The world is changing rapidly, and so is our relationship with the environment. Climate change is reshaping the very cities we live in. Urban flooding, exacerbated by extreme weather, is among the most devastating consequences, threatening lives, infrastructure, and economies. The World Resources Institute (WRI) predicts that by 2030, 15 million people and $177 billion in urban property will be impacted annually by coastal flooding, while riverine floods could affect 132 million people and $535bn in urban property annually. With urbanization on the rise, and climate-induced disasters becoming more frequent and intense, we are at a tipping point.
Bangladesh is among the 10 most disaster-affected countries in the world, highly exposed to severe monsoon flooding and cyclones, and sits on the frontline of this battle. In fast-growing megacities like Dhaka and Chittagong, rapid urban sprawl, inadequate drainage systems, and the loss of natural floodplains have made the situation even more precarious. According to the World Economic Forum, 55% of the global population now resides in urban areas, and by 2050, that number will rise to nearly 80%. For Bangladesh, urbanization will climb to 56% by mid-century. Without urgent action, the future of these cities is bleak.
We stand at the edge of a climate emergency and business-as-usual is no longer an option. A report by the World Bank warns that, by 2030, annual flood losses in the world’s largest coastal cities could exceed $1 trillion. Policymakers, often reluctant to depart from the familiar, tend to lean on grey infrastructure -- concrete floodwalls, drainage pipes, and levees -- because these methods feel predictable. However, these conventional solutions are falling short as the climate crisis intensifies. These solutions are often expensive, have high carbon footprints, and require constant maintenance.
We stand at the edge of a climate emergency and business-as-usual is no longer an option
Amidst the rising disaster, nature-based solutions (NbS) offer a change in thinking about how we manage urban flooding. NbS presents not only a technical solution but a vision for a future where human resilience and environmental stewardship work hand in hand. NbS is about securing the future for millions of people. If scaled up, these solutions have the power to transform urban landscapes, making them more resilient, equitable, and sustainable.
What are nature-based solutions?
NbS is best considered as an umbrella concept that covers a range of ecosystem-driven approaches that restore, protect, or enhance natural landscapes to mitigate the impact of climate-related disasters. These solutions leverage forests, wetlands, rivers, mangroves, and urban green spaces to reduce flood risks, sequester carbon, and enhance biodiversity. What makes NbS particularly powerful is their adaptability -- they work in harmony with natural processes and can regenerate over time.
Research by the International Union for Conservation of Nature (IUCN) indicates that NbS can substantially reduce flood intensity when integrated into the urban planning framework. They also offer broader co-benefits like improved air and water quality and increased resilience to heatwaves. Furthermore, NbS are cost-effective: According to the World Resources Institute (WRI), every $1 invested in flood protection infrastructure in Bangladesh can yield $123 in avoided damages (when moving from 3-year flood protection in 2010 to 10-year flood protection in 2050) and reduces the likelihood of floods occurring from 20% to 4%.
Green v grey: A quantitative comparison
The benefits of NbS over grey infrastructure are clear. According to World Bank studies on flood risk reduction, NbS like wetlands and urban green spaces can reduce a significant flood volume compared to grey infrastructure. Moreover, studies show that the cost of implementing NbS can be significantly lower than conventional flood defenses when factoring in long-term maintenance.
Metric | Nature-Based Solutions (NbS) | Grey Infrastructure |
Cost | 30-50% lower over time due to lower maintenance | High upfront and maintenance costs |
Flood Mitigation | More effective when integrated with urban planning | Effective short-term but less resilient to climate change |
Environmental Impact | Positive -- restores ecosystems, reduces emissions | Negative -- high carbon footprint, disrupts ecosystems |
Longevity | Adaptive and self-sustaining with minimal upkeep | Requires frequent maintenance and upgrades |
Co-benefits | Carbon sequestration, air and water quality, biodiversity | Few additional benefits beyond flood control |
Blending grey and green
While NbS offer clear advantages, they are most effective when combined with traditional grey infrastructure. The Sendai Framework for Disaster Risk Reduction highlights the importance of integrating NbS into national resilience strategies. It emphasizes the need for community involvement, leveraging local knowledge, and investing in both green and grey infrastructure to protect against climate-induced disasters.
In high-density urban areas like Dhaka, space limitations make pure NbS challenging. Several studies confirm the success of these hybrid solutions in achieving optimal results. For instance, Singapore’s Active, Beautiful, Clean Waters (ABC Waters) program blends NbS with grey infrastructure, significantly reducing flood risks while enhancing biodiversity. Similarly, New York City’s Green Infrastructure Program incorporates green roofs, bioswales, and permeable pavements into existing grey infrastructure, reducing stormwater runoff, and eliminating the need for costly large-scale drainage systems.
This pragmatic combination of grey and green ensures that cities benefit from the adaptability of NbS without sacrificing the reliability of engineered solutions. Bangladesh can follow suit, especially in cities like Dhaka, Chittagong, and Sylhet, where a hybrid approach could enhance urban flood resilience and sustainability.
Learning from global case studies
The restoration of traditional water tanks in Sri Lanka has reduced flood risks and provided drought relief. This NbS approach not only manages floods but also improves agricultural productivity and water security during dry seasons. Bangladesh can learn from this experience to restore its water bodies for flood mitigation.
Globally, initiatives such as the Room for the River program in the Netherlands demonstrate how widening floodplains and restoring natural waterways can reduce flood risks. By combining NbS with grey infrastructure, this €2.2 billion program has decreased flood risks in the Rhine Delta. These examples illustrate how protecting and restoring wetlands, riparian buffers, and floodplains offer cost-effective solutions for flood management. Bangladesh can implement similar hybrid strategies, especially in riverine regions.
Localizing NbS in Bangladesh
Bangladesh presents both an acute challenge and a massive opportunity for NbS implementation. Yet much of the conversation around NbS continues to focus on success stories from developed nations -- New York, Singapore, and the Netherlands -- while overlooking the untapped potential in Bangladesh.
Urban wetlands: Wetlands are among the most powerful natural flood defences, acting like sponges to absorb excess rainwater and releasing it slowly, thus reducing the volume of floodwaters. Urban wetlands like Gulshan Lake and Begunbari Beel in Dhaka play a critical role in stormwater retention, significantly mitigating urban flooding.
Similarly, riparian buffers -- vegetated areas along rivers -- are essential for absorbing floodwaters and preventing erosion. Yet, rapid urbanization is encroaching on these vital ecosystems, diminishing their capacity, and highlighting the urgent need for policy reform. Additionally, other vulnerable regions like Sylhet -- which faces repetitive flooding despite not being heavily urbanized -- also require NbS to safeguard their future.
Bioswales and floating gardens: In southern coastal regions, where flooding is a persistent threat, traditional floating gardens -- constructed from water hyacinths -- offer a natural flood defense while providing food security. This is a clear example of how local knowledge and NbS can converge to create sustainable solutions for flood management. Similarly, bioswales -- vegetated channels designed to manage runoff -- offer an innovative NbS approach. In cities like Seattle, bioswales have reduced stormwater runoff by 99%. Bangladesh’s adoption of such techniques could significantly bolster urban flood resilience.
Green roofs and permeable pavements: Green roofs -- rooftops covered with vegetation -- can absorb up to 70% of rainwater during peak storm events, reducing the burden on drainage systems. In Dhaka, small-scale green roof projects have already shown promise, retained large amounts of water, and lowered the urban heat island effect. Similarly, permeable pavements allow water to seep into the ground, reducing surface runoff and preventing flash floods. Studies indicate that permeable pavements can reduce runoff by 60-90% during heavy rains, depending on soil conditions and design.
Bridging the gap
NbS are praised for their adaptability, but in the face of escalating climate threats, we need to ensure that they remain effective in the long term. Rising sea levels, more frequent storms, and prolonged droughts could overwhelm certain NbS like wetlands and riparian buffers. To meet this challenge, we need to embrace cutting-edge technology.
Incorporating smart monitoring systems and remote sensing technologies into NbS can dramatically enhance their performance. These tools can track water levels in real-time, anticipate flood risks, and allow cities to adjust their defenses accordingly. Predictive analytics powered by artificial intelligence (AI) can model future climate scenarios, enabling planners to identify high-risk zones and implement NbS preemptively.
Globally, big cities like Singapore, New York and many more are already using these technologies to enhance their flood defenses. Bangladesh, too, must invest in these technologies to optimize the effectiveness of NbS. By doing so, we can create a future where cities are pro-active, not reactive, in the face of climate disasters.
Governance and policy challenges
Implementing NbS at scale requires more than technical expertise -- it demands coherent policy reform, streamlined governance structures, and financial incentives. In Bangladesh, fragmented governance and overlapping responsibilities among agencies have hindered NbS implementation. The National Adaptation Program of Action (NAPA) acknowledges the importance of NbS, but it lacks concrete mechanisms for cross-sector collaboration, leaving implementation fragmented and ineffective.
To address these issues, NbS must be integrated as the core component of urban planning strategies rather than treated as standalone projects, shifting NbS from reactive solutions to pro-active climate adaptation measures. Incorporating NbS into city planning frameworks, transportation networks, and housing policies will create more resilient urban systems. To secure a resilient future, policymakers must set clear targets for NbS integration in urban resilience plans.
Establishing a dedicated task force that bridges environmental and urban development ministries could further streamline NbS efforts, ensuring their integration across all sectors. Furthermore, public-private partnerships and innovative financing models like blended finance can help unlock NbS potential, particularly in low-income areas where upfront costs are prohibitive. International climate funds, such as the Green Climate Fund, can also play a critical role in supporting NbS projects in vulnerable regions.
Social justice
NbS are not just about protecting cities from floods -- they are about social justice. Marginalized and low-income communities are the most vulnerable to flooding, and NbS can play a crucial role in protecting these populations. But to truly protect the vulnerable, we must change how NbS is perceived.
Shifting public and political perception away from grey infrastructure and towards nature-based solutions is critical. Education and awareness campaigns are needed to highlight the long-term benefits of NbS. Communities must be involved in every step of the process, from planning to implementation, ensuring that these solutions are not only sustainable but also equitable.
A vision for the future
The conversation around climate change often centres on numbers and statistics, but at its core, this is a human crisis. What will Dhaka or Chittagong look like in 2050 if we don’t act now? What happens to the communities living on the edge of these cities, the ones who suffer the most when floods hit?
The stakes are clear. The decisions we make today will determine the fate of our cities tomorrow. The cost of inaction is far greater than the cost of investing in resilience today. If we embrace NbS -- if we harness the power of nature -- we can build a future where our cities are not just surviving but thriving.
What will Dhaka or Chittagong look like in 2050 if we don’t act now? What happens to the communities living on the edge of these cities, the ones who suffer the most when floods hit?
Md Zahurul Al Mamun is a Climate Change Researcher & Analyst. E-mail:moon.ju.153@hotmail.com. Uswatun Mahera Khushi is Assistant Professor, Local Government and Urban Development, Jatiya Kabi Kazi Nazrul Islam University. E-mail: umkhushi@gmail.com.