Porous paving to help ease flooding

By MELISSA CLARKE

Porous paving that filters stormwater is to be tested in central Auckland as a way of reducing pollution and flooding.

Herne Bay residents will today receive information on an initiative welcomed for its potential to alleviate ponding on low-lying properties and improve water quality in the harbour.

Metrowater spokesman Brian Sharman said the result of the trial could be less water on paved areas and cleaner water returning to the soil.

"Normal pavement, be it concrete or tarmac, is designed to have water run off it," he said.

But porous concrete allowed water to run through and into the soil underneath.

There were also larger and more gaps between the concrete blocks, to aid drainage.

"It minimises the run-off and collects some of the pollutants, for example oil."

Other contaminants in rainwater include faeces from cats and dogs, petrol, herbicides, pesticides and heavy metals from building and housing materials.

Porous concrete paving has been used overseas in newly developed areas, but this will be the first time it has been used in established neighbourhoods.

However, cost-effectiveness will be an issue. Porous concrete costs up to $200 per square metre, four times the normal price. It would also have to be maintained or replaced because the "filtering" clogs the concrete.

The paving is also less hard-wearing, and thus unsuitable for high-use areas.

On the cost-saving side, fewer pipes and drains would be needed.

Auckland, like many other major cities, faces the prospect of becoming a "turtle city", where nearly all of the surface is covered by concrete.

What may seem like a small problem of too much water on the footpath can have huge environmental consequences.

Senior lecturer in civil and environmental engineering at the University of Auckland, Dr Carol Boyle, hoped the project could help prevent flooding.

"Soil absorbs the water and trees assist in slowing down the speed of falling water," she said.

"When you have heavy rainfall in urban areas ... the water's velocity increases and the volume increases and the risk of flooding is higher.

"It also causes increased erosion and increased silt in the water that is flowing into rivers and seas."

Westhaven Marina manager Keith Hogan is someone who sees this regularly.

"Whenever there's heavy water downtown we notice it here."

He welcomed any initiative to improve water quality.

Dr Boyle said excess run-off was also a problem for residents downhill of subdivided or high-density housing.

Construction is expected to begin in Sarsfield St and Shelly Beach Rd early next year.