The team mapped the proportions of gray (concrete and buildings), green (vegetation), and blue (ponds and streams) in the seven cities using geographic information systems (GIS), satellite imagery, and machine learning. These maps were then combined with information about soil types and runoff potential to determine each city’s natural absorbency.
Deep on the coast, Auckland is New Zealand’s largest city with a population of 1.4 million and an average annual rainfall of 1210 mm (48 inches) – slightly more than New York and twice what London normally receives in a year.
The researchers calculated that 50% of Auckland’s area was green or blue, even excluding ports (London had the fewest of the seven cities at 31%). In one intense rainfall event – 50mm fall in 24 hours – they estimated that 35% of the water falling on Auckland would be absorbed by these spongy blue and green bits, leaving 65% to be treated by engineered stormwater systems – or else overflow and flood.
“It’s a measure of how this city was developed and the natural morphology underlying its core,” says Fletcher.
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Nairobi came in spongy second place with 34%. The Kenyan capital has even more green and blue expanses than Auckland – mostly parks and urban courtyards – but higher runoff potential due to its clay-dominated soils, which absorb less water than sand or gravel. In the lowest-ranked city, London, only 22% of the water would be absorbed in a similar rainfall event – a risk made clear in real life in July 2021, when 47.8mm of rain fell in an hour and became widespread led to flooding of streets. houses and subway stations.
Kathy Waghorn, an urban researcher at Auckland University of Technology’s School of Future Environments, is unsurprised that her city was rated as relatively fuzzy. “We have low urban density, we still have a lot of one-story apartments, we still have gardens,” she says.
Auckland’s geomorphology also plays a role, she says: the narrow strip of land surrounded by two huge harbours, the dozens of small dormant volcanoes dotting the city, the streams cascading down its green sides, and beneath them the legacy of its lava – Basalt and cinder caves and sinkholes. “The volcanic field shaped part of this open space,” says Waghorn. “Even our stone is kind of spongy.”