New Palmerston North subdivision prepared for rainy days
One of Palmerston Norths newest subdivisions is providing a trial ground to test the capacity of rain gardens that limit stormwater problems when the rain pours down. Rangitane o Manawatu Investment Trust has had a frustrating three years bringing its 32-section subdivision in Ashford Ave to fruition, but works are now nearing completion, and the network of rain gardens has been tested in Wednesdays downpour. Trust chair Ruma Karaitiana said when he started talking about rain gardens in 2019, people looked at him as if he was strange. That changed fast, with the city councils own Tamakuku Terrace subdivision one of several to adopt new techniques as climate change puts the spotlight on stormwater management. READ MORE: * The medium density housing developments that defied the Auckland floods - this is how they did it * Construction of Palmerston North Quest Hotel back on track as lockdown lifts * Section shortage becoming critical in Palmerston North Karaitiana said the trusts motivation was to do something that was aesthetically pleasing and environmentally friendly. The site was a challenging one, having sat largely idle for 75 years, initially intended for a school, and latterly used for some grazing and a community garden. It was set aside for Treaty settlements, which was when Rangitane bought the land. It was a low-lying, undulating site, with part of it identified as flood prone, which saw the imposition of conditions to build up ground levels and raise minimum floor levels. But as soon as earthworks began, it became obvious the site had also been used as a dumping ground. The discovery of barrels of bitumen that had been buried there some 70 years ago prompted an extensive recovery and remediation exercise. Then the stormwater management infrastructure could be put in place. Consultant civil engineer for the project Daniel Barr explained there was a network of 11 rain gardens created on the berms around Ashford Ave, its extension through to Sterling Cres, and the new Tuere St, which provided access into the second stage. The gardens were lined with rock and planted with flaxes and other hardy plants that helped to slow down, filter and partly treat the run off. But the really heavy lifting went on below. At each garden was a large, open pipe covered by a dome to prevent large items of debris falling in. Those were connected to soak pits that went down about 4 metres, to a layer within the river gravels deep below the site. And that was where the stormwater would go over a course of days, draining away slowly through the gravels into the Manawatu River system. Without them, all the run-off would be draining into the citys stormwater network and into the river almost instantly. Barr said the system was designed to cope with all the subdivisions run-off within the site for all but one-in-50-year rainfall events. If the systems capacity was exhausted, there was a secondary flow pipe under the road that would take the overflow, and an overland path towards Sterling Crescent and the Hokowhitu lagoon. Once the subdivision was complete, the rain garden network would be vested in the city council, which would take over responsibility for maintenance. Barr said there was still a lot of learning to do about rain gardens, and it would take time to understand just how effective they were, and how much maintenance would be needed. Council city planning manager Jono Ferguson-Pye said rain gardens were one means of achieving compliance in the management of stormwater through the councils subdivision engineering standards. There are also a range other engineering methods that can be used depending on the development context.