Will Interceptor reduce flooding?
Auckland is building a $1.2 billion wastewater pipe designed to prevent 80% of wet weather overflows – but would it have curtailed the latest 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 during the past 10 days.
The giant pipe is not a flood mitigation device, Watercare’s Central Interceptor executive programme director Shayne Cunis said.
‘‘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.
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 the Friday before Anniversary Day, 11 Auckland beaches had already been ‘‘black-flagged’’ because of sewage overflows.
Downpours that day and overnight on Tuesday last week 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 and Herne Bay – and take it to the Māngere Wastewater Treatment Plant for processing, Cunis said.
The $1.2b 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.
Neither the Central Interceptor tunnel nor branch sewer C tunnel were flooded in the latest storms, but two shafts under construction were flooded, Cunis said – pump station 25 site at Miranda Reserve, Avondale, and at the Mt Albert War Memorial site at Wairere Ave. ‘‘Inspections have taken place and further monitoring will continue . . . Both rain events occurred after most sites had finished for the day. We ensured all staff were accounted for and were safe and well.’’
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, Shamseldin said he did not think the Central Interceptor would make much difference.
Shamseldin said moves needed 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.’’