Ōwairaka Te Auaunga Stormwater upgrade brings multiple community benefits

PlanningThree Waters

The stormwater upgrade in Ōwairaka, an Auckland suburb just 8 kilometres from the CBD, is one of the Piritahi Alliance’s largest infrastructure projects. It’s also one of the most innovative using custom-designed rain gardens to treat stormwater in a way that hadn't been done before in Aotearoa New Zealand.

The Ōwairaka underground system was built in the 1950s and its stormwater and sewerage systems were linked as a combined network, so rainfall caused regular overflows involving raw effluent. With the planned increased density to Ōwairaka, upgrading and separating the network was essential to service the neighbourhood.

Costing $33m at completion in 2022, the Ōwairaka Te Auaunga stormwater upgrade involved removing aged stormwater/wastewater and upgrading the existing networks to provide the necessary infrastructure to enable 1,000 new warm, dry, homes to be built in the neighbourhood.This included a 5km new stormwater piped network and upgrades to the original network. It involved coordination with numerous parties, including homeowners, Healthy Waters, Watercare and Auckland Transport to accommodate backbone infrastructure within a ‘dig-once’ policy. 

One wrong move could cause massive power blackouts across Auckland.

Critical thinking to overcome complex challenges

Designing and constructing a dedicated stormwater system to operate separately from sewerage and doing it with the least possible impact and disruption was hugely challenging.

The upgrade met significant challenges at every step, including six-metre-deep basalt rock excavations in a densely populated community, the need to protect mature trees and the sensitive balancing act of only touching once. Space confinement meant that installing large stormwater treatment devices, considered standard practice, wasn’t an option in Ōwairaka. 

Further obstacles to be navigated included a complex network of tree roots, live underground and overhead services, and old and brittle cast iron water mains. With the site intersecting with a network of large electrical transmission cables that power the adjacent Waterview tunnel, careful design and construction methodology was required. One wrong move could cause massive power blackouts across Auckland.

The new combined technology and the complexities associated with the construction process also meant that planning and resource consenting were challenging. It required months of work and collaboration across the Piritahi Alliance, mocking up 3D models to get this innovation off the ground.

Overarching everything was the need to think strategically about future growth in the area. Sensible design was needed to avoid digging up the same land in the future, with a dig once policy ensuring that where people’s properties were impacted, we would only touch once, ensuring the impact on their homes and their lives was minimised.

Innovative design solution

A New Zealand-first design solution saw custom-designed Filterra rain gardens installed alongside new 1.65-metre-wide stormwater pipes. The bespoke rain gardens require about one-third of the space of regular rain gardens. They have a two-part rain garden design, with the treatment devices stacked on top of a second concrete box which is empty and bottomless. This lower box acts as a retention device, allowing water to soak into the natural basalt rock layer, recharging the groundwater network below.  While it was designed specifically to overcome a number of challenges and obstacles regarding the stormwater network at Ōwairaka, the innovation will be used across other neighbourhoods at Piritahi.

Additionally, the unit’s smaller footprint allows the mature trees in Ōwairaka their best chance at survival. Ongoing maintenance of the rain gardens is also minimal with fewer associated maintenance costs.  Concentrated devices treat and retain stormwater simultaneously in a far smaller space than traditional rain gardens. 

How to best manage community impact was a major consideration throughout the project. While the Filterra raingardens required less rock breaking and excavation, additional methods employed including pepper drilling, fracture expansion and rock breaking, an approach that reduced noise compared to commonly used methods, all the while, protecting localised services.

Multiple community benefits

Overcoming many challenges and delivering effective solutions for the project has delivered multiple benefits to the community, including cleaner water through creeks and rainwater systems and an overland flow path, providing protection from regular flood damage. The system was properly tested in the 100- year flood event that caused heavy flooding across the Auckland isthmus in March 2022 and again in January 2023.  With unprecedented amounts of rain (539mm) including one day’s rainfall doubling the monthly rainfall, there was extensive flooding and damage across the city.    We are pleased that the infrastructure we have constructed performed as it was designed to.

Kāinga Ora and Piritahi worked together on every aspect of the project to deliver this complex work, including extensive community engagement throughout the construction period. The project is an exemplar project, presenting achievements Kāinga Ora and Piritahi are making together with our Alliance partners.  

The project was a finalist in the 2022 Civil Contractors NZ Hirepool Construction Excellence Awards in the $20-$100m project value category.