Planning for a central storage tank
For a healthier harbour and coastal waters, we need to reduce the amount of untreated wastewater overflowing from the network. Overflows happen when wet weather increases the amount of water flowing through the network beyond the amount the pipes can carry.
To reduce overflows, we’re planning to build a new storage tank in central Porirua (see conceptual videos below), located north of the railway station (see map below for location). The tank will:
Capture and hold wastewater at times of peak flow, such as after heavy rain, then gradually release it at levels the network can carry to the treatment plant.
Store up to 7 million litres of wastewater, almost equivalent to three Olympic size swimming pools.
Significantly reduce overflows into Porirua Stream.
While the Porirua central storage tank will not stop all overflows from the wastewater network, it is an important part of the solution.
This map indicates the proposed location and position of the tank.
Here are some conceptual videos of the site and tank, but please note, the design is still a work in progress. So what you see here is not a true representation of the finished product.
The tank can be described as either storage or retention. Both terms have been used to describe it and it means a structure that temporarily holds wastewater when there is heavy rainfall. So, regardless of the term, the tank will only hold wastewater temporarily until it can be pumped on to the wastewater treatment plant.
Currently when it rains heavily, pipes and pump stations can get overwhelmed by the volume of water, and some wastewater may overflow into the environment.
The tank will capture and hold stormwater and wastewater at times of peak flow, then gradually release it at levels the network can carry to the treatment plant.
The current City Centre pump station “20” is a bottleneck and the largest source of overflows into the Porirua Stream. The tank, and new pump station linked to the tank, will take the pressure off pump station “20”.
The tank will only hold stormwater and wastewater temporarily, at times of peak flow during and after heavy rain.
Find out more about the wastewater network here
Construction is scheduled to begin in the first half of 2022, subject to the resource consent process being completed.
The fence is to make the site safer for staff who are on site to undertake investigations and detailed planning.
There is currently no formal access to the site. At the start of construction we will create a new entry point from State Highway 1. We will also be constructing a bridge across the Kenepuru Stream from the railway station carpark, to enable safe exit from the site and ongoing safe access after construction.
Yes, we are planning extensive planting, with a combination of kowhai/ngaio coastal forest, kahikatea/ marsh ribbonwood swamp planting, and carex, tussock and rush on stream margins. Planting will screen the tank from view.
We are working with Ngāti Toa and Porirua City Council on the landscaping planning.
Yes, Ngati Toa has been consulted and is supportive of the tank because it will contribute to reducing pollution of the harbour from wastewater overflows. Ngati Toa continues to be closely involved in detailed design and planning.
Several sites were considered in and around the centre of Porirua and assessed against a number of criteria. The current site was preferred for a number of reasons including:
- It is currently unused, so there is no disruption of current users
- It is publicly owned
- It is well located for network integration as the three main sewers to feed into the storage are both located within the land parcel.
The tank project is estimated to cost $46 million.
We are carefully planning construction to minimise the impact on the environment, including both the Kenepuru and Porirua Streams. Over time, the tank, pipe upgrades and other improvements will reduce wastewater overflows which will be good for the health of the streams and the harbour.
Construction of the tank and related pump station and pipes is being planned to avoid the wetland as much as possible. The existing wastewater pipes that sit under the wetland are being replaced and diverted around the wetland. We will restore areas impacted by construction.
We are working with ecologists from Beca and Greater Wellington Regional Council to assess the ecology of the site and make plans to minimise disturbance. The assessment will reflect the proximity of the site to the railway line and State Highway 1, which means there is an existing level of background noise.
Details will be set out in the resource consent application.
Wellington Water is aware of the estimated location of known fault lines in the Porirua area, and these do not go directly through the proposed tank site. We have completed extensive geotechnical investigations on the tank site to assess the seismic risk from liquefaction and geological hazards, so that we can take this information into account in the design. New Zealand engineers are well-experienced in designing infrastructure in areas of potential seismic risk, and the tank will meet all relevant standards.
This tank, as with all new Wellington Water infrastructure, is designed to take account of climate change predictions, including predicted sea level rise and increased intensity of rainfall.
The tank is one of a number of improvements to the wastewater network that will together ensure the network is able to manage predicted population growth.