Harbour tunnelling gets underway as part of the transformational southwest wastewater scheme

Next week we will start tunnelling a new harbour outfall at Clarks Beach as part of the southwest wastewater servicing scheme. This will improve the quality of the Manukau Harbour with significant benefits for the community and environment.

On Monday, May 12 a 1.2-metre-wide tunnel-boring machine (TBM) will get the ground moving on the $22 million Clarks Beach Outfall wastewater pipeline.

Programme delivery manager Dave Kennerley says the treated wastewater will be discharged approximately 100 metres into the Waiuku Channel – a highly dynamic part of the harbour where it will quickly disperse.

“The outfall is a crucial part of a wider programme of work that will support the projected population growth of Auckland’s south-west which is expected to grow to around 30,000 people by 2050.

“Initially, it will carry highly treated wastewater from the Clarks Beach Wastewater Treatment Plant, which we’re currently upgrading. But it has been designed and sized to support future population growth in the wider area.”

Project manager Jason Salmon says the outfall installation is expected to take six or seven weeks.

Photo: The tunnel boring machine (TBM) will take about seven weeks to finish its journey

"To ensure the resilience of the outfall, the pipeline will be double layered.

“The carrier pipeline, which is made from high-density polyethylene, will sit inside a steel pipe to prevent any leaks or ground and seawater intrusion.

“To install the outfall the TBM will cut through the ground and install the exterior steel pipeline at the same time.

“Once it’s reached its destination, the carrier pipe will be pulled through and plugged until it is brought into service. A 66-metre-long diffuser will then be installed.”

The diffuser includes 22 rubber nozzles called ‘duck bills’, due to their shape resembling a duck’s bill.

The nozzle design allows the periodic release of treated effluent to flow out but stops sea water flowing in.

Salmon says once the TBM finishes its journey it will arrive at a receiving pit 10 to 15 metres under the sea.

“The TBM will be lifted out by a team of divers who will unbolt it from the carrier pipe in the outfall, attach lift bags to it and winch it out on to a pontoon.

“Once it’s on the pontoon it will be towed to Onehunga Port and lifted back onto land.”

Update on Clarks Beach Wastewater Treatment Plant project

The upgrade to the Clarks Beach Wastewater Treatment Plant introduces sophisticated technology that will allow the plant to produce exceptionally high-quality treated wastewater, which ultimately benefits the Manukau Harbour.

It also supports population growth in Clarks Beach and Glenbrook Beach in the short term.

Project manager Sophia Chan says work has progressed well with all the main tanks now on site and the civil construction of the inlet structure nearing completion. A new power supply has also been installed and fit-out has begun.

Photo: Bird's eye view of the launch site at the Clarks Beach Golf Club

“We’ve also built a small temporary wastewater treatment plant, which will be operational in June.

“This will allow us to treat wastewater to a high standard uninterrupted while we continue to build the main wastewater treatment plant.

“Both the treatment plant upgrade and the outfall are on track to be completed by June next year.”

Meanwhile, our team has been engaging the community on several short-listed options for the wider programme of work that will support the projected population growth to 30,000 people by 2050. We will continue to share this work with the community as the optioneering process concludes.

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