Back in the 1940s when the majority of Ōwairaka was developed, combined networks were a common option to take both stormwater as well as wastewater away in the same pipes. Over the years as the community has grown, these pipes have reached their capacity, often resulting in overflows and flooding during heavy rain. Piritahi is now in the process of separating the wastewater and stormwater networks to better service the growing community.
When designing new stormwater networks, it’s essential to consider how water will be treated and retained when it ends up in our stormwater pipes. Normally, the solution is installing large treatment devices called rain gardens. However, space confinement meant this wasn’t an option in Ōwairaka, so Design Lead and Civil Engineer, Kane Willcox and his team had to get creative.
Ultimately, the design solution came in the form of 32 industry-first concentrated rain gardens, which were custom-designed using Stormwater360 Filterra biofiltration devices. These were to be installed alongside the new 1.65-metre-wide stormwater pipes.
What are rain gardens?
Rain gardens are drainage structures used to filter rainwater through plants, soil and other materials. They help to absorb contaminants that could otherwise end up in our waterways and help in slowing down and regulating the flow of rainwater into connected stormwater networks to help enhance water quality.
What is unique about Ōwairaka’s custom Filterra rain gardens?
Where a normal rain garden is around the size of a car park, one of these concentrated rain gardens requires about a third of that space.
• Two-part design.
As well as being significantly smaller in size, Ōwairaka’s rain gardens have also been customised and feature a two-part design. Ordinarily, a Filterra rain garden would discharge treated stormwater directly into the wider stormwater network. In Ōwairaka, the treatment devices are being stacked atop a second concrete box which is empty and bottomless. The 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 box below in this way is not just a first for Piritahi, but also the first time this has been done in New Zealand.
What are the benefits?
The rain garden’s smaller footprint allows for quicker installation with less disruption than traditional rain gardens and minimal maintenance costs once installed. As much of Ōwairaka is built on hard basalt rock, this means less rock breaking and excavation is required causing less disruption during installation and in the future. The reduced size also means mature protected trees in the neighbourhood have more room to grow.
What were the obstacles?
The new combined technology and the complexities associated with the construction process meant that planning and resource consenting was no easy feat. It required months of work and collaboration between teams across the alliance, mocking up 3D models to get this innovation off the ground.
And the obstacles didn’t stop there. The complex network of tree roots, live underground and overhead services, and old and brittle cast iron water mains, mean the actual installation has been another challenge that our construction team continue to successfully navigate.
The installation of these innovative new rain gardens is well underway in Ōwairaka and we are now exploring their use across our other neighbourhoods.
A massive well done to the team - Kane Willcox, Avi Achary, Hamza Al-Mufty, Hamish Sandilands, Mandy Price, Michael O’Shea, Neo Li, Philip Bellard, Sean Lin, Ben Chester and Sarah Karlsen for getting this innovation off the ground, and a huge thank you to all those involved in this complex project over the last two and a half years, including the team at Stormwater360.