Case Study: Nature-Based Solutions — A City or Utility Pilot and the Results So Far

Since launching its Active, Beautiful, Clean (ABC) Waters Programme in 2006, Singapore has certified 124 projects that integrate nature-based stormwater management into urban landscapes—transforming concrete drains into naturalized waterways while treating over 40% of development footprints in gold-certified projects. This city-state's approach offers a replicable model for densely populated regions across Asia-Pacific seeking to balance rapid urbanization with climate resilience.

Why It Matters

Cities worldwide face a dual challenge: managing increasingly intense rainfall events while maintaining livable urban environments. Traditional gray infrastructure—pipes, drains, and underground storage—struggles to keep pace with climate change. The costs of inaction are staggering: Copenhagen's 2011 cloudburst caused €1 billion in damages; New York City's combined sewer overflows release billions of gallons of untreated water annually; and across Asia-Pacific, urban flooding affects millions each monsoon season.

Nature-based solutions (NbS) offer a fundamentally different approach. Instead of channeling water underground as quickly as possible, NbS slows, stores, and treats stormwater at the surface through green spaces, wetlands, bioswales, and naturalized channels. These systems deliver multiple co-benefits beyond flood protection: improved water quality, urban cooling, biodiversity habitat, recreational space, and enhanced property values.

For cities in the Asia-Pacific region—where 60% of the world's population lives and urbanization continues at unprecedented rates—Singapore's ABC Waters Programme demonstrates that nature-based approaches can work at scale in tropical, land-constrained, and highly developed environments.

Key Concepts

Active, Beautiful, Clean (ABC) Waters: Singapore's framework integrating stormwater management with community spaces. "Active" emphasizes recreational use, "Beautiful" focuses on aesthetic integration, and "Clean" ensures water quality treatment through natural processes.

Blue-Green Infrastructure: The combination of water features (blue) with vegetation (green) to manage urban hydrology. This includes rain gardens, bioretention swales, constructed wetlands, and naturalized channels that work together as interconnected systems.

Stormwater Treatment Percentage: A key metric in ABC Waters certification. Standard certification requires treating 25% of a site's area; Gold certification requires 40% or more—meaning rainfall from nearly half the development passes through natural treatment systems before discharge.

Sponge City Principles: The concept of designing cities to absorb, store, and purify rainwater like a sponge, rather than rapidly draining it. Singapore's ABC Waters predates but aligns with China's national Sponge City initiative launched in 2015.

Combined Sewer Overflow (CSO): In older cities like New York, stormwater and sewage share the same pipes. During heavy rain, these systems overflow, discharging untreated waste into waterways. Green infrastructure reduces CSO volumes by keeping stormwater out of sewers entirely.

What's Working

Policy Integration and Mandatory Requirements

Singapore's Housing and Development Board (HDB)—responsible for 80% of the population's housing—has mainstreamed ABC Waters features into all new projects since 2018. This top-down mandate ensures consistent implementation across the city-state. As of 2024, HDB accounts for 62 of the 124 certified projects, representing 50% of all certifications.

The certification scheme, launched in 2010, creates clear benchmarks. Projects earn points across four categories—Active, Beautiful, Clean, and Innovation—with minimum thresholds in each category required for certification. This prevents gaming the system by excelling in one area while neglecting others.

Demonstrated Cost-Effectiveness

Copenhagen's Cloudburst Management Plan provides compelling economic evidence. The city calculated that traditional underground sewers alone would deliver negative return on investment, while combining blue-green surface solutions with targeted gray infrastructure generates net savings. Their analysis showed the "cost of doing nothing" at €16 billion in damages over 100 years versus €1.4 billion for the combined approach.

New York City's program similarly demonstrates scale: $3.5 billion committed through 2045 to manage 1.67 billion gallons of CSO reduction annually. Demonstration projects showed 20-23% stormwater runoff reduction for storms of one inch or less—proving the approach works in real urban conditions.

Measurable Environmental Outcomes

Singapore's Bishan-Ang Mo Kio Park, completed in 2012, remains the flagship example. Engineers transformed a 2.7-kilometer concrete drainage channel into a naturalized river with vegetated banks, creating 62,000 square meters of new park space while increasing flood conveyance capacity by 40%. Biodiversity surveys recorded a 30% increase in plant species and significant growth in bird and butterfly populations.

NYC has built over 9,000 green infrastructure assets as of 2019, with 2,500 more added in 2024. The program achieved its 2025 milestone of 669 million gallons per year CSO reduction ahead of schedule, demonstrating that nature-based approaches can deliver at metropolitan scale.

What Isn't Working

Maintenance and Long-Term Performance

Early green infrastructure installations in New York revealed maintenance challenges. Rain gardens require regular sediment removal, vegetation management, and repair after damage from vehicles or pedestrians. Without adequate maintenance budgets and trained personnel, performance degrades significantly within years of installation.

Singapore addresses this through its three-year certification validity, requiring ongoing compliance. However, smaller municipalities across Asia-Pacific often lack the institutional capacity to sustain maintenance programs, limiting replicability.

Underground Infrastructure Conflicts

New York's program encountered significant obstacles from existing underground infrastructure—subway tunnels, gas mains, water pipes, and electrical conduits—that constrain where surface installations can be placed. In some areas, high bedrock or water tables make infiltration impossible, requiring alternative approaches like infiltration basins that provide less visible greening.

Private Property Integration

Both New York and Copenhagen struggle to extend green infrastructure onto private property, where much impervious surface exists. NYC allocated $53 million for incentive programs targeting properties over 50,000 square feet, but voluntary uptake remains limited. Copenhagen's plan acknowledges €320 million must come from private homeowners—a significant coordination challenge.

Real-World Examples

1. Singapore ABC Waters Programme

Context: Singapore launched ABC Waters in 2006 to transform utilitarian drains and canals into vibrant community spaces while improving water quality and flood resilience.

Implementation: The program operates through voluntary certification for developers, with HDB mandating compliance for public housing. The 2024 update introduced 22 new certifications at a single seminar—the highest ever—including four Gold certifications for Tengah district projects treating over 45% of their development footprints.

Results: 124 certified projects as of 2024. The Kallang River at Bishan-Ang Mo Kio Park demonstrates the approach's potential, combining flood management with recreation space that attracts over three million visitors annually. Studies estimate non-market benefits—including aesthetic appreciation, recreational value, and wellbeing improvements—at significant levels across completed projects.

2. New York City Green Infrastructure Program

Context: NYC launched its program in 2010 to reduce combined sewer overflows polluting local waterways. The program represents the largest municipal green infrastructure initiative in the United States.

Implementation: The city builds rain gardens, bioswales, green roofs, and permeable playgrounds across combined sewer areas in all five boroughs. Priority watersheds include Newtown Creek, Jamaica Bay, and the Gowanus Canal—all historically impaired by CSO pollution.

Results: 669 million gallons per year CSO reduction achieved by 2024, meeting the 2025 milestone early. Demonstration projects documented 20-23% stormwater runoff reduction. The program manages approximately 1,230 acres of green space as of 2021, with a goal of 8,000 acres by 2030.

3. Copenhagen Cloudburst Management Plan

Context: A catastrophic 1,000-year storm in July 2011 caused €1 billion in damages and flooded 50,000 homes, catalyzing Copenhagen's Cloudburst Management Plan adopted in 2012.

Implementation: The plan combines 300 surface projects with two major cloudburst tunnels over 20 years (2015-2035). Blue-green solutions handle everyday and moderate storms; gray infrastructure activates only during extreme events. Karen Blixens Square exemplifies the approach—a five-acre public plaza with undulating concrete domes and gardens that capture and store rainwater.

Results: 60 projects under development as of 2024, with 11 completed. The city designed for 100-year storm management at surface level, up from the previous 10-year capacity. Co-benefits include biodiversity corridor creation, 10-15% property value increases near new parks, and urban heat island reduction.

Action Checklist

• Conduct baseline assessment: Map current stormwater infrastructure, flood-prone areas, and available green space. Quantify current CSO volumes or flooding frequency as baseline metrics.

• Calculate cost of inaction: Following Copenhagen's model, estimate 50-100 year damage costs under current climate trajectories to build the economic case for investment.

• Develop certification or standards framework: Create clear benchmarks for nature-based stormwater treatment, using Singapore's tiered approach (25% standard, 40% gold) as a template.

• Integrate with housing and development policy: Engage public housing authorities and planning departments to mainstream NbS requirements into new development, following HDB's 2018 mandate.

• Establish maintenance protocols and funding: Budget for long-term maintenance from project inception. NYC's experience shows that maintenance failures undermine performance within years.

• Design for multiple benefits: Ensure projects deliver recreation, biodiversity, and aesthetic value alongside flood management to build community support and demonstrate return on investment.

• Monitor and publish results: Implement post-construction monitoring and share data publicly to build the evidence base and demonstrate accountability.

FAQ

Q: Can nature-based solutions work in tropical climates with intense monsoon rainfall?

A: Yes. Singapore's ABC Waters Programme proves NbS effectiveness in tropical conditions with annual rainfall exceeding 2,300mm. The key is designing for local hydrology—Singapore's systems handle high-intensity rainfall through appropriate sizing and the integration of multiple treatment stages (bioswales, rain gardens, constructed wetlands) in sequence.

Q: How do nature-based solutions compare in cost to traditional gray infrastructure?

A: Copenhagen's analysis found that combining blue-green solutions with targeted gray infrastructure delivers net savings compared to gray-only approaches. Surface solutions cost approximately €700 million versus potentially €2+ billion for equivalent underground capacity. Additionally, NbS generate co-benefits—recreation, property values, cooling—that gray infrastructure cannot provide.

Q: What institutional capacity is needed to implement city-scale nature-based solutions?

A: Successful programs require: (1) a lead agency with clear mandate (Singapore's PUB, NYC's DEP); (2) legal frameworks allowing utility funding for surface projects (Copenhagen amended laws in 2011); (3) design standards and guidelines (all three cities publish detailed technical manuals); and (4) dedicated maintenance personnel and budgets. Smaller cities can start with pilot projects in priority watersheds while building institutional capacity.

Sources

• PUB Singapore. (2024). "ABC Waters Certification." Singapore's National Water Agency. https://www.pub.gov.sg/Professionals/Awards-and-Certification/ABC-Waters-Certification

• HDB Singapore. (2024). "Four Tengah BTO Projects Awarded Top-tier Active, Beautiful, Clean Waters Certification." Press Release, June 21, 2024.

• NYC Department of Environmental Protection. (2024). "Green Infrastructure Annual Report." https://www.nyc.gov/site/dep/water/green-infrastructure.page

• NYC DEP. (2024). "CSO Quarterly Progress Report Q2 2024." Combined Sewer Overflow Long Term Control Plan.

• City of Copenhagen. (2012). "Cloudburst Management Plan." Copenhagen Climate Adaptation Plan.

• Climate-ADAPT. (2024). "The Economics of Managing Heavy Rains and Stormwater in Copenhagen." European Environment Agency. https://climate-adapt.eea.europa.eu

• Lim, H.S. and Lu, X.X. (2016). "Sustainable urban stormwater management in the tropics: An evaluation of Singapore's ABC Waters Program." Journal of Hydrology, 538, 842-862.

• ASLA. (2016). "The Copenhagen Cloudburst Formula: A Strategic Process for Planning and Designing Blue-Green Interventions." Professional Awards. https://www.asla.org/2016awards/171784.html