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Invercargill stormwater drainage systems below New Zealand standards

May 02, 2016

{Invercargill consulting engineer Nathan Surendran is calling for greater public debate around stormwater drainage and management in the face of global climate change. ROBYN EDIE/FAIRFAX NZ}

Invercargill's storm drainage systems are below the New Zealand standard - but city leaders say improving them would cost too much money.

The Invercargill City Council (ICC) presently spends $1.42 million per year on a drainage system that has a one-in-five chance of flooding, known as the annual exceedence probability (AEP).

The probability is twice as high as the one-in-ten recommended by Standards New Zealand.

ICC drainage and solid waste manager Malcolm Loan said upgrading the system to a 10 per cent AEP would cost about $140,000 per year.

He said the standards were not compulsory.

"The predictions for sea level rise and climate change are quite alarmist," he said.

"We don't want to put in a system that's going to bankrupt the city."

Loan said the ICC's current standards - which apply to new construction and the upgrading of existing storm drains - were developed in 1985, and were adequate for dealing with storms that produced a large amount of moisture in a short time.

"We don't see any particular reason to change that. The reality will be some storms may result in ponding in some places. Any system we design can be beaten by a greater rainfall."

A March storm that resulted in water flooding Stead St was particularly concerning because water flowed over floodbanks without much rainfall, Loan said.

The water mostly came from a combination of a high tide and strong westerly winds, he said.

Invercargill consulting engineer Nathan Surendran presented a submission at the ICC's Infrastructure and Services meeting on April 26 calling for greater public debate about the city's drainage system.

"The conversation we need to have as a city is, what are the benefits of that system? What are the costs to ratepayers?"

It was concerning the ICC had not changed its storm drain standards since 1985, particularly in the face of climate change, he said.

"There will be things that we'll have to let go of as we adapt to climate change," he said.

"One of those things is being able to do what we want, wherever we want, how we want. They're looking at it on a capital cost basis, versus a life cycle cost."

Surendran pointed to an April report by the Royal Society of New Zealand, "Climate change implications for New Zealand," that states damaging floods are expected to increase in frequency and intensity.

"The current estimated Annual Exceedence Probability of a 1-in-440 year event would increase to around a 1-in-100 year event by the end of the century in a low carbon world, and closer to a 1-in-50 year event in a high carbon world," the report says.

Standards New Zealand is a part of the Ministry of Business, Innovation and Employment (MBIE).

MBIE did not respond to queries in time for publication.

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