Tree-garden plan in flood fight

WILL HARVIE
Last updated 05:00 28/07/2014
Rain garden
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IMPROVING WATER QUALITY: Dean West of Evergreen Landcare plants a rain garden, or swale, beside the Christchurch Coastal Pathway, on the Causeway.

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Christchurch residents can expect to see more rain gardens as the rebuild progresses.

Rain gardens were recently planted on the Causeway, between Mt Pleasant and Redcliffs, and experimental rain gardens will be installed later this year at The Commons, as the former Pallet Pavilion site is now called.

Rain gardens, also called swales, are primarily used to improve stormwater quality, although they can play a minor role in flood prevention, Christchurch City Council engineers said.

On the Causeway, for example, passing vehicles drop hydrocarbons, bits of tyres and brake linings and various other materials onto the road. On the old Causeway, these materials drained into the estuary.

Now, rain or stormwater drains take them into the rain garden between the road and the Coastal Pathway.

Soils in the rain garden, and to a lesser extent, plants, capture the contaminants and prevent perhaps 60 per cent to 70 per cent of them entering the estuary, council water environmental engineer Ken Couling said.

"There's a big future for rain gardens in the suburbs," Couling said.

"They are our most significant tool to improve stormwater quality."

Plans are afoot to retrofit the Avon River catchment with the gardens, he said.

The increased cost is marginal, especially if landscaping is required. "If plants are going in, why not make it a rain garden?" the council's unit manager of capital investigations Keith Davison said.

Council engineers are also looking at stormwater tree pits - rain gardens with a tree in them.

The council hasn't planted any yet, but they are common in Melbourne and Couling expects them to feature in the central city. Underground pipes and cabling mean "you can't always fit what you want where you want", Couling said.

Rain gardens are "part of the toolkit available to council to clean stormwater", Davison said. At large new subdivisions such as at Wigram, earthworks can create or restore wetlands.

Where space is tight tree pits and rain gardens are suitable.

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