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Cost savings from green-grey complementary infrastructure likely to be huge: new research

Image: UCCRTF has worked to encourage a mix of green and grey infrastructure in New Clark City, Philippines (photo by Bases Conversion and Development Authority).

October 2022

Increasing the use of green infrastructure and Nature-based Solutions (NbS) to complement grey infrastructure investments is highly cost-effective, according to new research conducted in the People’s Republic of China. The 2019 infrastructure study in Nanjing, found that coupling green space, permeable pavement and green roofs with pipelines and stormwater detention cells saved 94% in costs over the project life cycle.[1]

The study evaluated the environmental and economic benefits of various types of coupled green and grey infrastructure systems. The authors acknowledged that the results are highly context-specific, and are based on the local conditions and characteristics, the coupled green and grey scenario was also found to perform better than other scenarios in urban residential drainage and flood control. In fact, for the coupled green and grey scenario, the simulation results showed that no flooding occurred during the ten-year return period rainfall.

The findings are further supported by a 2021, study from Suzhou, China, which found that a green-grey intervention costs thousands of dollars less per square kilometer than green-only while delivering higher reductions in pollutants and runoff.[2]

The studies provide evidence that can be used to increase the awareness of the potential benefits and complementarity of green infrastructure solutions in urban planning and infrastructure design. By integrating ‘green’ infrastructure (constructed wetlands, bioswales, permeable pavement and green roofs) with ‘grey’ infrastructure (pipelines, manmade reservoirs, water and wastewater treatment facilities) urban planners can save money, improve water quality, manage flood risk, and deliver social benefits to residents, businesses and urban economies.

Green-grey interventions deliver multiple benefits beyond direct cost savings

Each year, flooding causes an estimated $120 billion USD in urban property damage.[3] Losses are expected to rise with climate change and population growth – especially in South, Southeast and East Asia. [4] Simultaneously, urbanization increases pollution and stormwater runoff; a study of flood risk in Mumbai found that increasing demand on drainage systems contributed to a two- to three-fold increase in runoff in the city.[5]

There is a clear need to increase urban water storage, improve water quality and reduce flood risk. But overhauling cities’ water and stormwater systems can cost tens of billions of dollars, and cause significant disruptions to residents’ lives and businesses. Integrating green infrastructure can not only avert interruptions to lives and urban economies but also deliver cost savings.

The benefits of green-grey interventions are not limited to cost savings. Depending on the project type, green infrastructure components – notably bioswales, rain gardens, trees and wetland conservation – may help reduce the urban heat island effect, improve air quality, offset carbon dioxide emissions, or serve as publicly accessible green space.

Additionally, development experts say that in some locations, green-grey infrastructure can help stabilise or increase residents’ incomes – for instance by reducing runoff that damages fisheries and aquaculture, or by ensuring a consistent water supply for crop irrigation.[6]

Key considerations for advancing green-grey infrastructure efforts

ADB’s Urban Climate Change Resilience Trust Fund (UCCRTF) has sought to increase the uptake of Nature-based Solutions (NbS) and green infrastructure that can increase the efficacy and cost-effectiveness of the built environment. In the Philippines, for example, In the Philippines, UCCRTF support for a masterplan review for the 9,500-hectare New Clark City (NCC) paved the way for nature-based solutions to influence subsequent development activities to build a 45-hectare park , the largest park to be built in the Philippines in the past 50 years (see Box 1).

The completed pilot of the nonnetworked nature-based solutions for providing water supply and sanitation in a community in Makassar, Indonesia will be replicated in six other sites within the city through funding support from the Government of Australia equivalent for A$4 million (about $2.8 million).

Box 1: The case of New Cark City, Philippines.

The New Clark City River Study

The New Clark City (NCC) River Study in the Philippines aims to support evidence-based decision-making on the potential of the river and the green space surrounding NCC to become integral to urban resilience and recreation. The study evaluated NCC’s planned systems and their interlinkages by analysing climate risks, vulnerabilities of anticipated assets, local topography, and hydrology. The study finds that the proposed engineering solutions for the greenfield city would require massive earthworks and maintenance, and would not allow for leveraging the river as one of the most beautiful natural features of the site. Additionally, the drainage capacity was found not to be sufficient when accounting for climate change.


The study recommends the enhancement of certain riverbank conditions at multiple scales to provide urban functions and ecological services. This recommendation was taken forward by the Bases Conversion and Development Authority (BCDA), the project proponent. In the proposed 200-hectare National Government Administrative Center, which is in its pilot phase, BCDA will maintain the river function by establishing a river park that fulfills multiple purposes. This nature-based approach prevents badly planned river alignments, thereby minimizing erosion and flooding risks.


Furthermore, there is a strong economic rationale for this approach due to significant cost savings as a result of reduced earthworks and ecological modification, and from the future savings from mitigated damages caused by flooding. For the private sector, such solutions also enhance land value capture.

Combining green and grey infrastructure can be a cost-effective way to address water supply challenges and enhance cities’ climate resilience, while also delivering environmental and social co-benefits. However, to ensure proposed projects deliver outcomes that are equitable and sustainable, planners and policymakers must proceed with care. From UCCRTF’s experience, it is essential to:

  • Conduct robust public engagement, especially with historically marginalized communities. People living on cities’ margins may depend on non-traditional water sources – e.g., canals, reservoirs or outdoor drinking fountains – for bathing, drinking and washing. Especially for any infrastructure project that may change the availability of these kinds of water sources, planners must speak with all stakeholder groups to ensure their concerns are heard and integrated into project plans.

  • Respect land rights and land use. Green infrastructure tends to require more land than traditional grey infrastructure, so planners should take care to understand and respect jurisdictional boundaries, as well as the ways in which communities use land – even if the use is not officially sanctioned. For instance, if skateboarders skate in an empty reservoir, or if community members have planted a garden in an unoccupied green space, planners should include these groups in public engagement efforts.

  • Use multicriteria analysis (MCA) to assess value. Because green-grey projects deliver co-benefits beyond monetary cost savings, planners should look beyond traditional economic cost-benefit analysis to weigh the pros and cons of a proposed green-grey project. Multi-criteria analysis is a way of systematically assessing projects, by analysing proposals against several different criteria (both quantitative and qualitative) weighted against their relative importance.

  • Consistently monitor green-grey infrastructure performance. Cities that consistently gather data and report on infrastructure performance make it easier for planners to demonstrate benefits and identify areas for improvement. Over time, robust data collection leads to better projects; it can also empower cities to engage in public-private partnerships with financiers and funders who want to see evidence of efficacy.

  • Engage with policymakers. Policymakers who understand the benefits of green-grey infrastructure promote it through policies, laws and regulations; depending on the jurisdiction, they may also allocate funding for green-grey infrastructure projects, or advocate for it in city budget decision-making.

  • Seek out partnerships for sustainable finance. Infrastructure upgrades are costly. Green-grey infrastructure has not only been shown to be less costly than grey-only infrastructure; it also has become a space for public-private partnerships and finance innovation, as demonstrated by UCCRTF’s work in New Clark City. Cities including Quito, Ecuador and Washington D.C. have developed water funds[7] and piloted environmental bonds[8] to pay for green-grey water management projects, delivering environmental, economic and social benefits in a way that’s sustainable for cities’ budgets.

Urbanization, population growth and climate change are driving flood risks and water supply challenges in many Asian cities. Green-grey infrastructure – when undertaken with care – can be a cost-effective way of responding, while also delivering multiple environmental, social, and economic co-benefits.



[1] Xu, C., Tang, T., Jia, H., Xu, M., Xu, T., Liu, Z., Long, Y. and Zhang, R., (2019) ‘Benefits of coupled green and grey infrastructure systems: Evidence based on analytic hierarchy process and life cycle costing’, Resources, Conservation and Recycling, 151, pp. 1-10.

[2] Leng, L., Jia, H., Chen, A.S., Zhu, D.Z., Xu, T. and Yu, S. (2021) ‘Multi-objective optimization for green-grey infrastructures in response to external uncertainties’, Science of the Total Environment, 775, pp. 1-10.

[3] Sadoff, C.W., Hall, J.W., Grey, D., Aerts, J.C.J.H., Ait-Kadi, M., Brown, C., Cox, A., Dadson, S., Garrick, D., Kelman, J., McCornick, P., Ringler, C., Rosegrant, M., Whittington, D. and Wiberg, D. (2015) Securing Water, Sustaining Growth: Report of the GWP/OECD Task Force on Water Security and Sustainable Growth. Oxford, U.K.: University of Oxford, p. 175.

[4] Sadoff, C.W., Hall, J.W., Grey, D., Aerts, J.C.J.H., Ait-Kadi, M., Brown, C., Cox, A., Dadson, S., Garrick, D., Kelman, J., McCornick, P., Ringler, C., Rosegrant, M., Whittington, D. and Wiberg, D. (2015) Securing Water, Sustaining Growth: Report of the GWP/OECD Task Force on Water Security and Sustainable Growth. Oxford, U.K.: University of Oxford, p. 175.

[5] Ranger, N., Hallegatte, S., Bhattacharya, S., Bachu, M., Priya, S., Dhore, K., Rafique, F., Mathur, P., Naville, N., Henriet, F., Herweijer, C., Pohit, S., Corfee-Morlot, J. (2009) ‘An assessment of the potential impact of climate change on flood risk in Mumbai’, Climatic Change, 104, pp. 139-167. DOI 10.1007/s10584-010-9979-2

[6] Browder, G., Ozment, S., Bescos, I.R., Gartner, T. and Lange, G. (2019) Integrating green and gray: Creating next generation infrastructure. Washington, D.C.: World Bank and World Resources Institute.

[7] ‘Quito water fund’ (2022) Water funds toolbox. The Nature Conservancy.

[8] ‘DC Water: First ever environmental impact bond’ (2021) Quantified Ventures.


  • UCCRTF Secretariat
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