Motorway work cleaner and greener

By ARNOLD PICKMERE

Can new motorways be built, then filled with vehicles, without damaging the environment?

The short answer is no. But encouraging evidence is emerging around Auckland that they can be built and operated in a much more environmentally friendly way.

Motorway developments and other earthworks can allow harmful sediments into drains and waterways. And contaminants can wash off from road surfaces and pollute waterways and harbours.

The Auckland Regional Council, charged with stopping such pollution problems under the Resource Management Act, identifies sedimentation and stormwater as two major pollution issues for Auckland.

In the Auckland region, Transit NZ is part-way into a 10-year, $2.6 billion raft of state highway projects with a priority of completing the motorway network.

Two good examples of a smarter environmental approach being adopted by Transit (watched by the regional council) include the greatly improved approach to stormwater handling in the newly completed $67 million Grafton Gully project.

The other is the $94.4 million Greenhithe Deviation started this year. It will run from the Albany Highway to the upper harbour bridge, a 5km part of what will be the new upper harbour motorway between Albany and the Northwestern Motorway at Westgate.

As part of the project, a swathe has been cut through a bush area of ridges, gullies and streams in the sensitive upper Waitemata Harbour catchment.

Environmental protections at the site include:

Earthworks

During this stage earthworks are limited to essential work areas, minimising disturbance of the site. All runoff from the construction area passes through temporary erosion and sediment control devices and other arrangements to minimise the amount of sediment-laden runoff entering waterways.

Sediment retention ponds will catch run-off from the site. The sediment will settle, allowing clean water to be drawn off at the surface.

With the approach of winter, when the risk of erosion is greatest, most of the Greenhithe site is being closed down. Straw and grass seed are being spread over the earthworks to minimise run-off or erosion.

Streams

There are 16 streams which will have culverts taking them under the new motorway.

Three have been identified as having native fish species including the banded kokopu (a small whitebait species), the long-finned eel and freshwater crayfish. The streams have been degraded by agricultural use, dam building and sedimentation from residential developments. The bottom of culverts in the streams are being designed to make sure the fish can continue their natural migration. Culverts are being installed at a slope fish can manage. The floor of the culverts mimic the characteristics of the stream bed including rocks and stones and small pooling areas.

Motorway completion


When the motorway is finished there will be permanent facilities (ponds and wetland areas and other devices) to catch sediment and treat contaminated road run-off. Such measures will also allow run-off to streams to be controlled to avoid erosion problems during storms.

Geckos

Rather like people living in the path of a future motorway route, some 55 lizards have lost their territory. Captured before the bush clearance, they included 44 forest geckos, 10 Auckland green geckos and one copper skink. They have been released in a bush reserve, where they are being monitored.

Trees

Seed gathered from the bush before it was cleared will produce 250,000 native trees to landscape the 5km Greenhithe Deviation.

Noise

Plans are in place to screen local people from the noise of the new motorway. Where necessary, earth bunds (banks) are being used to do the job, planted with vegetation.

Where noise fences are also required on top of these bunds, they will be low, where possible, and probably be hidden by trees in time. People developing or subdividing land near the motorway after official motorway designation (probably mid-2007) will be responsible for sound protection.