This section discusses the trend of urbanization and declining natural LULC of a few major Indian cities and their suburbs, emphasizing the need for resilient cities in India. The increase in built-up area in the National Capital Territory (NCT) of Delhi was 162.7 sq. km to 531.2 sq. km from 1993 to 2018 (Bondwal and Bisht, 2019). The same study found that the forest cover in the NCT of Delhi decreased from 155.8 sq. km to 130 sq. km between 1993 and 2018. Land-use transitions are not only limited to the cities but also greatly affect suburban regions (Naikoo et al., 2020). Mumbai city experienced a significant decline in natural land use and land cover (LULC) from 1977 to 2017, with a 60 percent reduction in vegetation and a 65 percent reduction in waterbodies (Udas-Mankikar and Driver, 2021). Another study reported that the built-up area of Mumbai rose from 28 to 57% of the city’s total area from 1991 to 2018, and it is projected to reach 66% by 2030 (Naikoo et al., 2023). Similarly, the Chennai Metropolitan Area (CMA) witnessed an increase in built-up areas from 18 to 48% and a decrease in vegetation from 57 to 26% between 1988 and 2017 (Mathan and Krishnaveni, 2020). Developments have taken over 90 per cent of the wetlands of Chennai city (Ahmad and Hassan, 2024). Bengaluru city experienced a significant decrease in the green cover of the city, more than 50% from 2003 to 2021, while the built-up area almost doubled during the same period (Keerthi-Naidu and Chundeli, 2023).

The consequences of the rampant land use transition in urban and suburban landscapes in India can be seen as increased urban pressures, i.e., flood risk, water stress, water pollution, urban heat island (UHI) and air pollution. The natural drainage systems in most cities are facing threats from encroachment, inadequate maintenance, poor solid waste management, and lack of adequately designed stormwater drainage infrastructure. For example, Chennai city suffered the most disastrous flood of the century in 2015, causing more than 400 human casualties, nearly 2 million severely affected and about USD 80 billion of estimated loss (Vojinovic, 2015). On the contrary, four years later, in June 2019, the city was unexpectedly hit by ‘Day Zero,’ and all of its major reservoirs dried up. The city has a minimum of 108 Litre per capita per day (LPCD) of water supply, much less than the WHO minimum criteria of 150 LPCD (Rajaveni et al., 2016). The study confirms the presence of UHI in Chennai with an intensity of 4.5°C in winter and 2.5°C in summer (Rajan and Amirtham, 2021).

The annual economic impact of urban flooding in India is disastrous, ranging from USD 1.1 billion to USD 5 billion (Sharief and Vangipuram, 2022). The highest UHI intensity recorded internationally is as high as 12°C, while the observed maximum UHI intensity in India is 8–9°C. UHI can deteriorate the urban environment in multiple ways – increase in energy and water consumption, higher emissions of pollutants into the atmosphere resulting in the greenhouse effect, heat-related health discomfort, and degradation of water quality in streams, rivers, and other water bodies (Jain and Sarkar, 2017; Veena et al., 2020; Vujovic et al., 2021). Every city and its suburbs face the abovementioned urban pressures with varying intensities depending on the type of city (like coastal city, riverine city, or mountainous city). The lack of effective streamlining, regulation, and monitoring of urbanization processes is a key factor contributing to significant environmental degradation. Taking account of climate change, environmental risks, and socio-economic vulnerability, there is an urgent need for a paradigm shift in urban developments in India and the adaptation of nature-based solutions for the development of resilient cities. NBS strategies optimize the climate-related risks with other objectives to achieve multiple benefits regarding ecological, socio-economic and overall urban well-being (Roumeau et al., 2015; Vojinovic, 2015). The concept of nature-based solutions and the associated ecosystem services have been discussed in detail in the next section.

The World Bank introduced the NBS concept in the late 2000s to address increasing climate-related risks, promoting ecosystem-based approaches (World Bank, 2008). The International Union for Conservation of Nature (IUCN) has been taking the lead in conserving the ecosystem and promoting nature-based solutions (NBS) globally by formulating core principles and frameworks for mainstreaming NBS. Figure 1 highlights the timeline of major milestones in developing the NBS concept. In recent years, the definition and scope of nature-based solutions have become vast and diverse. The diversity of concepts and definitions has led to challenges in achieving conceptual clarity, making the term more subjective (European Commission, 2021; Cohen-Shacham et al., 2016; Albert et al., 2017; Dhyani et al., 2020). Table 1 enlists the two most widely accepted definitions of NBS. Fundamentally, NBS is a novel approach that primarily focuses on using ecosystems to address climatic, environmental, and socio-economic challenges (Balian et al., 2014; Cohen-Shacham et al., 2016; Depietri and McPhearson, 2017; Dorst et al., 2019). Figure 2 depicts a distinct range of ecosystem and natural capital-based approaches under the NBS concept (Dhyani et al., 2020). Ecosystems demonstrate a remarkable ability to mitigate the adverse effects of climate-related risks and safeguard communities (Diaz et al., 2015; Lo, 2016).

NBS encompass the use of natural processes and ecosystems to create infrastructure, provide services, and develop comprehensive strategies to enhance urban resilience. World Bank identifies some NBS typologies like urban forests, urban farming, green corridors, river and stream renaturation, river floodplains, bioretention areas, and wetlands (World Bank, 2021). These approaches typically transcend traditional boundaries and necessitate collaborative efforts across various sectors. Nature-based solutions offer diverse advantages for cities, such as mitigating disaster risks, strengthening climate resilience, ensuring food-water security, promoting biodiversity restoration, and overall community well-being. Numerous terminologies under nature-based concepts have been developed in different parts of the world, such as ‘green infrastructure’ (GI), ‘blue-green infrastructure’ (BGI), ‘natural infrastructure’ (NI), ‘low impact development’ (LID), and ‘ecosystem-based adaptation’ (EBA). The terms NBS, GI, BGI, LID, EBA and sustainable measures are interchangeably used in the present study.

There is a hierarchy of approaches for implementing the NBS umbrella concept as strategic planning, i.e., ‘protection and sustainable management of existing natural infrastructure’, ‘restoration and rehabilitation of degraded one’ and then ‘creation of new NBS’ (Cohen-Shacham et al., 2019). It is essential to consider this hierarchy when identifying and prioritizing nature-based solutions opportunities at a strategic level, such as when evaluating investment options for a city. These three approaches must be applied to plan and prepare NBS projects across various spatial scales. Generally, NBS is implemented at three spatial scales – neighborhood scale, city scale and river basin scale. At each scale, various NBS families can be implemented. For instance, floodplain restoration projects that restore natural hydraulic and hydrological connectivity can effectively mitigate flood hazards at the river basin level, while green roofs and bioswales can be strategically designed for implementation at the neighborhood scale (Liberalesso et al., 2020; Puchol-Salort et al., 2021). Measures (like urban forests, constructed wetlands and rivers and streams renaturation) at the city scale aim to enhance urban land use planning and strengthen the city’s disaster risk management. Figure 3 depicts the schematic section of NBS at the neighborhood, city, and river basin scales.

The study presents a constructive exploration of NBS and its associated ecosystem services, featuring a range of successful regional implementations, as shown in Table 2. The reviewed literature included technical reports, project summaries, academic publications, government publications, conference proceedings, and resources from web search engines and academic databases (‘Web of Sciences’ and ‘Scopus’) for regional and city-scale NBS interventions. Search keywords include various NBS measures – green corridors, green roofs, urban forests and parks, urban agriculture, bioswales, rain gardens, retention ponds, permeable pavements, natural wetlands, constructed wetlands, stream renaturation and floodplain restoration. Further, five long-term city-scale NBS projects were selected to gain a worldwide perspective and foster the development of resilient cities in India (as mentioned in section 3.5). The number of NBS measures adopted simultaneously and the wide range of ecosystem services were used as selection criteria for the city-scale NBS exemplars examined in the study. It is worth mentioning that while no formal surveys were conducted with city officials, the findings still offer valuable insights for future discussions and research in the context of the development of resilient cities in India.

The natural ecosystem can deliver multiple environmental and socio-economic benefits, which are called ecosystem services. Figure 4 depicts the pertinent processes related to ecosystem services for urban resilience. A few major ecosystem services related to NBS are briefly discussed below, along with the role of NBS in achieving SDGs.

In recent decades, there has been a growing trend toward using nature-based solutions for sustainable and resilient cities. Countries like the United States, the United Kingdom, the Netherlands, Germany, and many more have seen notable success in managing urban pressures through nature-based practices. The European countries have tremendously succeeded in developing the NBS strategies for urban resilience. In Asia, countries like China, Malaysia, Singapore, Japan and Thailand have also been working on the NBS implementation for various urban ecosystem services for a long time. As research into nature-based intervention expands, many such projects have been launched and completed successfully in cities worldwide. Therefore, five NBS projects from different countries (USA, Canada, The Netherlands, China, and Australia) have been selected to understand a worldwide perspective. All the selected projects have been critically analyzed, and their major objectives and the key lessons learnt have been presented in Table 4. The key learnings from these city-scale NBS projects include the effective combination of green and grey infrastructure to optimize urban water management and enhance urban resilience, implementing adaptive management practices, community engagement and awareness, and a strong emphasis on a multi-benefit approach.

As the demand for resilient cities grows and NBS intervention plans are being developed, several challenges have emerged in implementing NBS in urban areas. These challenges span technical, social, and institutional factors. Indian cities face unique urban pressures due to their diverse socio-economic culture, demography, and climate. Despite the vital role of blue and green spaces for the environment and community well-being, these natural spaces are decreasing, and impermeable surfaces are rising in the cities. Analyzing the factors responsible for increasing urban pressures in Indian cities, some of the major challenges in NBS intervention that need to be addressed are as follows.

Various studies have been conducted worldwide to develop strategies to address the challenges (social, institutional and technical) associated with NBS interventions. Financial constraints are recognized as an institutional challenge. Studies highlight that social and institutional barriers tend to outweigh technical barriers (O’Donnell et al., 2017; Thorne et al., 2018; O’Donnell et al., 2021). Case studies of numerous cities emphasize the importance of a comprehensive approach to overcome these challenges. It is crucial to raise awareness, secure diverse funding sources, integrate green infrastructures into new developments, and increase overall funding for nature-based projects within cities (Iojă et al., 2018; O’Donnell et al., 2021). Drawing insights from multiple studies and frameworks addressing the challenges in NBS intervention in urban areas worldwide (O’Donnell et al., 2017; Melville-Shreeve et al., 2018; Amaral et al., 2021; Toxopeus and Polzin, 2021; Suleiman, 2021; Castelo et al., 2023), six key steps have been identified for formulating strategies to tackle the implementation challenges in India. These steps include – (a) amending legislation and developing the policies to establish guidelines for nature-based measures with specific standards, (b) highlighting and promoting the numerous co-benefits of NBS-integrated multifunctional spaces, (c) encouraging collaborative efforts from research and planning to execution (d) increasing awareness through education, community events, and activities, (e) securing sustainable funding by involving the private sector (f) promoting the advanced scientific research on retrofitting NBS measures in existing urban settings and creating new ones.

Researchers also presented the strategies at various stakeholder levels to conceptualize and streamline the framework for NBS intervention (Qiao et al., 2018; Landscape Institute and the Construction Industry Council (LI and CIC), 2019; O’Donnell et al., 2020). Similarly, the stakeholders can be selected to institutionalize the NBS in the urban landscapes in India. It will define a framework at each stakeholder level to overcome these challenges. These stakeholders include the central government, state governments, academia, practitioners, and individuals. The central government should acknowledge the benefits of NBS and promote its nationwide adoption by revising legislation, establishing technical standards, empowering states to create regional policies, and facilitating collaborative research. State governments should promote the benefits of blue-green infrastructures at the local level, invest in outreach programs to educate communities, develop policies for adoption and maintenance, and secure funding through private-sector collaboration.

Taking on financial constraints and investing in blue-green infrastructure necessitates a proactive approach, including conducting a thorough cost–benefit analysis. For example, developing a robust framework to assess the socioeconomic impact of a flood event can provide valuable insights for comparing the economic benefits of blue-green infrastructure to the local community and the investors. The role of academia and practitioners in the development of strategies to overcome technical and financial issues is inevitable. The academia should enhance scientific understanding of the nature-based approaches through rigorous collaborative research with international organizations to fill the knowledge gaps and provide evidence regarding the hydrological and ecological performance of NBS. Researchers can also advocate for policy changes that support the implementation of NBS by publishing the research outcomes. With the collaboration of academia, practitioners need to develop low-maintenance blue-green infrastructure measures, assess their suitability for regional environments, create open-source toolkits to assess and monetize the ecosystem services and raise public awareness about the co-benefits. Assessing the ecosystem services and further monetizing them will also draw the attention of the private sector to invest in nature-based projects for urban sustainability and resilience.

Citizen engagement can facilitate cities through the problem-based model to identify the local challenges, connect people with nature and increase the sense of ownership of NBS-intervened places to overcome the challenges. Communities working with natural processes and systems can facilitate better adaptability of nature-based projects (Brown and Mijic, 2019). Individuals should take part in local stewardship efforts for nature conservation, actively engage in the ongoing NBS projects, provide valuable support, and strive for sustainable development. Encouraging behavior change among fellow citizens is also important, as it raises awareness about the multifaceted NBS-integrated urban spaces.