Auckland has 137,000 buildings at risk of flooding

Auckland has 137,000 buildings prone to flooding and there will more torrential rain and flash flood events like those that swept through homes and businesses in Piha and nearby Kaiaua in the past month.

The scale of Auckland's vulnerability to floods is set out in a council document that went online at the weekend around the time a Piha park ranger reported the worst and fastest flood he has seen in 30 years.

Sixty people were evacuated by boat down the two-metre-high waters running down a Piha road on Saturday evening. A MetService weather station recorded 27.5mm of rain at Piha in one hour.

"Flooding is one of the most significant and frequent natural hazards in the Auckland region," says a resource consent application by Auckland Council to discharge stormwater.

The document estimates that 137,000 buildings are prone to some form of flooding, of which 16,000 are at risk of flooding above floor level. There is a 1 per cent chance of a flood event in any year.

Climate change is predicted to result in the same or less rain in the Auckland region, says the document, but alter the intensity and frequency of extreme rainfall events and lead to rising sea levels. When Auckland was hammered by ex-tropical cyclone Fehi on Thursday, Tamaki Dr was closed to rush hour traffic after a king tide.

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The consent application said flood risk needs to managed and mitigated to avoid the loss of human life, protect buildings and property, and minimise disruption to infrastructure, and social and economic costs.

A key to managing and mitigating flood risk is accurate flood models to help predict the extent and height of flooding, said the application.

It also warned that intensification potentially increases the risk of flooding, but provides significant opportunities to reduce flood risk.

Lynn Yeager, owner of The Pink Shop in Kaiaua, begins the clean up after January's floods. Photo / Jason Oxenham

he council is seeking a resource consent to put Auckland's "patchwork" of consents from pre-Super City days under a single consent for 35 years with six-yearly reviews.

It is required for the diversion and discharge of existing and future stormwater from the public network and the impervious areas that contribute to them.

The city's public stormwater network comprises 6000km of pipelines, 145,000 manholes and more than 900 stormwater detention and treatment facilities.

As well as flood effects, the resource consent addresses the effects on streams, the coast, groundwater, wastewater, growth and managing assets.

Council declined a request to interview Craig McIlroy, the head of its stormwater division Healthy Waters, but issued a statement saying it was working to help create communities to withstand storms, flooding and other climate change effects.

A spokeswoman said it was unable to give details of the most flood prone areas in Auckland given the timeframe and access to the data on a Sunday.

The statement said council had commissioned a report by Niwa that found Auckland's climate will get hotter over the next 100 years with more extreme bursts of rainfall and rising sea levels.

Council is very conscious of these changes and using them to inform regional planning and design standards for stormwater, coastal erosion and coastal defences, the statement said.

David Abbott, of the Stop Auckland Sewage Overflows Coalition (SASOC), said the more heavy torrential downpours there are, the more important it was to get new systems in place to upgrade infrastructure to trap stormwater before it gets mixed with wastewater.

Auckland faces a huge problem with its old combined stormwater and wastewater pipes, which overflow from 41 points around the inner city suburbs into the Waitemata Harbour every times there's more than 5mm of rain.

Mayor Phil Goff, who was unavailable for comment yesterday, has proposed a targeted rate of $66 for the average ratepayer in this year's new 10-year budget to raise $400 million for spending on stormwater to improve the city's water quality.