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Industry first for stormwater management


9 Sep 2021

In October 2019 the Government brought together KiwiBuild, Housing New Zealand (the country’s largest landlord), and its development subsidiary, HLC, to become Kainga Ora (Homes & Communities) to enable a more cohesive, joined-up approach to delivering the Government’s priorities for housing and urban development. One area currently being transformed by Kainga Ora is Owairaka, an area that is part of the greater Mt Roskill suburb in central Auckland.

According to the 2018 consensus, Owairaka has a population of 5268 and has the highest percentage of overseas-born Kiwis (47.5 percent). Asians make up almost 40 percent of the local residents. Owairaka has also been in a state of development for almost a decade as it is next to the Waterview Tunnel which finished construction in 2016. Then, in 2018, the government started pulling down hundreds of state houses in the area for a new development called the Roskill Development.

Construction of Owairaka’s stormwater network began in late 2019 and is one of Piritahi’s largest infrastructure projects.


This Kainga urban development project is being carried out by Piritahi (which translates as coming together), an alliance of companies delivering over $1 billion of land development and infrastructure works for the Government.

The Roskill Development is separated into new neighbourhoods and Piritahi has been preparing sites in areas of Mt Roskill for a massive housing development involved and designing and constructing new and upgraded infrastructure for the area.
The Roskill Development will see 11,000 new terraced and apartment homes delivered over the next 15 years. These new homes will be a mix of state, market and ‘affordable’homes. Infrastructure also involves new parks and public spaces and new stormwater networks.

The stormwater project

Construction of Owairaka’s stormwater network began in late 2019 and is one of Piritahi’s largest infrastructure projects. It’s also one of the most innovative, using custom-designed rain gardens to treat stormwater in a way that hasn’t been done before in this country. Most of the area was developed in the 1940s, following World War II. Back then, combined wastewater/stormwater networks were common, but can lead to sewage run-off into waterways during heavy rain and flooding. With the planned increased density to Owairaka, upgrading and separating the network was essential.

The design solution

When designing new stormwater networks, they must comply with stormwater management requirements such as considering treatment and retention options. In other areas where Piritahi is working, the solution to managing new stormwater capacity issues is often found in installing large stormwater treatment devices.

However, space confinement meant this was not an option in Owairaka, so Kane Willcox, Piritahi’s design lead for this project, and his team had to get creative. The design solution came in the form of 32 industry-first, custom-designed Filterra rain gardens, to be installed alongside new 1.65-metre-wide stormwater pipes. The most significant difference between standard rain gardens and Filterra concentrated rain gardens is their size. Where a normal rain garden is around the size of a car park, a Filterra rain garden requires about a third of that space.

The design solution for space confinement came in the form of 32 industry-first, custom-designed Filterra rain gardens, to be installed alongside new 1.65-metre-wide stormwater pipes.

Two-part rain garden design

A Filterra rain garden would usually discharge treated stormwater directly into the wider stormwater network. In Owairaka, the treatment devices are being 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. Combining the Filterra rain garden with retention below in this concentrated way is not just a first for Piritahi, but also the first time this has been done in New Zealand.

The protection of mature trees had to be considered with this project and the unit’s smaller footprint allows the trees their best chance at survival.

Ongoing maintenance of the rain gardens is also minimal with fewer maintenance costs associated with it.

Concentrated devices treat and retain stormwater simultaneously in a far smaller space than traditional rain gardens. This means they require less rock breaking and excavation and can be installed faster with less disruption to surrounding residents. The installation of these rain gardens is now underway and the Piritahi team is now exploring their use across other neighbourhoods and projects.

The design solution for space confinement came in the form of 32 industry-first, custom-designed Filterra rain gardens, to be installed alongside new 1.65-metre-wide stormwater pipes.


This Story is from the September/October issue of Water