Will Auckland's giant new wastewater tunnel reduce flooding?

Thursday, 2 February 2023

Exclusive look inside Watercare's Central Interceptor tunnel, when it reached a milestone of 1km bored underneath Auckland. (Video published June 2022)

Auckland is building a $1.2 billion wastewater pipe designed to prevent 80% of wet weather overflows – but would it have curtailed Friday’s flooding?

The answer is both yes and no.

The Central Interceptor will significantly reduce the number of wet weather overflows in central Auckland when it’s operational in 2026, but no infrastructure is calibrated to cope with the record-breaking volume of rain that fell on the city in the past week.

The giant pipe is not a flood mitigation device, Watercare’s Central Interceptor executive programme director Shayne Cunis said.

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Hiwa-i-te-Rangi Tunnel Boring Machine completed the Manukau Harbour crossing in December 2022.

“But the level of wastewater that would have been released into waterways would have been greatly reduced, so there would still be considerable benefits to the environment,” he said.

The 4.5m diameter tunnel is designed to prevent four out of five wet weather overflows, but will not make much difference to overall flooding in overwhelming events such as those seen this week, Cunis said.

Floodwater on Brewster Ave, Pt Chevalier, was high enough for Aucklanders to kayak.

The project, which includes the 14.7km Central Interceptor tunnel with an overall capacity of 233,000m³ and two branch sewers, was designed to solve the problem of wastewater and stormwater mixing in a combined network of pipes in central Auckland.

In heavy rain, stormwater overwhelms the pipes – resulting in wastewater and stormwater overflowing into streams. Even before the worst of the rain hit the region on Friday, 11 Auckland beaches had already been “black-flagged” because of sewage overflows.

A chart in a Watercare presentation on the Western Isthmus Water Quality Improvement Programme shows the improvement expected in the wastewater network from 2017 to 2030.

Downpours on Friday and overnight on Tuesday saw stormwater flood Auckland properties and health authorities warn people to avoid potentially contaminated floodwater.

The Central Interceptor will capture wastewater and stormwater in older parts of Auckland – such as Mt Albert, Mt Roskill, Grey Lynn and Herne Bay – and take it to the Māngere Wastewater Treatment Plant for processing, Cunis said.

The $1.2 billion project is on budget and on track for completion in 2026, although it will take longer if resource consent is granted to extend the tunnel to Pt Erin in Herne Bay. The May Rd, Mt Roskill, south section is due to begin operating before the main tunnel reaches its destination.

Associate professor Asaad Shamseldin at the University of Auckland said moves need to be made to manage stormwater more sustainably.

Neither the Central Interceptor tunnel nor branch sewer C tunnel were flooded in this week’s storms, but two shafts under construction were flooded, Cunis said – pump station 25 site at Miranda Reserve, Avondale, and the other was the Mt Albert War Memorial site at Wairere Ave.

“Inspections have taken place and further monitoring will continue in coming days. Both rain events occurred after most sites had finished for the day. We ensured all staff were accounted for and were safe and well,” Cunis said.

The tunnel boring machine Hiwa-i-te-Rangi was due to resume tunnelling on Thursday and micro-tunnel boring machine, Domenica, will restart on link sewer C in Avondale next week.

University of Auckland associate professor Asaad Shamseldin, a specialist in urban water management and climate change said combined wastewater and stormwater systems were standard at the time the infrastructure was built in central Auckland.

However, things have changed and in most parts of Auckland wastewater and stormwater are separate, Shamseldin said.

In terms of flooding, he didn’t think the Central Interceptor would make much difference.

Shamseldin said moves need to be made to accelerate the sustainable management of stormwater in order to reduce the impacts of rainfall and improve water quality.

Changes should include better management of stormwater at the source, reducing impervious areas and preserving natural hydrology (the movement of water), he said.

Flood mitigation measures would come at a cost and in some cases it might be cheaper for authorities to buy flood-prone land than make infrastructure changes, Shamseldin said.

And with extreme rainfall events expected to increase in frequency and magnitude, he said there needed to be a new design standard.

”Should we adopt one standard for all or tweak the design for certain parts of the city? That will be the question that has to be addressed. There has to be a discussion between government, local government and the community.”