GLOBAL SPONGE CITIES SNAPSHOT: THE POWER OF NATURE

BY VINCENT LEE

As cities face increasing threats from climate change, they need to fully understand how naturebased solutions and green infrastructure can mitigate these threats. Through the AI and land-use analysis tool Terrain came the Global Sponge Cities Snapshot, which evaluated cities’ capacity to absorb stormwater to determine their “sponginess.” The snapshot aims to get cities to leverage technology and think more about nature as an asset to be protected and enhanced.

Our cities aren’t just concrete jungles. Every blade of grass, tree, pond, lake, and lump of soil together form vital infrastructure. As cities face increasing threats from climate change—including heavy rainfall and extreme heat events—they need to fully understand this natural infrastructure and how to enhance it. But nature-based solutions have previously been seen as more difficult to implement and prohibitively expensive.

In our report with the World Economic Forum, BiodiverCities by 2030: Transforming Cities’ Relationship with Nature, we highlighted that nature-based solutions are, on average, 50% more cost-effective than man-made alternatives and deliver 28% more added value. Cities must commit to understanding and quantifying this natural infrastructure and learn how to enhance it. But advanced digital tools have been game-changers, transforming our ability to assess cities’ preparedness for future climate risks and identify opportunities for improvement. Thanks to AI and machine learning, we can now quantify the case for nature-based solutions, helping to better understand a city’s natural “sponginess” and how to enhance it.

Projects around the world—from Shanghai, China, to Mansfield, U.K.—are showing what is possible, and now is the time to accelerate their adoption on a global scale. That’s why we’ve produced our Global Sponge Cities Snapshot: to get people talking about how we can move beyond concrete solutions as we help cities cope with increasingly heavy rainfall and other impacts of climate change.

The Case for Nature-Based Solutions

The term “sponge city” was coined in 2013 by Professor Kongjian Yu of Peking University, describing cities that work with nature to absorb rainwater instead of using concrete to channel it away.

Natural infrastructure is not only extremely effective in managing floodwater, but it also brings greater benefits than traditional engineered “gray” infrastructure and can contribute positively to biodiversity and carbon reduction.

While cities have a natural “sponge” quality, this is a baseline absorbency that can be improved through interventions. Our survey is not intended as a scorecard but to show cities how they can use digital tools to quickly establish a better understanding of their natural assets. Even the most “spongy” cities can enhance their absorbency and work with nature to deliver maximum resilience.

Nature-based solutions can also be applied to a host of climate change problems, such as extreme heat and drought. While they have previously been seen as more difficult to implement and prohibitively expensive, advanced digital tools have been game-changers.

How We Rate Cities’ “Sponginess”

We originally examined cities around the world with different urban profiles—from the densely packed Mumbai to Auckland, known for its generous public parklands. Each city was given a sponginess rating based on three major factors: the amount of green and blue space within the urban environment, the hydrogeological properties of the soil, and the water runoff potential for green areas.

1. Using Terrain to Measure the Amount of Green and Blue Space

Terrain, our AI and machine learning tool, helps cities rapidly understand how land is used. It harnesses the power of data analytics, machine learning, and automation to accurately digest large quantities of data and satellite imagery—80% quicker than a manual approach. It recognizes patterns, producing detailed land-use maps and accurately calculating a region’s coverage percentage of different land types, such as grass, trees, hard-paved impervious land, buildings, and water. This automates the traditionally time-consuming task of deciphering a city’s typology. Terrain is 5x quicker than a manual approach, analyzing 20,000 square meters of land data per second. The technology is also highly accurate and can even distinguish between a tree nursery and a forest.

Terrain is being used worldwide to help planners and authorities understand how land is being used, as well as to conduct global surveys such as the Global Sponge Cities Snapshot, which assessed the natural ability of nine major global cities to absorb water and mitigate urban flooding problems based on their green and blue spaces.

For this study, we used Terrain to calculate the amount of green and blue areas in approximately 150-square-kilometer snapshots over the urban centers of Auckland, London, Mumbai, Nairobi, New York, Shanghai, and Singapore.

2. Accounting for Soil Types

Once we had the percentage of blue, green, and gray spaces for each city, we used a global database of hydrologic soil groups to calculate the amount of each major hydrological soil type in each city. Soil types have a significant impact on the amount of water runoff and, therefore, a city’s sponginess. This can be due to soil type and texture (e.g., sandy soils are “spongier” than clay-based soils) as well as depth of soil and depth to the water table. For example, a groundwater table close to the surface reduces the sponge capacity of the soil.

3. Calculating Water Runoff Potential for Green Areas

With the impact of soil type factored in, we used the Curve Number method to calculate the amount of runoff from a defined rainfall event. Imagine rain falling onto a surface: It’s either absorbed or “stored” in the soil or slowly makes its way to water bodies. Alternatively, it runs off the land and contributes to storm flow and potential flooding. The runoff is affected by vegetation cover, whether it’s a grasscovered open space or covered by trees. It also varies depending on the amount of rain. For our calculations, we looked at the runoff potential for 50 mm of rainfall in a day.

How Can This Help Cities?

We hope this snapshot starts a conversation and gives people an understanding and confidence that nature-based infrastructure solutions can be deployed, bringing more resilient, cleaner, healthier, and happier cities. Some specific areas of conversation include: Review spongy (and non-spongy) areas against flood risk maps to identify priorities for implementing green infrastructure.

• Identify the distribution of green space to help alleviate stormwater flooding, which generally aligns with the distribution of the public realm in cities (i.e., is absorption across the city distributed and equitable or occurring in one centralized area?).

• Trees that are mature with large canopies can help to increase absorptive capacity (as demonstrated by Singapore), which can support the case that urban trees provide myriad benefits.

Cities can learn from each other and get insight into how other cities with comparable land use and ground conditions are performing.

Over time, cities can track the performance of green infrastructure programs and land use planning. By leveraging readily available data and having an understanding of the existing “sponginess” in cities, the Sponge Cities Snapshot can help to influence and prioritize green infrastructure, stormwater resiliency, and land-use programs within cities.

Vincent Lee Water Engineer, Arup New York, New York

Vincent Lee has a wide range of experience in sustainable development, green infrastructure, and water management and serves as the Arup Americas East Civil + Water Engineering Team Leader and Arup’s Global Water Skills Leader. His engineering background has enabled him to implement water-resilient cities and communities worldwide. This expertise is built on a deep foundation of related work, including supporting NYC with its green infrastructure program for the last decade, developing Shanghai’s urban drainage masterplan, and leading the engineering effort for the Buoyant City: Miami Beach Historic District Resilience and Adaptation Guidelines.