new auckland

The billion dollar pipe future-proofing Auckland

Design work is nearly complete on one of the biggest infrastructure projects New Zealand has ever seen. 

Auckland’s $1 billion central interceptor is a 13-kilometre underground sewage and stormwater pipe running between Western Springs and Mangere which will take the pressure off century-old pipelines. 

Most of Auckland’s wastewater is treated at Mangere, and it flows into the plant there via seven major pipes (interceptors). However in places that structure is ageing, and combined with the huge growth predictions for the city, the new interceptor is needed. 

It will have enormous wastewater storage capacity, and will reduce wet-weather overflows into the Waitemata and Manukau Harbours by up to 80 percent. 

Any questions over the necessity for such a the project must have been nullified this week after the only fuel pipe between Marsden Pt Refinery and Auckland broke. Watercare design manager Dave Ward says the organisation's bread and butter is thinking ahead, and as every new project and its costing has to be justified repeatedly to Auckland Council, everything goes under the microscope.

"We are always looking at duplication, augmentation and repairs with wastewater infrastructure ahead of time, before failings arise," he says. "We look at growth projections and resilience .. and our plans have to be timely, not too early (so that ratepayer money is tied up) and not too late. There's a considerable amount of head-scratching and desk-based work before we get to the build stage."

So is it frustrating having a job where you plan for massive projects that no one really thinks about, no one will see when they're finished, and no one will notice unless they fail?

"Welcome to the world of public infrastructure! How many times do you hear about sewage engineers being celebrated? Things you take for granted every day - the water to your bath, your toilet, the water you drink - we are the unsung heroes of the city!" 

"We’re building the future for Auckland,“ says Watercare’s general manager of infrastructure delivery, Steve Webster. 

“It’s not just pouring concrete or putting steel in, we are making Auckland a vibrant city and helping New Zealand as a whole.”

The central interceptor is the most spectacular of a number of projects over the next 10 years that will cost $4.9b. They're the sort of schemes that are decades in the planning and design. Auckland's Unitary Plan has helped give certainty about their future path, and Watercare's local board/stakeholder engagement manager Brent Evans says they're blessed to be answerable to just one body. "It means we can plan on a 10-20 year horizon."

Other huge projects at the moment include a new pump station and giant underground storage tank in Takapuna ($30m); a new pump station nearly finished at Wynyard Quarter ($14m); a $141m expansion of the Mangere wastewater treatment plant (pictured above); a new wastewater pipeline along Wairau Rd ($14m); and the Hunua 4 watermain which future-proofs water supply to the fast developing Flat Bush and airport areas, but will have a much bigger reach when it's finished in 2020. It's worth $376m. 

But when the Central Interceptor is talked about there’s a sense of awe about the size and price of the project. The pipe will cross the Manukau Harbour at about 15 metres below the seabed, and will connect along the route to the existing wastewater network, which will divert flows and overflows into the tunnel. It will be drilled down to depths of between 22 and 110 metres under the city but with little disruption - boring machines will largely be used. 

It does not replace the Auckland isthmus' hundred year old combined stormwater and sewage system that is often blamed for nasty discharges into the Waitemata. But Evans says it's an enabler - it will take some of the pressure off that old system until it can be assessed, repaired or replaced. "There will be a solution put forward," he promises. 

Tendering for the central interceptor job comes next, construction should start in 2019, and is due to be completed in 2025. Ward says for projects of this size and scale you can't just find a contractor in the Yellow Pages - "there have to be rigorous and robust processes, and that takes time. They have to be built to all the consent requirements".

A new emergency pressure relief outfall structure at the new pump station will also be built, but Watercare insists it is unlikely to be activated more than once every 50 years for a massive storm. 

It could happen if there is a significant power failure or outage, and the onsite generator fails, and there’s a significant storm. 

It says there is no practicable alternative place or method for discharges, and there’s a very low probability of them happening. If it is activated, it will “have very localised and short-term ecological, environmental and visual effects on the Manukau Harbour with no adverse cumulative effects".   

Watercare is blunt about the state of the current pipe.

“The lower section of the Hillsborough Tunnel and Manukau Siphon is estimated to have a residual life of between 15 and 25 more years. If it fails, it could mean continuous untreated wastewater discharge into the Manukau Harbour for an unknown period of time from over 200,000 customers, including industrial flows presently treated at the Mangere Wastewater Treatment Plant.” 

Such arguments did not convince the Manukau Harbour Protection Society, Onehunga Enhancement Society and other groups which were incensed at resource consents being granted for the pipe and lodged an appeal against it, with the help of grants from the Ministry for the Environment. 

They managed to get strengthened requirements around emergency discharges and backup emergency equipment. The groups are still opposed to the project because of the damage they say such large volumes of freshwater pose to bird and marine habitats. 

Watercare has posted an explainer on the finer details of the project to YouTube. 

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