Sponge cities soak it up

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Flash floods in cities worldwide are becoming commonplace, but what is being done to cope with the heavy downpours that will only increase as climate change bites?

EUROPEAN AND CHINESE CITIES are getting a jump on climate change hazards by implementing a so-called sponge city strategy to deal with situations such as flooding from the increasing number of rainstorms.

Catching and reusing water

Sponge cities work by making road and pavement surfaces permeable and using the collected groundwater for other purposes. Some might be used to recharge depleted aquifers or to irrigate gardens or urban farms. The water could also be used to flush toilets or even be processed to make it clean enough to drink.

‘It’s a new way of thinking about stormwater, not as a problem but as an opportunity and a resource to augment our water supply,’ says Richard Luthy, Professor of Civil and Environmental Engineering at Stanford University in California.

Wider climate-adaptive measures

In Berlin, where the sponge city philosophy has been embraced in a shared commitment with Barcelona, Lyon and Liverpool, the term ‘sponge city’ incorporates a raft of other measures.

There, green roofs, wetlands, light-coloured buildings that reflect heat and sidewalk awnings and tree plantings to provide shade are being recommended as climate adaptation strategies.

While the recommendations do not have the status of regulations, and so are not binding on developers, they do have to be considered in development plans for Berlin, says Heike Stock, the municipal officer in charge of the programme.

‘The city will use its powers to negotiate with real estate developers over the details of projects to encourage climate-adaptive measures like green rooftops,’ she says.

‘We want to avoid new buildings that aren’t adapted to a hotter climate, which would result in people installing electricity-hungry air conditioning.’

China’s ambitious plans

With 16 pilot projects in the making, sponge cities are a booming business in China where rapid urban migration and development is encroaching on areas of limited stormwater drainage.

China’s aggressive programme was launched in 2015 and is funded by the central government. It requires 20% of the chosen sponge cities to be built to a sponge city standard by 2020 and be absorbing and reusing at least 70% of their rainwater. Plans are for the concept to be rolled out nationally.

Lingang, a planned city in Shanghai’s Pudong district, has embraced typical sponge city measures. These include green rooftops, wetlands for rainwater storage and permeable pavements.

While the sponge city programme in China is facing challenges, including technical and management issues, there are positives. ‘Previously, in the past 30 years, people just followed conventional national standards … we are shifting to a sustainable way to manage water issues in cities. It’s a good sign,’ says Michael Zhao, an expert in water management at Arup in Shanghai.

India faces similar problems to China. With urban flooding a recurring problem in cities such as Bangalore, which had its wettest year in 2015, there have been calls to follow China’s example.

An editorial in the Hindustan Times on 18 October noted, ‘One of the key reasons why cities are collapsing due to natural calamities is because we have gone against the natural systems. Experts point out that a solution to urban flooding is to mimic nature by making cities act as a sponge or becoming sponge cities.’

Coping with flooding here

While the sponge city approach has not been widely discussed in New Zealand, cities here face similar challenges. Auckland Council says 16,000 Auckland homes are stormwater and flood prone, and climate change projections foresee more flooding as stormwater drains fail to cope with heavy downpours.

However, Water Sensitive Design for Stormwater, the city’s guide to urban planning, is likely to help mitigate against future floods, at least in parts of the urban sprawl.

Rules that came in with the Unitary Plan mean new developments cannot produce more run-off than would have happened prior to the build. The design model is already being used in developments such as Hobsonville Point.

Houses there are fitted with large 3,000 L rainwater tanks that collect excess water and are plumbed to use this non-potable water in the homes.

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