How do “sponge” cities make the most of nature to prevent floods and manage urban water supply?


Abundant urban natural areas such as forests, trees, plant-edged sidewalks, lakes, and parks mitigate the effects of storms and flooding events by  acting like sponges that absorb water when it is plentiful and release it when it is scarce. The new AI-based research ranks seven major cities on their ‘sponginess.’ 


This is the first study that used the AI to rank seven major cities on their "sponginess", by comparing how they were covered by ‘blue and green infrastructure’ including grass, trees, ponds and lakes, and how much was covered in ‘grey infrastructure’ such as concrete, pavement and buildings.


Researchers also looked at the type and texture of urban soil to assess how much water it could hold, as well as plant cover, which can help retain water and prevent runoff. They used satellite imagery, artificial intelligence and machine learning to make the calculations. 


The research comes as a growing number of urban areas are experiencing devastating floods, and climate change brings heavier rainfall and growing flood risk.


700 million people already live in areas where rainfall extremes have increased, a number expected to grow as global temperatures rise, according to a recent Intergovernmental Panel on Climate Change (IPCC).


With already 16 cities in China that embrace their “sponginess” through inner-city gardens, improved river drainage and plant-edged sidewalks, China is a great example of making the most out of nature. These cities are keeping their forests healthy and actively incorporate trees and other green infrastructure to manage stormwater and flooding risks and improve infiltration rates across their area. This is not only the case in China, the trend of  incorporating “nature-based solutions” to tackle climate shocks has grown in popularity in recent years everywhere in the world!


Landslide-hit Freetown, Sierra Leone’s capital, for instance, is planting trees to help prevent future disasters, while Tirana in Albania is creating a ring forest to clean the air and halt urban sprawl.


So what were the findings? 


The seven cities analysed were New York, London, Singapore, Mumbai, Auckland, Shanghai, and Nairobi.


Each was given a “sponginess” percentage of 1-100 per cent. Cities with higher ratings can absorb more water during rainfall.


New Zealand’s Auckland came out top with a 35 per cent sponge rating - largely thanks to its stormwater systems, many golf courses, green parks and good-sized residential gardens.


It was followed by Nairobi at 34 per cent, while New York, Mumbai and Singapore tied third with 30 per cent, and Shanghai fourth with a 28 per cent sponge rating. In last place was London, at 22 per cent, mainly due to high levels of concrete and poor soil absorbency.


The role of urban green spaces in water supply management and climate risk mitigation


The root systems filter out pollutants, and slow down the flow of water towards creeks, rivers, lakes, and ultimately cities. Trees require a lot of water to grow, so as rainwater soaks into the soil, trees absorb substantial amounts through their roots. This uptake slows water down, stopping it from crashing downstream and flooding cities along the way.  It's also important to preserve the urban forests as chopping down large swaths of trees to make way for other land uses makes water absorption impossible and creates a much higher risk of flooding. This also reduces risks from combined sewage overflow, which threaten the health of communities and aquatic ecosystems.


For example, 50% of Jakarta's main watershed has been converted  to agricultural land or urban development. Without the trees to stop it, throughout the rainy season water fills the many rivers running through the city of more than 10 million people, forcing many to evacuate their homes. This deforestation of upstream watersheds resulted in the loss of 40 lives and cost US$2.4 billion in 2013.


The impact of deforestation on a city’s water supply became  clear also in Brazil in 2014 and 2015. Riots broke out in Sao Paulo and the surrounding municipalities when a record-breaking drought brought reservoir levels down to just 5% - only a months supply for an area with more than 21 million people.


Urban green spaces have a crucial role in providing potable water, reducing utility costs, and can even save people's lives and livelihoods. Moreover, they improve the air we breathe, provide innumerable health benefits, and support biodiversity. The flooding in Jakarta and the drought in Sao Paolo show us that the deforestation can disrupt the ecosystems and the resulting changes can be devastating.


References: How can 'sponge cities' use nature to tackle climate-fuelled floods? | News | Eco-Business | Asia Pacific