make it an indispensable tool for building resilience in urban Spain, ensuring that cities are better equipped to face future challenges.

However, another important aspect that must not be overlooked is that, while BGI can provide substantial social and environmental benefits, these benefits are not always distributed equally across different socio-economic groups within cities. In many cases, as seen, for example, during the COVID-19 pandemic [62,63], wealthier neighborhoods tend to have greater access to well-maintained green spaces, parks, and other nature-based amenities, while lower-income areas often remain underserved, lacking adequate infrastructure or facing environmental hazards such as poor air quality and flood risks. This disparity can exacerbate existing inequalities, as access to green spaces has been linked to improved mental and physical health, increased social cohesion, and overall well-being [64].

Furthermore, a major challenge for BGI projects is ensuring equitable access while mitigating unintended negative consequences, such as environmental gentrification [65]. As urban areas introduce new green spaces and improve existing infrastructure, property values may rise, leading to increased rent and living costs that can displace long-term residents, particularly those in vulnerable communities. This creates a paradox where the very people who could benefit most from improved urban environments may be forced to relocate due to economic pressures [66,67].

4. Examples of BGI Projects in Spain

Spain has been at the forefront of implementing BGI to enhance urban resilience from the 70s and preserve historical and natural monuments from climate change consequences, like floodings. Burgos, Valencia, and Bilbao could be consider the pioneers, followed by big cities like Seville with flooding problems from 19th century and the Guadalquivir River Green Corridor in 1992–1993 [68], Girona—Ter River Ecological Corridor during 2000s—ongoing [69], Murcia with the Segura Riverfront Restoration between 2001 and 2013 [70,71,72], Zaragoza with the Ebro River Corridor between 2005 and 2010 [73,74], Valladolid with the urban and ecological restoration of the Pisuerga Riverbanks between 2004—ongoing [75], or San Sebastian with the Flood Control and Urban Green Spaces Project between 2004 and 2020 [76]. Following the trend during the 2000s, the biggest Spanish cities, Madrid and Barcelona, were not an exception.

Additionally, the importance of maintaining natural and cultural spaces and boosting urban resilience has gradually become vital in Spain. According to existing categories [18] and indexes [19], Blue–Green Infrastructure (BGI) in Spain, in general, and particularly in cities like the proposed cases, can be categorized into river corridors, riverfronts, and sustainable drainage systems based on their design, function, and ecological impact.

1

Burgos, the pioneer in Spain

During the 19th century, Burgos faced significant flooding problems primarily due to the overflow of the Arlanzón River. The city’s geographical location and lack of adequate flood management infrastructure made it particularly vulnerable to heavy rainfall and snowmelt from surrounding mountains. These floods often caused severe damage to homes, bridges, and farmlands, disrupting daily life and economic activities. In addition, poor urban drainage systems and limited river regulation exacerbated the impact of these floods, leading to repeated cycles of destruction and reconstruction. Historical records indicate that several major flood events in the 1800s affected not only Burgos but also nearby rural areas, emphasizing the need for an improved hydraulic infrastructure, which was gradually developed in the late 19th and early 20th centuries. Burgos was one of the first cities that took early steps to address water problems, even when BGI did not exist [55,77]. The impetus for sanitation and urban reorganization was closely linked to the floods the city endured during the late 19th century. Burgos was a pioneer in these efforts, setting a model for other cities facing similar challenges.

To tackle these issues, the city undertook several restructuring projects, including the canalization of water in 1888 and the installation of street lighting. A civil honor was requested to recognize this work. However, the absence of a comprehensive drainage system for waste evacuation meant that sanitation and reorganization goals were not fully achieved. These goals became increasingly urgent, as it was anticipated that Burgos would soon reach a population of 40,000. Without proper urban reorganization and sanitation measures, the city risked continued flood damage and deteriorating monuments at the city center [3].

A detailed study of a drain and sewer network required a precise topographic map to analyze elevation levels, as these would influence the speed of water flow and its ability to transport solid debris. Additionally, since some drains already existed in parts of the city, another map was needed to integrate previous work into the new system, minimizing costs. On 28 July 1890, Aparicio Mendoza, a member of the Municipal Corporation and the Water Board’s executive committee, called for the creation of a general town map and a separate map of the existing drains as a foundation for planned improvements.

The project proposed, with a topographic map (Figure 1)—due to the lack of topographic knowledge in most cities in Spain—and a sewer (Figure 2) was presented in 1894 by civil engineers Mariano Martín Campos and Eduardo Lostau. It served as a pivotal example of water management in Spain. It addressed multiple challenges of the time that seem to be repeated nowadays, including flooding, street restructuring, and sanitation, offering a comprehensive solution to these interconnected urban issues. Right now, the maintenance works of this plan continue, with the improvement of the Arlanzón River. The city has focused on restoring riverbanks and maintaining green areas along the water, promoting both ecological conservation and urban cooling effects.

Burgos’ early efforts in water management, particularly in response to flooding, sanitation, and urban restructuring, align with modern concepts of urban resilience. According to the urban resilience index of Spanish cities [19], the resilience of Burgos can be assessed through key operational indicators across multiple dimensions and explained because of 19th century operations, yielding a theoretical result of three.

Hydrological resilience is reflected in flood risk reduction, measured by the frequency of flooding events and the percentage of water effectively absorbed by infrastructure, particularly through historical canalization efforts since 1888. Sanitation resilience is demonstrated by the efficiency of waste evacuation, evaluated through the presence and capacity of sewer networks, including those planned in the late 19th century.

Urban planning resilience is indicated by the expansion of city access and connectivity, reflected in the development of new routes and railway station access. Cultural heritage protection is assessed through measures to safeguard historic sites, including the restructuring of streets in the city center to prevent flood damage to monuments like the Cathedral. Finally, institutional resilience is reflected in municipal coordination, with decision-making processes involving engineers, planners, and officials, as seen in the collaboration between Aparicio Mendoza, Saturnino Fernández, and Ramón Aguinaga Arrachea in the 1890s. These indicators provide a structured way to evaluate how Burgos’ historical strategies contribute to urban resilience and inform modern Blue–Green Infrastructure (BGI) implementation.

2

Madrid: “Landscape of Light”

Madrid led, from the 2000s, one of the most significant urban resilient initiatives, whose benefits are clear nowadays. Madrid has invested in the preservation of its cultural landmarks, such as the Royal Palace and museums along the Paseo del Arte. The Prado Museum, Reina Sofía, and Thyssen-Bornemisza continue to attract millions of visitors annually.

But the most important changes are those related to green areas. The city implemented a low-emission zone called “Madrid Central” to reduce air pollution and promote sustainable mobility. This policy restricts access to older, more polluting vehicles in the central areas, encouraging cleaner transportation methods.

But it was not the only proposal in order to blend green areas with urban life. A regenerative initiative was also applied. Projects like expanding parks, rooftop gardens, and integrating blue–green infrastructure have gained momentum to combat the urban heat island effect and manage stormwater. For instance, Retiro Park remains a key focus for conservation and sustainable practices. However, it is not the most important green project. Another project, and the biggest one, was designed to reconnect the city with the Manzanares River [78,79]. In the past, it was an area of parks and empty lands, where industrialization took place (Figure 3).

The project, which began in the late 2000s, sought to address the long-standing separation between the city and its river, caused by the M-30 highway, which ran alongside and above the river for many years. This highway effectively isolated the riverbank from the urban fabric, creating a barrier to public access and hindering the potential for the riverfront to become a vibrant public space. The goal of the Madrid Río Project was to transform this underutilized area into a dynamic, accessible, and environmentally sustainable urban zone [79,80,81,82].

One of the project’s most notable features was the creation of an extensive green park, originating next to the Royal Palace and its gardens, that stretches along the river for several kilometers. This park includes new green spaces, pedestrian pathways, cycling lanes, and recreational areas that improve the quality of life for both residents and visitors. The project aimed to enhance the environment by introducing biodiversity and green infrastructure, including the planting of numerous trees and the establishment of sustainable drainage systems. The revitalization of the riverbanks was intended to serve as both a recreational hub and an environmental asset, offering new opportunities for outdoor activities, social gatherings, and relaxation within the urban context.

The removal of the M-30 highway from the surface level was a critical part of the project (Figure 4). The highway, which had previously disrupted the connection between the city and the river, was buried underground in a tunnel system, freeing up valuable space along the river for the new park. This operation required significant engineering efforts, including the construction of tunnels and the reconfiguration of surrounding infrastructure. The demolition of the highway was not only a technical challenge but also a symbol of the city’s commitment to reclaiming public space and promoting sustainable urban development.

The Madrid Río Project also focused on improving the city’s urban resilience by addressing environmental issues such as air pollution, urban heat islands, and water management. The creation of green spaces and the introduction of sustainable design elements, like permeable surfaces and water filtration systems, contribute to the mitigation of the city’s environmental challenges. Additionally, the project included the construction of pedestrian bridges that connect the two banks of the river, further enhancing mobility and encouraging active transportation, such as walking and cycling. By integrating these green and blue infrastructure elements, the project has significantly improved the city’s ecological health and the well-being of its residents (Figure 5).

Beyond its environmental and infrastructural achievements, Madrid’s initiatives had a profound social and economic impact [83,84,85]. The new green areas have become a beloved destination for locals, providing much-needed green space in an otherwise densely built city. It has fostered community interaction, wellness, and social cohesion by offering venues for outdoor activities, sports, and cultural events. Moreover, the regeneration of the area has attracted both tourism and investment, leading to the revitalization of the neighborhoods surrounding the river. Madrid is widely regarded as a model of urban regeneration, demonstrating how cities can creatively repurpose infrastructure to improve public spaces and promote sustainability while fostering economic growth and community well-being.

According to the urban resilience index of Spanish cities and its indicators [19], Madrid had a low resilience index (1). However, this does not mean that the city’s resilience has not improved over time [53]. Madrid’s resilience can be assessed through several key operational indicators that address its environmental, infrastructural, and governance challenges. Hydrological resilience is reflected in the city’s stormwater drainage capacity, permeable surface coverage, and the efficiency of its reservoirs, which help mitigate flooding and water scarcity. The Madrid Río project has played a crucial role in restoring the Manzanares river, integrating green spaces for better water absorption and reduced runoff. These changes have contributed to climate resilience, measured through urban tree canopy coverage, temperature reduction in green spaces, and the accessibility of parks, as seen in projects like the Madrid Río, which counteract the urban heat island effect.

Urban planning and infrastructure resilience is evident in the promotion of sustainable mobility, pedestrian-friendly spaces, and the expansion of public transport, exemplified by the Madrid Central low-emission zone and renovations of major streets like Gran Vía. Lastly, institutional and social resilience is reflected in multi-sectoral collaboration, government funding for sustainability projects, and citizen engagement in climate adaptation programs, particularly through Madrid’s Climate Change Adaptation Plan. Together, these indicators provide a comprehensive framework for evaluating and enhancing Madrid’s capacity to adapt to environmental and urban challenges.

3

The renewal of Barcelona

In the case of Barcelona, during the 2000s, various projects were carried out to renew the city and make it more resilient. Environmental degradation and pollution have affected Barcelona since the 1960s, primarily due to the rapid expansion of population and the industrialization of surrounding areas, which placed significant pressure on its ecosystem.

The Diagonal Mar Parc was the first example of how urban spaces can be transformed into multifunctional areas that serve both environmental and recreational purposes (Figure 6). Designed by the visionary architects Enric Miralles and Benedetta Tagliabue, the park integrates contemporary landscape architecture with sustainable practices. Situated in one of the city’s rapidly developing districts, the park plays a crucial role in enhancing the urban fabric by offering a green oasis amid the bustling cityscape [86].

A key feature of Parc de Diagonal Mar is its sophisticated water management system. The park incorporates an innovative network of channels, ponds, and irrigation mechanisms that collect and recycle rainwater. This system ensures that the park’s plant life remains hydrated without overburdening the city’s water supply. By utilizing collected rainwater for irrigation, the park promotes sustainability and demonstrates a practical approach to resource conservation in urban environments.

Beyond water conservation, the park is also designed to function as a flood mitigation zone. During periods of heavy rainfall, the park’s infrastructure effectively manages stormwater, preventing flooding in nearby areas. The landscape is engineered with a series of basins and wetlands that temporarily store excess water, gradually releasing it to minimize the risk of overflow. This dual role as both a public park and a flood management system underscores the park’s innovative design and its importance to the city’s infrastructure [87,88].

Parc de Diagonal Mar is not only a technical achievement but also a social hub. It offers a range of recreational facilities, including walking paths, playgrounds, and open spaces for various activities. These features make the park an inviting space for residents and visitors alike, fostering a sense of community and encouraging outdoor engagement. The park’s design emphasizes accessibility and inclusivity, making it a popular destination for people of all ages [89].

Following this intervention, new approaches were taken regarding the Besòs river area, the extension of the industrial area near Diagonal Park. The result was a key green space that spans the final 9 km of the Besòs River, stretching from its junction with the Ripoll River to where it meets the Mediterranean Sea [90,91].

The parks’ holistic approach to design and function set a benchmark for urban planning projects worldwide. By blending environmental sustainability with community-centric spaces—Barcelona and border areas such as Santa Coloma de Gramenet, Sant Adrià de Besòs, and Montcada i Reixac—Diagonal Mar Park and the Besos Park demonstrate how cities can innovate to meet the challenges of modern urban living. Its success illustrates the potential for green spaces to contribute to both ecological resilience and the quality of life for urban dwellers [92,93], positioning it as a model for future developments in sustainable urban design.

According to the index created in 2016 [19], Barcelona has one of the lowest indices (0). However, Barcelona’s resilience strategies, particularly thanks to these initiatives, are reflected in key environmental, social, and economic indicators. Environmentally, these initiatives enhance flood risk reduction, improve air quality, and promote biodiversity by integrating green corridors and natural water retention measures. In terms of climate adaptation, they contribute to temperature regulation through increased vegetation and facilitate sustainable water management with permeable surfaces and restored wetlands. Social resilience is strengthened by ensuring equitable access to green spaces, which improves public health and well-being by encouraging physical activity and reducing environmental inequalities [94]. Additionally, these projects support economic and urban resilience by integrating sustainable urban planning and attracting investment without excessive displacement. Together, these initiatives demonstrate Barcelona’s commitment to climate resilience, social equity, and sustainable urban development through nature-based solutions.

4

Turia Gardens (Valencia)

Valencia has been the center of news at national and international level for months after the natural disaster that took place at the end of October, when the floods resulted in not only economic loss but deaths. The Turia River has played a pivotal role in the historical development of Valencia. Originating in the province of Teruel, the river flows for 280 km before emptying into the Mediterranean Sea. For centuries, it provided essential water resources for agriculture and was crucial for trade and industry. However, the river was also known for its instability and frequent flooding.

Before the devastating flood of 1957, the Turia River had already caused several major floods, including those in 1517 and 1776. These floods, which occurred in cycles, wreaked havoc on crops, buildings, and livelihoods [38]. Both events underscored the city’s vulnerability, particularly as its proximity to the river became an increasing challenge amidst urban expansion. The city’s growth is evident in various historical maps and views, including 16th-century depictions and 17th-, 18th-, and 19th-century maps. In the 19th-century map, the historical center is clearly marked, with visible city walls—the Muslim wall and the larger medieval wall constructed under Pedro the Ceremonious between 1356 and 1370. By this time, the city center had already become overcrowded, with buildings extending beyond the walls [95].

The 1957 flood marked a key point, far surpassing previous flood magnitudes and making it clear that the natural course of the Turia River was no longer sustainable for an expanding city like Valencia. On 14 October 1957, Valencia was hit by torrential rain resulting from an extreme meteorological phenomenon. Known as gota fría, this intense Mediterranean rain event caused an accumulation of up to 300 L of water per square meter within hours. The Turia could not contain such a volume, and in just a few hours, the water breached the dikes and channels, inundating the city.

The Plan Sur was developed as a direct response to the 1957 flood disaster and to avoid possible future damage to the historical city center. Valencia’s mayor at the time, Tomás Trénor Azcárraga, Marqués del Turia, worked tirelessly from late 1957 to devise solutions for the city’s recovery. This effort followed the adoption decree granted to Valencia by Francisco Franco after the flood, which was issued by the Ministry of Housing on 23 December 1957. However, the plan initially faced challenges due to a lack of funding [10,96,97,98,99,100,101,102,103,104].

The Plan Sur was created to protect the city from future floods by constructing a new riverbed that diverted the Turia River’s waters southward, away from the urban areas. This measure has been effective for decades, as Valencia’s city center has not faced severe flooding from the Turia since its implementation. However, the heavy rains of 2024 tested this infrastructure, revealing ongoing risks in areas adjacent to the diverted riverbed. The construction of the new Turia channel required extensive engineering, including the excavation of large channels and the building of massive dikes and bridges. A 12 km long, 175 m wide canal was dug to redirect the river’s waters while safeguarding the urban core.

One of the main challenges was designing a channel that could withstand floods equal to or more intense than the catastrophic flood of 1957. To address this, the project incorporated expansion zones along with channels and gates to regulate water flow effectively. The work was completed in the 1970s, standing as a testament to Valencia’s resilience through innovation and collective effort.

One of the most significant and distinctive aspects of this intervention was the abandonment of the Turia River’s original riverbed. Following the river’s diversion, the old channel was emptied, leading to numerous proposals for repurposing the space. Initially, the government suggested constructing an urban highway to connect northern and southern Valencia. However, this plan was met with strong opposition from residents and environmental groups, who argued that the old riverbed should be transformed into a green space for the people of Valencia to enjoy [105,106].

After extensive debate and negotiation, the decision was made to create an urban park in the former riverbed. This outcome was seen as a triumph for the public and represented a shift in how urban spaces were managed. The result was the creation of the Turia Gardens, a project that began to take shape in the 1980s (Figure 7). Today, the park spans over 110 hectares, making it one of the largest urban parks in Spain and a prime example of urban revitalization and environmental management. Notable features of the park include Gulliver Park, the City of Arts and Sciences, and the Palau de la Música, each highlighting Valencia’s cultural and architectural heritage [3]. Together with the historical center, the city is one of the most remarkable treasures of Spain.

However, recent urbanization in the southern areas of the channel has altered flood impacts. The heavy rains of 2024 have shown that, while Valencia’s city center remains protected, the southern region is still vulnerable, affecting neighborhoods and municipalities that have grown in these areas (Figure 8). But the future is uncertain. As we present here, this is not new, but these episodes have become increasingly frequent and severe in the Mediterranean, exacerbated by climate change. This event has demonstrated that the previous land management and planning were adequate at that moment but due to the expansion of the city, other solutions need to be implemented to prevent future events from resulting in the same consequences [107].

Experts are now evaluating whether the previous reconfiguration of the river remains effective, regarding also the index of resilience (1) [19]. They question whether its current design should be adapted to better retain and manage rainwater, especially in light of the more frequent intense rainfall driven by climate change in the Mediterranean region. This includes assessing whether the changes implemented in the past are sufficient or if new strategies should be developed to enhance the park’s Blue–Green Infrastructure (BGI). Valencia’s Turia Gardens, of the first examples of repurposing an old riverbed into a green corridor, illustrates the transformative potential of BGI. Expanding the scope of such infrastructure could significantly enhance Valencia’s flood resilience and urban sustainability.

5

Bilbao Green Belt (Bilbao)

Bilbao’s industrial past is deeply tied to the evolution of the Ría de Bilbao, the estuary that shaped the city’s economic and urban development. From the late 19th century through most of the 20th century, Bilbao became a major industrial hub, driven by shipbuilding, steel production, and heavy manufacturing. The estuary served as the city’s lifeline, facilitating trade and transport while also suffering severe environmental degradation due to unchecked pollution from factories and urban expansion. By the late 20th century, as industry declined and economic priorities shifted, Bilbao embarked on an ambitious transformation, focusing on environmental restoration, urban regeneration, and the revitalization of the Ría.

The Bilbao Green Belt is an ambitious urban planning initiative designed to encircle the city of Bilbao with a cohesive network of green spaces and ecological corridors. This project represents a strategic effort to enhance the city’s environmental sustainability and improve the quality of life for its residents. By linking various natural areas around Bilbao, the Green Belt aims to restore degraded habitats, promote biodiversity, and create a more resilient urban ecosystem (Figure 9).

A central objective of the Bilbao Green Belt is the restoration of natural habitats that have been disrupted by urban development [77]. Through reforestation and the rehabilitation of wetlands, the project seeks to recreate the natural landscapes that once flourished in the region. This not only supports local wildlife but also contributes to the city’s broader environmental health. The creation of ecological corridors allows for the free movement of species, thereby strengthening the region’s biodiversity and ecological balance.

Improving air quality is another significant focus of the Green Belt project. The expansion of green spaces helps to filter air pollutants, reducing the impact of industrial emissions and urban traffic. Trees and vegetation act as natural air purifiers, absorbing carbon dioxide and releasing oxygen, which can lead to a noticeable improvement in the city’s air quality. This aspect of the project is particularly beneficial for the health and well-being of Bilbao’s residents, offering a cleaner and more pleasant urban environment [19].

In addition to its environmental benefits, the Green Belt plays a critical role in enhancing Bilbao’s capacity to manage stormwater and mitigate flood risks. The green spaces and restored wetlands function as natural water retention areas, reducing the likelihood of flooding during heavy rains. This natural infrastructure complements the city’s existing stormwater management systems, providing an additional layer of protection against extreme weather events, which are becoming increasingly common due to climate change [107].

Finally, the Bilbao Green Belt significantly enhances the city’s recreational offerings by providing residents with accessible green areas for leisure and outdoor activities. Parks, trails, and open spaces within the Green Belt offer opportunities for walking, cycling, and relaxation, promoting healthier lifestyles and fostering a stronger connection between the community and nature. This integration of environmental and social benefits underscores the Green Belt’s role as a model for sustainable urban development, demonstrating how cities can harmoniously blend natural restoration with urban living [108,109].

Regarding the resilience index of Spanish cities [19]. Bilbao would have the same index as Barcelona (0). However, the Green Belt Corridor is known as a vital initiative aimed at enhancing urban resilience through ecological restoration and sustainable land use, and which is continuously evolving [110]. By strengthening ecological connectivity, it links green spaces, forests, and water bodies, promoting biodiversity and habitat continuity. The project also mitigates flood risks by restoring riverbanks and wetlands, improving stormwater management.

Additionally, increased vegetation helps improve air quality and counteracts the urban heat island effect, supporting climate adaptation efforts. The repurposing of former industrial and degraded lands fosters sustainable urban development, while the creation of new habitats aids in biodiversity conservation. Furthermore, the project enhances community well-being by providing access to natural spaces, encouraging outdoor activities, and supporting mental and physical health. Through citizen engagement, it promotes environmental awareness and strengthens social cohesion, demonstrating a comprehensive approach to resilience that integrates ecological, social, and climate adaptation strategies into Bilbao’s urban landscape.

5. Conclusions

BGIs offer a promising pathway for cities to become more resilient, sustainable, and livable. Spain’s successful examples demonstrate the potential of BGIs to transform urban landscapes, enhance ecosystem services, and improve the quality of life for residents. As cities continue to grapple with climate change and urbanization pressures, the adoption of BGIs will be crucial in building resilient and adaptive urban environments. Policymakers, urban planners, and communities must collaborate to harness the full potential of BGIs, ensuring a sustainable future for urban areas.

The implementation of Blue–Green Infrastructure (BGI) in cities like Barcelona, Madrid, Bilbao, and Valencia has shown significant progress in addressing urban sustainability challenges, improving resilience, and enhancing quality of life for residents. While each city has its unique context and approach, the overall trend in these urban centers reflects the growing recognition of the importance of integrating natural systems into urban planning.

Overall, the experiences of these four cities demonstrate that BGI is a helpful component of modern urban planning. It addresses a wide range of environmental, social, and economic challenges, fostering resilience, improving quality of life, and enhancing urban sustainability. While each city has tailored its BGI strategies to local conditions, the shared successes underline the importance of nature-based solutions in the future of urban development.

However, one of the main limitations of BGI projects is the high initial investment required for their planning, design, and implementation. While BGI can provide long-term environmental and social benefits, cities may face challenges in securing the necessary funding for such large-scale projects. In some cases, public budgets and financial constraints can hinder the development of comprehensive BGI solutions, particularly in the context of competing urban priorities. To address these issues, BGI planning and implementation should incorporate inclusive and participatory approaches, ensuring that communities most in need are prioritized in green infrastructure investments. Policies such as affordable housing protections, community-led urban planning, and equitable funding mechanisms can help prevent displacement while maximizing the social and environmental benefits of BGI for all urban residents.

Also, BGI systems, such as parks, green roofs, and urban forests, require regular maintenance to remain effective. Without proper care and management, the ecological benefits of these green spaces can diminish over time. In some cities, lack of resources for the long-term upkeep of these infrastructure elements can lead to degradation, reduced biodiversity, and failure to meet their intended climate adaptation goals. The sustainability of BGI depends on ongoing investment in maintenance, which is not always guaranteed.

Other aspect that we cannot forget is that, although BGI can provide significant social and environmental benefits, the distribution of these benefits across different socio-economic groups within cities can be uneven. In some cases, wealthier neighborhoods may have better access to well-maintained green spaces, while low-income areas may be underserved or lack adequate infrastructure. Ensuring equitable access to green spaces and mitigating the risk of gentrification is a key challenge for BGI projects, as certain neighborhoods may experience displacement or increased costs as a result of these improvements.

Moreover, the effectiveness of BGI in addressing climate-related challenges, such as extreme heat and flooding, is sometimes constrained by the unpredictable nature of climate change as we have seen in Valencia in October, 2024. In some cases, extreme weather events, such as heavy storms or prolonged droughts, may exceed the capacity of BGI solutions to provide relief. While BGI is a valuable tool for climate adaptation, it should be part of a broader, more comprehensive strategy that includes other climate resilience measures, such as gray infrastructure and policy changes, to ensure cities are fully prepared for future challenges.

Finally, it seems that the resilience index of Spanish cities was made at a moment of first steps in a lot of Spanish cities. Revising the resilience index annually is essential to ensure that cities and regions effectively adapt to evolving environmental, social, and economic challenges. Climate change, urban development, and socio-economic shifts continuously alter risk factors, making periodic assessment crucial for maintaining preparedness and response capacity. Regular updates allow for the integration of new data, emerging risks, and the effectiveness of previously implemented resilience measures. Additionally, annual revisions help policymakers and urban planners identify gaps in adaptation strategies, improve resource allocation, and ensure that resilience-building efforts remain aligned with current needs. By refining indicators each year, cities can proactively enhance their ability to withstand and recover from shocks such as extreme weather events, economic crises, or infrastructure failures, fostering long-term sustainability and well-being.

In summary, while BGI offers numerous environmental, social, and economic benefits in big cities like Barcelona, Madrid, Bilbao, and Valencia or medium-size cities such as Burgos, it is not without its limitations. Addressing these challenges requires careful planning, adequate funding, long-term maintenance, and a focus on social equity to ensure that the full potential of BGI can be realized in urban areas.