Can a sponge city really prevent flooding?

Is a sponge city the answer to Auckland's flooding woes? The Detail finds out what the concept is all about.

With the cleanup in full swing all over Auckland after this week's catastrophic flooding, people are starting to talk about throwing out the old building rules and "unengineering" our city - to help it cope with future devastating downpours.

It's about uncovering natural streams that have been tar-sealed and paved over, and replanting natives that will absorb the water, says Tim Welch, who teaches urban planning at Auckland University and is co-director of the Future Cities Research Hub. 

But it's also about moving people and their homes from the riskiest places.

Auckland is behind other parts of the world where so-called 'sponge cities' have been created as a solution to flooding. At least 30 cities in China have been redeveloped in this way, likewise cities in the US and in Singapore, Welch says.

"In Singapore a lot of the natural springs and rivers have been uncovered, with lots of tree plantings along the sides and lots of native plantings to absorb the water. That creates a natural channel that's much higher capacity than any pipe we could build. That's just one of a list of many things we could do," Welch says.

He explains to The Detail why Auckland wasn't able to absorb the record rainfall: traditional town planning and building design, such as metal or shingle roofing, allows the water to flow into the gutters, through the downspout, along a concrete driveway into the road and gutters, then into ageing stormwater pipes and out to streams and beaches.

By contrast, Welch points to two new spongy suburbs in Auckland - Hobsonville Point and Stonefields - which he says fared well. 

As one example, each new house at Hobsonville Point has to have its own catch basin for rainwater captured from the roofs, to reduce the amount of water running down the road. 

At Stonefields a giant, tiered floodable park has been created by preserving some of the existing wetlands, preventing floodwaters flowing into properties.

Over in Mount Roskill, something similar is happening at Te Auaunga Creek.

"This is definitely a sponge city approach here and there are more and more of these types of things happening," says Auckland councillor Julie Fairey.

It's part of a $25 million Auckland Council Healthy Waters project, started in 2016, to prevent flooding in nearly 200 homes, mainly owned by Kainga Ora. 

The river park includes a natural waterfall, which has been redeveloped from an ugly culvert, and also features a specially designed fale, footbridges and an outdoor classroom.

Fairey shows The Detail how the planting of hundreds of natives on tiered banks on both sides of the stream absorbs heavy rainfall. 

Some nearby properties were hit badly by this week's flooding, she says, but overall, there's been less impact on homes in the area since the new infrastructure was built.

"What we've had over the last few days is extreme. Yes, we could do better and we have to build better like this

"But whether the infrastructure here has made a difference or not in these particular circumstances, it's too soon to tell to be honest, but we will do all that data and investigation in due course," she says.

Welch says some developed parts of Auckland may be beyond help. 

"Areas like west Auckland have been built on wetlands. There are two things we can do. We can try to shore up those areas, make sure that water doesn't intrude as much by building up a lot of the sponge city concept and just slow down the amount of water that would naturally go there.

"Or, we can retreat and that may be for some people the only option which is to unbuild our housing, unbuild our neighbourhoods and move back," he says.

Listen to more about the work that's gone on at Te Auaunga Creek in the full podcast episode.

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