Green roof to soak up city's problem runoff

Tottering home in Awanui St, Birkenhead. Picture / Dean Purcell

It might make most people yawn but Earl Shaver loves talking about stormwater.

The Auckland Regional Council staffer's pet project is a $350,000 trial "green roof" on top of Auckland University's Engineering Faculty - a patchwork of gardens with various plants and soils aimed at finding the best combination for New Zealand conditions.

Overseas, green roofs have been shown to reduce stormwater runoff by up to 90 per cent.

Plants and porous soils absorb rainwater, filter out contaminants and allow water to evaporate.

One problem, particularly bad on the North Shore, was covering the land with "impervious" surfaces - material that doesn't soak up water such as concrete - including the house, driveway, paving, paths and barbecue areas.

Mr Shaver said: "They make an assumption it's 60 per cent [of a section] but it's often more, so then the pipes being put in are not big enough, the water overflows and who knows where it goes?"

No-one has any idea how widespread the problem of illegal stormwater drains or pipes might be.

Mr Shaver said: "You could be talking hundreds. People want to solve a nuisance so they just do it and they don't tell anyone and there's no way of knowing until something happens."

Stormwater is a billion dollar problem in Auckland but one councils are shy of tackling given the likely ratepayer reaction to the bill.

Two reports in 2004, by consultants PricewaterhouseCoopers and Boston Consulting, estimated the cost of fixing the stormwater problems over the next 20 years as being in the "low billions".


Going, going gone ...

* Soil forms on top of rock as rain and sun help to break down vegetation over hundreds if not thousands of years.

* Around Auckland, this soil or clay layer can be as shallow as 2m and sits like a blanket on top of sandstone, the rock common around the Waitemata Harbour.

* Heavy rain, coupled with a lack of large tree roots holding the soil together or efficient drainage, saturates the soil layer, weakening its structure by separating particles.

* The weight added from the water is like soaking a woollen blanket. With some or all of those conditions, the soil simply slides off the rock and plunges to the bottom of the cliff.