New outfall for the Rosedale Wastewater Treatment Plant

This remarkable picture of the portal to the tunnel is taken from the bottom of a 45 metre excavation. A tunnelling machine will be lowered into the pit to cut the deep tunnel under roads and public areas of the city from Rosedale to Mairangi Beach. A high resolution enlargement of the photo is available as a PDF ( 323 KB) for those who wish to examine the construction at closer quarters. Map of new outfall route

Work is under way on North Shore City Council’s new Rosedale Wastewater Treatment Plant outfall, which will discharge high quality treated effluent from the treatment plant, 2.8km out to sea into the Rangitoto Channel. The new outfall is required to cope with the future needs of our growing city, and will also discharge further out to sea than the existing outfall. It needs to be completed by the end of 2010 to meet a deadline imposed by the Auckland Regional Council.

Changes to part of proposed route

The outfall route stretches 2.6km from the treatment plant to Mairangi Bay, running deep beneath streets, parks and reserves, a limited number of commercial and residential properties, as well as council owned land. It will then run under the seabed 2.8km out into the channel. The route for the western part of the outfall has been simplified and straightened, resulting in a $7m cost saving for ratepayers, and allowing the whole length of the landbased portion of the outfall to be tunnelled at least 25 metres underground. The deep tunnel option will be much less disruptive to Mairangi Bay residents and businesses than the trenching method proposed in earlier consultation, which at times could have resulted in significant traffic issues. The council undertook extensive consultation with affected residents and businesses as part of the process for seeking land owner approvals and consent variations from the Auckland Regional Council.

Need for new tunnel and outfall

The existing outfall is nearing the end of its lifespan, is too small to cope with further growth in the city and has had major repairs in recent times. The new tunnel will have a lifespan of at least 100 years, and will have up to six times more capacity than the existing outfall. At its discharge point, diffusers will ensure the high quality treated effluent is widely dispersed, unlike the present outfall which discharges the treated effluent directly from the end of the outfall pipe just 600 metres from shore.

Construction method

The proposed onshore section of the new outfall is planned to be totally built by underground tunnelling, much of it deep underground. The tunnel will be bored from the treatment plant to a point offshore, where it will rise to join a section of pipe buried in the seabed, minimising disruption at Mairangi Bay beach. Excavated materials from the tunnel will be carried back along the tunnel to the Rosedale Treatment Plant.

Map of new outfall route