Urban flood risk management needs nature-based solutions: a coupled social-ecological system perspective

Urban flooding is a significant and growing global problem exacerbated by climate change and increasing urbanization. By 2050, the frequency of 100-year flood events is projected to double in 40% of the world. In urban areas, floods cause substantial damage, economic losses, and disruption to both human society and natural ecosystems. Current urban flood risk management (FRM) strategies often focus on improving infrastructure to accommodate floodwaters and mitigate exposure, but these approaches can overlook the potential of natural ecosystems. Green and blue spaces offer ecological measures for FRM, but these measures, if implemented without considering social and ecological contexts, can have unintended negative consequences. For example, rain gardens might disrupt animal habitats, and restoration projects might displace vulnerable communities. Nature-based solutions (NbS) aim to harness the natural services of ecosystems to mitigate flood hazards while simultaneously enhancing biodiversity and climate adaptation. However, the extent to which NbS achieves multiple benefits for both society and nature remains unclear. This study utilizes a coupled social-ecological systems (SES) perspective to explore the win-win potential of NbS for urban FRM, aiming to address the critical need for equitable and sustainable flood management strategies that enhance both human well-being and ecosystem health.

The authors conducted a scoping review of literature published between 2000 and 2023 on ecological measures in urban FRM. The review classified ecological measures into three categories: restoration measures, engineered measures, and hybrid measures. Flood risk aspects addressed were classified based on hazard, exposure, and vulnerability components. The search strategy initially yielded 2758 studies, with 1271 ultimately included after a screening process following the PRISMA protocol. The review revealed an uneven global distribution of studies, with a concentration in developed countries. It also highlighted the unintended social and ecological impacts of FRM measures focused solely on flood mitigation. The review further analyzed emerging research on NbS in urban FRM, published predominantly after 2018, emphasizing the integration of social and ecological perspectives during design, implementation, and governance stages to achieve multiple benefits.

The study employed a two-pronged methodological approach. First, it conducted a comprehensive scoping review of existing literature on ecological measures for urban flood risk management (FRM). This review used a structured search strategy across multiple databases, focusing on articles published between 2000 and 2023, employing keywords related to various types of ecological measures (restoration, engineered, hybrid), flood risk aspects, and NbS. The review followed the PRISMA protocol for systematic reviews, involving multiple stages of screening and selection based on titles, abstracts, and full texts. Data extracted from the selected studies included geographical location, type of FRM measures, unintended social and ecological effects, and information on NbS-related research published after 2018. Second, the study developed a conceptual framework based on a coupled social-ecological system (SES) perspective. This framework extended Ostrom’s (2009) general SES framework to the context of urban FRM, dividing the NbS-FRM into four sub-systems: NbS Decision-making and Rules, Citizens and Stakeholders, NbS-related Ecosystem, and NbS Hydrological Performance. The interaction among these sub-systems was analyzed through three dimensions: coupling social and ecological factors (D1), linking human activities with hydrological responses (D2), and balancing potential trade-off effects (D3). The framework was used to structure the findings from the scoping review and identify key knowledge gaps in achieving social and ecological co-benefits from NbS-FRM.

The scoping review revealed an uneven geographical distribution of studies on ecological FRM measures, with a disproportionate focus on North America and Europe. While many countries in the Global South face significant flood risks, fewer studies address ecological solutions there. The review further showed that ecological measures solely focused on flood mitigation can have negative social and ecological consequences. The study then showed that integrating social and ecological perspectives throughout the NbS lifecycle (design/planning, implementation, governance) is crucial for successful implementation and achieving both flood risk reduction and broader social and ecological benefits. The findings highlighted several key aspects: * **Dimension 1 (Coupling Social and Ecological Factors):** Biophysical factors (soil properties, plant traits, climate) influence NbS hydrological performance, and social factors (risk awareness, investment schemes, stakeholder knowledge) significantly affect social adaptation. However, coupled social-ecological studies are scarce and often lack consideration of NbS resilience to flood hazards. * **Dimension 2 (Linking Social Processes and Hydrological Responses):** Land-use change and urban development strongly influence hydrological responses to NbS. Collaborative implementation and knowledge co-production significantly enhance NbS effectiveness. Resident engagement in practices like rain gardens can supplement large-scale NbS projects, but few studies investigate NbS responses to flooding across various hydrological processes. * **Dimension 3 (Balancing Trade-offs):** Performance evaluations often focus on hydrological benefits and costs without considering social equity or ecological impacts. Unintended social inequities and biodiversity loss can occur. Long-term governance and transboundary collaboration are critical for managing trade-offs. Successful projects emphasized ecosystem connectivity and transboundary governance, synergizing flood mitigation, habitat quality, and human well-being.

The study's findings highlight a crucial gap in the current understanding and application of NbS for urban FRM. While the potential for co-benefits is evident, current practices often fall short due to a lack of integrated social-ecological perspectives. The uneven global distribution of research underscores the need for more attention to vulnerable regions. The conceptual framework provides a valuable tool for researchers and policymakers to better integrate social and ecological factors into decision-making processes. By incorporating resilience thinking, dynamic modelling of social-ecological interactions, and collaborative governance approaches, NbS can be effectively designed, implemented, and managed to achieve both flood risk reduction and wider social and environmental benefits. The study's emphasis on long-term monitoring and adaptive management recognizes the inherent uncertainties and complexities of urban systems.

This study underscores the significant potential of NbS for achieving equitable and sustainable urban flood risk management by providing both social and ecological co-benefits. The research highlights critical gaps in existing approaches, including the need for greater focus on resilience, more robust methods for capturing dynamic social-ecological interactions, and enhanced collaborative governance mechanisms. The proposed conceptual framework offers a valuable tool for future research and practical implementation, moving towards a more holistic and effective approach to urban flood management. Future research should focus on developing and testing specific methods for assessing and enhancing the resilience of NbS and on further refining tools for collaborative management and long-term monitoring.

The study's scoping review, while comprehensive, is limited by the available literature. The uneven geographical distribution of studies may bias the findings. The conceptual framework, while offering a valuable structure, requires further empirical testing and refinement. The study predominantly focuses on the potential of NbS; actual implementation challenges and success rates require further investigation. The economic aspects of NbS implementation, beyond simple cost-effectiveness analyses, also require further research.