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Creating ‘sponge cities’ to cope with more rainfall needn’t cost billions – but NZ has to start now

Public Policy / opinion
Creating ‘sponge cities’ to cope with more rainfall needn’t cost billions – but NZ has to start now

By Timothy Welch*

Tune into news from about any part of the planet, and there will likely be a headline about extreme weather. While these stories will be specific to the location, they all tend to include the amplifying effects of climate change.

This includes the wildfire devastation on the island of Maui in Hawaii, where rising temperatures have dried vegetation and made the risk that much greater. In Italy, summer temperatures hit an all-time high one week, followed by massive hail storms and flooding the next.

Flooding in Slovenia recently left three people dead and caused an estimated €500 million in damage. At the same time, rainfall in Beijing has exceeded a 140-year record, causing wide-scale flooding and leaving 21 dead.

These northern hemisphere summer events mirror what happened last summer in Auckland, classified as a one-in-200-year event, and elsewhere in the North Island. So far this year, rainfall at Auckland Airport has surpassed all records dating back to 1964.

Given more rainfall is one of the likeliest symptoms of a changing climate, the new report from the Helen Clark Foundation and WSPSponge Cities: Can they help us survive more intense rainfall? – is a timely (and sobering) reminder of the urgency of the challenge.


Pipe dreams

The “sponge city” concept is gaining traction as a way to mitigate extreme weather, save lives and even make cities more pleasant places to live.

This is particularly important when existing urban stormwater infrastructure is often already ageing and inadequate. Auckland has even been cutting spending on critical stormwater repairs for at least the past two years.

Politically at least, this isn’t surprising. Stormwater infrastructure, as it is currently built and planned, is costly to develop and maintain. As the Helen Clark Foundation report makes clear, New Zealand’s pipes simply “were not designed for the huge volumes they will have to manage with rising seas and increasing extreme rainfall events”.

The country’s current combined stormwater infrastructure involves a 17,000 kilometre pipe network – enough to span the length of the country ten times. The cost of upgrading the entire water system, which encompasses stormwater, could reach NZ$180 billion.

This contrasts starkly with the $1.5 billion councils now spend annually on water pipes. The report makes clear that implementing sponge city principles won’t wholly solve flooding, but it can significantly reduce flood risks.

Trees and green spaces

The real bonus, though, lies in the potential for sponge city design to reduce dependence on expensive and high-maintenance infrastructure.

There are already examples in Auckland’s Hobsonville Point and Northcote. Both communities have incorporated green infrastructure, such as floodable parks and planted wetlands, which kept nearby homes from flooding.

But the report’s recommendations are at odds with some of the current political rhetoric around land use policy – in particular “greenfields” development that encourages urban sprawl.

The report urges that cities be built upwards rather than outwards, and pushes back on residential infill development encouraged by the Medium Density Residential Standards.

Citing a recent report on green space from the Parliamentary Commissioner for the Environment, the Helen Clark Foundation report argues for the preservation of urban green spaces – like backyards – as part of the flood mitigation approach.

Preserving tree cover is another urgent priority. Trees help absorb rainfall, reduce erosion and provide essential shade and cooling in urban areas – counteracting the dangerous urban “heat island” effect. Citing data from Global Forest Watch, the report states:

Auckland has lost as much as 19% of its tree cover in the past 20 years, Dunedin a staggering 24%, Greater Wellington around 11% and Christchurch 13%.

Incentives for homeowners

Making Aotearoa New Zealand more resilient to extreme weather, the report says, need not break the bank.

It recommends raising the national minimum standards governing the percentage of the total area of new developments that must be left unsealed. This would ensure the implementation of sponge city concepts, and see buildings clustered to maximise preserved green space.

The government should also require local councils to plan for and provide public green spaces, and to develop long-term sponge city plans – just as they do for other types of critical infrastructure.

Neighbourhoods could be retrofitted to include green roofs, permeable pavements and unsealed car parks. Land use and zoning could also encourage more vertical development, rather than sprawl or infill housing.

The government could also provide incentives and education for homeowners to encourage minimising sealed surfaces, unblocking stormwater flow paths, and replacing lawns with native plants and rain gardens.

More extreme weather and intense rainfall is a matter of when, not if. As the Helen Clark Foundation report makes clear, spending future billions is less of a priority than acting urgently now.The Conversation

*Timothy Welch, Senior Lecturer in Urban Planning, University of Auckland. This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Fantastic piece. Followed through to it's logical conclusion, cities de-urbanise; they increase space per citizen; they increase solar energy-capture per person (and that aids house-temp, aids food-production, both of which are external demands demanding resources and energy).

Ex fossil energy, cities are in trouble; none made it to over 1 million, prior. Not only do we have to adapt to the exhaust-ramifications, we have to adapt to not having them too. More space per person is the only valid game in town.


The expansion of large carparks hasn't helped. Quite strange considering the value of the land they are built on.they should be smaller, and required to be multilevel.


Our legacy colonial city models, as seen in many countries, were established on harbours and waterways in line with the available transport access at the time. The ensuing ad hoc expansion has now been shown to have been a poor choice for the long term and future climate projections. A simple failure to actually plan by those charged with doing just that.

We could do much better than going through the very costly process of 'retrofitting' by calling time on these existing worn out locations. Leave them as quaint seaside antiquities for the nostalgic, maybe to succumb to sea level rise anyway, and actually plan efficient new cities.

They could be built to minimize risk from all the things our current major cities and towns are currently prone to with durable infrastructure, transport, energy savings and reduced environmental effects built in. Seems too easy really. 


This makes too much sense that councils will just ignore this advice


Since 70% odd of electorate think they shouldn’t pay more tax but want extra services, it won’t get done


My area of South-West Christchurch is getting quite spongy - several new storm water areas with new native plantings, new developments going in with large storm basins. The recent heavy rain didn't lead to any major flooding when a few years ago there would have been houses underwater. I think the Council has done a good job and we've gained some nice walk/bike spots. 

We are trying to do our bit in the backyard, adding drainage and water retention to keep more of what falls on us for the veggies. Half the lawn has turned into veggies or native planting. This is something we can all contribute to. 


Great article, water is a valuable resource and we need to look at opportunities to make use of it rather than to flush it away.  Urban Stormwater Management Manual
for Malaysia
  shows how they plan to move from a conveyance orientated approach to storage for storm water management.