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

Flooding is among the most severe climate-related disasters worldwide, with 100-year flood events projected to occur at least twice as frequently as today across 40% of the globe by 2050. In urban areas, extreme flooding affects both people and ecosystems, increasing damage and welfare losses while disrupting hydrological processes and biotic communities. Urban flood risk comprises hazard, exposure, and vulnerability; FRM aims to enhance infrastructure capacity to handle excess water, reduce exposure, and increase adaptation of vulnerable groups. Green and blue spaces are widely used as ecological FRM measures, but measures focusing solely on mitigation may create unintended ecosystem and social consequences (e.g., habitat disturbance from rain garden siting; displacement from poorly designed restoration projects). NbS can harness ecosystem services to mitigate hazards and enhance biodiversity and climate adaptation, offering potential to address linked climate, societal, and biodiversity challenges. Cities are complex social-ecological systems (SES), and coupled SES frameworks can help center human well-being and biodiversity and enhance resilience. Yet, NbS for urban flood regulation rarely achieve dual social and ecological co-benefits due to insufficient accounting for socio-ecological interactions, limited measurable biodiversity outcomes, and challenges in spatial-temporal upscaling and cost-effectiveness. Building on Ostrom’s SES framework, the authors adapt it for NbS-FRM with four sub-systems (NbS Decision-making and Rules, Citizens and Stakeholders, NbS-related Ecosystem, NbS Hydrological Performance) and articulate three interaction dimensions: coupling social and ecological factors, linking human activities with hydrological responses, and balancing trade-offs. The study asks: (1) What are current research trends and gaps regarding NbS-related ecological measures for urban FRM? (2) How can urban FRM leverage NbS to achieve social-ecological “win-win” outcomes? The objective is to provide a holistic SES-based tool to embed NbS in FRM research and strategies for equitable and sustainable outcomes.

The scoping review identifies an imbalanced global distribution of ecological FRM studies, concentrated in the United States (39%), China (19%), Australia (9%), and the United Kingdom (8%), with fewer studies addressing vulnerable Global South regions despite high exposure to coastal flooding and sea-level rise. Prior FRM ecological measures often targeted flood mitigation alone, leading to unintended negative social (e.g., displacement and marginalization) and ecological consequences (e.g., habitat disruption). While frameworks coupling social and ecological perspectives exist in broader NbS and resilience literature, their application to urban FRM has been limited, contributing to inadequate stakeholder well-being improvements, limited measurable biodiversity gains, and insufficient upscaling. Emerging NbS literature since 2018, spurred by global climate adaptation advocacy, reports NbS effectiveness for flood risk and SES issues, employing tools such as multi-criteria assessments, social learning, participatory decision-making, and performance evaluations. However, gaps remain: limited incorporation of resilience thinking, inadequate consideration of environmental and social change dynamics, sparse integration of social variables (risk awareness, investment schemes, stakeholder knowledge) with biophysical factors, and insufficient monitoring of long-term trade-offs (equity, biodiversity, costs).

The study conducts a scoping review of ecological measures for urban FRM (2000–2023) using Web of Science Core Collection and PRISMA procedures. Initial retrieval: 2758 records; inclusion of peer-reviewed English-language research articles and exclusion of non-research papers left 2636; title/abstract screening removed duplicates/irrelevant topics yielding 1978; full-text review resulted in 1271 studies included. Studies were structured by: (1) type of ecological measure—restoration (e.g., floodplain, wetland, river, riparian, parks), engineered (e.g., rain gardens, constructed wetlands, green/blue roofs, bioswales, bioretention, stormwater ponds), and hybrid (e.g., green/natural infrastructure, ecosystem-based adaptation/solutions, LID/SuDS/GSI, sponge city, water-sensitive planning); and (2) urban flood risk aspects (hazard, exposure, vulnerability; including flooding, stormwater runoff, waterlogging). Key features extracted included country distribution, measure types, unintended social/ecological effects, and emerging NbS studies by year, intervention stage (design/planning, implementation, governance), and addressed challenges. In parallel, the authors developed a conceptual SES-based framework for NbS-FRM by adapting Ostrom’s SES (Governance system, Users, Resource system, Resource units) into four FRM-relevant subsystems: NbS Decision-making and Rules, Citizens and Stakeholders, NbS-related Ecosystem, and NbS Hydrological Performance. The framework organizes evidence across three dimensions: D1 coupling social and ecological factors; D2 linking human activities with hydrological responses; and D3 balancing trade-offs through evaluation, monitoring, and governance.

• Global imbalance: Among FRM studies across 96 countries, ecological FRM measures are concentrated in the United States (39%), China (19%), Australia (9%), and the United Kingdom (8%), with underrepresentation in the Global South. • Unintended consequences of mitigation-only ecological measures include habitat disturbance (e.g., amphibian habitats affected by rain garden siting) and social displacement/marginalization when stakeholder engagement is lacking. • NbS effectiveness: 77 cases (all post-2018) explicitly report NbS benefits addressing flood risks and SES challenges, reflecting recent global advocacy for NbS in climate adaptation. • Coupled SES insights (D1): Biophysical variables (precipitation/ET across climate zones, vegetation traits such as plant height, canopy density, leaf area, species richness; soil hydraulic conductivity; habitat quality) are well-studied for hydrological performance. Social variables (risk awareness, investment schemes, stakeholder knowledge) are less frequently considered, yet they influence acceptance and willingness to pay for NbS, aiding uptake and cost-sharing. • Siting tools: Coupled studies often use multi-criteria evaluation based on environmental variables (land permeability, slope, stream network density) and social vulnerability (education, age, gender, race) to prioritize NbS. However, trait-level NbS design, local climate factors, and dynamic resilience to hazards are rarely integrated. • Social processes and hydrology (D2): Urbanization and imperviousness reduce interception, infiltration, and storage; reconnecting green/blue surfaces improves hydrological functions. Example: disconnecting downspouts and directing roof runoff to lawns produced 57–99% runoff volume reduction over nine months. In Syracuse, if 58–64% of households install rain gardens/barrels, additional reductions of 5.3% in peak runoff and 6.3% in total runoff are achievable. • Inclusive implementation and social learning can expand beneficiaries; market-based incentives and social networks can retrofit 20%+ of impervious surfaces with GSI in some contexts. • Changing environments: Few studies analyze fundamental hydrological processes of NbS under variable rainfall and climate/land-use changes, limiting understanding of maintaining effectiveness and equitable service provision for vulnerable groups. • Trade-offs and governance (D3): Centralized NbS may provide greater flood regulation but with higher investment/maintenance costs than distributed NbS; conventional cost-effectiveness analyses often omit biodiversity and equity outcomes. Documented trade-offs include green gentrification (e.g., a stormwater park in Atlanta leading to displacement) and partial biodiversity recovery (e.g., 8-year urban river project increased aquatic diversity but did not fully restore species composition/density). • Upscaling and connectivity: Watershed-level connectivity and transboundary governance can accumulate synergies, reduce costs, mitigate flood losses, and enhance ecosystem functions (e.g., nutrient cycling, carbon storage). • Overarching gaps: Lack of resilience thinking in NbS design/planning; inadequate consideration of climate/environmental/social change dynamics; limited collaborative, cross-boundary monitoring and management to address trade-offs.

Findings address the research questions by revealing that current ecological FRM measures are unevenly distributed and often focus narrowly on mitigation, creating unintended social and ecological outcomes. Evidence since 2018 demonstrates NbS potential to deliver co-benefits when designed and governed through a coupled SES perspective. The proposed framework operationalizes three dimensions to guide practice: (D1) integrate social (risk awareness, stakeholder knowledge, investment schemes) with biophysical factors (vegetation traits, soils, climate) for adaptive, resilient NbS siting and design; (D2) explicitly link human behaviors, land-use decisions, and collaborative implementation to hydrological responses using process-based monitoring and modeling; (D3) anticipate and manage trade-offs via performance evaluation that includes equity and biodiversity, long-term monitoring, and governance at watershed and transboundary scales. Prioritizing resilience thinking, process-based effectiveness assessments under changing conditions, and collaborative governance can shift NbS-FRM towards equitable, sustainable “win-win” outcomes that enhance human well-being and ecosystem health.

The study synthesizes global evidence on ecological measures for urban FRM and proposes a coupled SES framework to leverage NbS for dual social and ecological co-benefits. The scoping review (1271 studies, 2000–2023) identifies uneven global research distribution and mitigation-focused measures with unintended impacts, alongside post-2018 growth in NbS evidence for co-benefits. The framework outlines priority steps: (1) foster resilience thinking in NbS design and planning by assessing baseline ecosystem and societal capacities and building indicators across pre-, during-, and post-flood stages; (2) capture process-based NbS effectiveness in changing environments through integrated data, social surveys, field observations, remote sensing, and modeling (e.g., system dynamics with participatory inputs, climate and policy scenarios); and (3) enhance collaborative approaches for monitoring and managing trade-offs via context-based policy mixes, multi-stakeholder involvement, co-developed indicators, and tools such as Bayesian Belief Networks. Future research should focus on NbS resilience under varying and changing conditions, the coupled dynamics of hydrological responses and human activities, and governance mechanisms to balance trade-offs and upscale synergies across sectors and boundaries.

The evidence base is limited by an uneven global distribution of studies, with underrepresentation of the Global South, which may constrain generalizability. Many reviewed studies emphasize biophysical factors while underreporting social variables (risk awareness, investment schemes, stakeholder knowledge), and few analyze fundamental hydrological processes of NbS under variable rainfall and changing climate/land use, limiting understanding of dynamic effectiveness. Long-term monitoring of ecological and social outcomes is sparse, making trade-offs (e.g., equity impacts, biodiversity trajectories) uncertain and difficult to evaluate. The scoping review includes only peer-reviewed English-language studies and excludes non-research literature, which may introduce selection bias.