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Raingardens Transform Site from Mean to Green


Thursday 15 November 2007

Raingardens Transform Site from Mean to Green

Tarata Stream and the Manukau Harbour will be even cleaner thanks to innovative environmental initiatives for filtering stormwater at a construction site in Mangere.

Virtually every drop of water that falls on the carparks and driveways at 30 Hall Ave will be filtered through four ‘raingardens’ filled with over 350 native plants, and a grassy ‘swale’, before being discharged into the public stormwater system.

Not only is this great news for local waterways, but the community will benefit too. Before development, contaminated silt washed directly into an open drain, and then into the Tarata Stream. The open drain - now properly piped and hidden - was a dangerous hazard to children, a prolific mosquito breeding ground, and was often clogged with unsightly rubbish – including everything from car-parts to sofa-beds.

The 355 square metres of raingardens were designed by consulting engineer Mark Essex of Te Atatu, in consultation with the Auckland Regional Council (ARC). He says the main purpose of the gardens is to purify the rainwater runoff from the carpark and driveways before it re-enters the environment. Water from the roof will be clean enough to flow directly to the stormwater system.

“The gardens filter silt and chemicals from the water which would otherwise end up in the environment – clogging waterways, damaging native bird and animal life, and ultimately killing shellfish and other marine life,” says Mr Essex.”

Raingardens are a popular concept overseas, and are becoming increasingly incorporated into new urban developments in New Zealand as a way to decrease detrimental human impact on the environment. These deep, carefully mixed soil systems are densely planted with specially selected native trees and shrubs. When it rains, water collects in the raingarden’s shallow surface depression, where it filters down through the soil into a deeper scoria layer and out into the stormwater pipe system. This minimises the negative effect of harmful pollutants such as car oils, brake-pad dust, and associated heavy metals like lead, zinc, and cadmium that would otherwise enter the Tarata Stream, and ultimately the Manukau Harbour. The raingardens also slow the volume of water entering the stormwater system during periods of heavy rain.

Besides helping filter the water when it rains, the plants also remove a lot of the water and dissolved pollutants during dry weather by a natural process called ‘evapotranspiration’. Native plants such as coprosmas, flax, pittosporum, and carex grasses are used for their ability to effectively transpire water containing dangerous chemicals.

Landscape consultant and Lynfield resident Kim Keach says that native plants are remarkably tolerant and ideal for use in raingardens.

“Natives do well in both dry weather and in wet conditions, and the varieties we have selected should thrive because the roots don’t mind having ‘wet feet’. Another advantage of using natives is that these plants encourage the presence of native birds in the area,” says Kim.

In total there will be over 1000 native plants on the property, including 12 pohutukawas that have been planted to beautify the motorway boundary.

The project also includes a 90 metre grassy area called a ‘swale’. This is a sloping surface filter system which works in conjunction with the four raingardens to collect and trap over 75% of the suspended sediments in water run-off from the site.

Besides being clean and green, the project has plenty of positive spin-off for the local community.

“Not only have we improved the quality of water leaving the site and entering Tarata Stream,” says Mr Essex, “but we have increased the biodiversity of the area, eliminated the dangerous open drain, and removed over 3400 tonnes of contaminated soil. We’ve been able to beautify the neighbourhood with a nice green site without adding to flooding problems, and it’s likely that this will improve overall real-estate values in this area.”

On completion the site will be home to three Kingdom Halls for meetings of Jehovah’s Witnesses who will meet in English and five other languages. There will also be a freestanding two-bedroom unit along the front boundary to maintain the residential feel of Hall Ave. The project is being built entirely by volunteer labour, and site-works will continue until the end of November 2007. Major construction will take place over a 12 day period in February 2008.

Planting of the first raingarden will take place on Saturday 17 November.


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