Nature-Based Flood Defences in Urban Settings: Co-Benefits and Trade-Offs

As an experienced flood control specialist, I’ve seen how traditional grey infrastructure often falls short in providing comprehensive urban flood protection. Fortunately, a growing number of nature-based solutions (NbS) are emerging as viable alternatives that can deliver multiple benefits beyond just flood mitigation.

NbS leverage the power of natural ecosystems to manage water flows, reduce flood risks, and enhance urban resilience. From wetland restoration to urban greening and permeable surfaces, these nature-based flood defences offer a more sustainable and holistic approach to flood risk management (FRM) in cities.

However, whether NbS can truly achieve both social and ecological co-benefits remains a critical question. In this article, I’ll explore the potential of NbS in urban FRM, highlighting the opportunities for synergistic “win-win” outcomes as well as the trade-offs that might want to be carefully navigated.

Harnessing Nature’s Power for Flood Resilience

Flooding is one of the most severe climate-related disasters, affecting both people and ecosystems. By 2050, current 100-year flood events are projected to occur at least twice as frequently across 40% of the globe.¹ In urban areas, these extreme flood disasters exacerbate impacts, as growing populations are exposed to flooding that intensifies damage and welfare reduction.

Flood risk is determined by three key elements: flood hazard, exposure, and vulnerability.² To address these threats, urban flood risk management (FRM) aims to improve infrastructure capacity, mitigate population exposure, and enhance adaptation of vulnerable communities.

Traditionally, grey infrastructure like levees, flood walls, and storm drainage systems have been the go-to solutions. However, these engineered approaches often have unintended consequences, such as disrupting hydrological processes and associated ecosystems.³

That’s where NbS come in. By harnessing the natural services of local ecosystems, NbS can potentially mitigate flood hazards while also enhancing biodiversity and adaptation to future climates.⁴ This makes NbS a powerful tool for tackling the interlinked challenges of climate change, societal vulnerability, and biodiversity loss.

Integrating Social and Ecological Perspectives

Cities are complex social-ecological systems (SES) where human society and natural ecosystems are deeply intertwined. To fully realize the potential of NbS in urban FRM, we need to adopt a coupled social-ecological perspective that considers the interactions between these two domains.

Existing frameworks, such as Ostrom’s SES framework,⁵ provide a foundation for understanding the interplay between the governance system, citizens/stakeholders, the resource system (i.e., ecosystems), and the resource units (i.e., ecosystem services). By integrating this SES perspective into NbS research and practice, we can better identify the key variables and processes that drive both social and ecological outcomes.

For urban FRM, the NbS-SES framework can be interpreted as four interacting sub-systems:⁶

1. NbS Decision-making and Rules: The governance system and decision-making processes that guide NbS implementation.
2. Citizens and Stakeholders: The diverse range of individuals, communities, and organizations engaged with or affected by NbS.
3. NbS-related Ecosystem: The natural systems and ecosystems that provide the foundation for NbS interventions.
4. NbS Hydrological Performance: The water-related functions and services delivered by NbS.

By considering the reciprocal interactions between these sub-systems, we can better understand how NbS can be leveraged to achieve synergistic “win-win” outcomes for both people and nature.

Uneven Distribution and Unintended Consequences

My review of the current research landscape reveals some concerning patterns. The distribution of ecological FRM measures is highly uneven around the world, with the majority of studies concentrated in the Global North, particularly the United States, China, Australia, and the UK.⁷

This geographic disparity highlights the importance of expanding research and implementation of NbS-FRM in the flood-vulnerable regions of the Global South, where communities often lack the resources and guidance to adopt these nature-based approaches.

Moreover, even when ecological measures are implemented solely for flood mitigation, they may have unintended negative consequences for both society and ecosystems.⁸ For example, the selection of rain garden locations may disrupt habitats for amphibious animals, while failure to engage local stakeholders in restoration programs can displace and marginalize vulnerable communities.

My analysis shows a significant bias towards engineered and hybrid measures (e.g., Low Impact Development, Water-Sensitive Urban Design, Green/Blue Infrastructure) compared to restoration measures.⁹ While the latter have proved to offer broader co-benefits for habitat quality and flood loss reduction, they are underrepresented in the research.

These findings underscore the need to carefully consider the social and ecological implications of NbS, going beyond just their flood mitigation capacity. Integrating stakeholder engagement, equitable distribution of benefits, and biodiversity conservation into the design and implementation of NbS is crucial for achieving truly sustainable and inclusive urban FRM.

Emerging Trends in “Win-Win” NbS

Encouragingly, a growing number of studies are exploring the potential of NbS to address both social and ecological challenges in urban FRM.¹⁰ These NbS approaches, published primarily since 2018, reveal how integrating social and ecological perspectives can contribute to flood risk mitigation and adaptation while simultaneously tackling a wide range of issues.

By optimizing the configuration and composition of NbS across the city, ecosystem functions can be maximized to reduce flood risks. Meanwhile, proper public engagement, continuous maintenance and monitoring, and flexible funding mechanisms can help double-check that more equitable distribution of NbS benefits to at-risk individuals and communities.¹¹

Moreover, NbS that achieve multiple ecological functions beyond just flood regulation can provide higher values of ecosystem services to a broader range of beneficiaries.¹² For instance, constructing green roofs with diverse plant species can enhance substrate cooling and stormwater retention, while reconnecting streams and vegetation in floodplains can improve habitat quality and facilitate species migration.

These examples demonstrate the potential for NbS to deliver synergistic “win-win” outcomes that benefit both people and nature. However, key challenges remain in terms of understanding the dynamic capacity of NbS to withstand flood impacts, adequately considering environmental and societal changes, and effectively managing trade-off effects.

Navigating the Trade-Offs

While NbS hold great promise, they are not a panacea. Careful consideration of the trade-offs between different benefits and co-benefits is essential for optimizing urban FRM strategies.

My review of the literature reveals three critical dimensions of social-ecological interactions that might want to be addressed:

1. Coupling of Social and Ecological Factors: Integrating key design and planning variables for NbS that account for both social and ecological systems.
2. Linking of Human Activities and Hydrological Responses: Understanding how human-driven activities and behaviors influence the hydrological performance of NbS-related ecosystems.
3. Balancing of Potential Trade-off Effects: Managing the potential trade-offs between hydrological benefits, social well-being, and ecological consequences.

By understanding these dimensions, we can develop more comprehensive frameworks and tools to support decision-making and implementation of NbS in urban FRM. This includes fostering resilience thinking, enhancing the modeling of coupled social-ecological dynamics, and establishing collaborative approaches for monitoring and managing trade-offs.

Towards a Sustainable and Equitable Urban Future

As an experienced flood control specialist, I believe that leveraging NbS with a coupled social-ecological perspective holds immense potential for achieving sustainable and equitable urban FRM. However, significant gaps and challenges remain that might want to be addressed through further research and practical applications.

Key priorities include:

1. Expanding the Geographic Scope: Directing more research and implementation efforts towards flood-vulnerable regions in the Global South to address the current geographic disparities.
2. Prioritizing Restoration Measures: Increasing the focus on restoration-based NbS, which have demonstrated broader co-benefits for both people and nature.
3. Integrating Resilience Thinking: Developing frameworks and tools that can assess the dynamic capacity of NbS to withstand changing environmental and societal conditions.
4. Enhancing Modeling of Social-Ecological Dynamics: Improving the integration of human activities, behaviors, and preferences into the modeling of NbS hydrological performance and ecosystem services.
5. Fostering Collaborative Governance: Establishing cross-sectoral and transboundary approaches for the long-term monitoring and management of NbS trade-offs.

By addressing these priorities, we can unlock the true potential of NbS to deliver sustainable, equitable, and resilient solutions for urban flood risk management. As we face the escalating challenges of climate change, urbanization, and environmental degradation, nature-based flood defences offer a promising pathway towards a more harmonious and prosperous future for both people and the planet.

Tip: Regularly inspect and maintain flood barriers and drainage systems