New system keeps rubbish out of harbour

Rain falling on the new motorway lanes in Auckland's upper Grafton Gully now washes into new stormwater grates and pipes designed to cope with a 100-year flood.

If it has not rained for a while it will also wash rubbish and particles off the road. Engineers call it dirty flow.

Motorways get plenty of rubbish on them. Up in the Spaghetti Junction part of the motorway, which the lanes in the gully adjoin, they pick up about 50 tonnes of rubbish a year - tyres, bumpers, objects and parts that have fallen from cars and trucks.

In some ways that is the best of it. The worst nasties are often in the water like that which washes down the pipes and into Transit's new 85m by 10m sedimentation tank on the Grafton Gully floor, near the Stanley St tennis centre.

Transit NZ, which runs the motorways, says that despite design advances motor vehicles continue to leave contaminants.

This occurs mostly as a result of tyre wear, brake wear and exhaust emissions. The contaminants include heavy metals (lead, zinc, copper etc) and hydrocarbons (like those from diesel exhaust). And there may be grit and oils.

When the stormwater reaches the unobstrusive, enclosed tank it enters a forebay. Here bigger and heavier bits are trapped. The rest of the water spills into the tank which has baffles along its length to slow the water.

That allows the sediments to drop to the bottom of the tank which is at least 1.5m deep. The cleanest water then decants over a weir at the other end into a pipe and heads for the harbour.

It looks clear. The Auckland Regional Council stipulated that 75 per cent of long-term suspended solids be removed. Some poisons from the road surfaces may have dissolved in the water and cannot be separated, but the end result, according to engineers, has greatly reduced poisons and far less sediment.

The tank can also be used to isolate spillages on the motorway. Further down the gully, the catchpits under the stormwater grates have filters - wire cages with bags that remove sediments and the like.

Contractors will clear and replace these.

The big Grafton tank will need cleaning in its main length only once every 35 years. The first tank where the water enters will be cleared by a digger probably every three to five years.

Tests will determine how poisonous and hazardous the material is and how it will be disposed of at landills.

At least it is no longer going into the Waitemata Harbour.