Harbour being used as a rubbish tip

ROBERT KITCHIN/ The Dominion Post

SEA OF SLUDGE: Harbour strategy co-ordinator Keith Calder said the sea of tyres and rubbish-in Porirua Harbour-was visible from the city at low tide.

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Porirua has turned its back on its harbour, creating a sea of unsightly trash, including hundreds of tyres, shopping trolleys, road cones and rusted oil drums, a city council official says.

In an effort to clean up the harbour and address chronic silt build-up clogging vital waterways, officials have started the biggest mapping expedition in Porirua Harbour's history.

Harbour strategy co-ordinator Keith Calder said the sea of tyres and rubbish was visible from the city at low tide and reinforced a perception that it was acceptable to throw things into the harbour.

"We've too long turned our back on the harbour. We're trying to reverse not only the physical damage but address the community's attitudes toward the harbour."

Information from the mapping expedition will form the basis of a computer model to help predict how the growing tide of silt will build up in the harbour.

Early results from the survey suggest the silt in the Pauatahanui Inlet may have risen by as much as a metre in the past 30 years a "scary" figure, Mr Calder said.

"There are indications that it's possibly silting up much quicker than had previously been understood."

Silt and heavy metals had built up over decades from tidal flows, runoff from earthworks, and erosion, but could increase with a rise in housing development around the harbour basin, he said.

The silt threatened marine life and recreational use of the harbour.

A $90,000 bathymetric survey, part of Porirua's $2.4 million funding to clean up the harbour, had taken Discovery Marine just over a month to complete. Surveyors used sonar to map every inch of the harbour, to within one centimetre of accuracy.

Mr Calder said their technology adjusted for changing factors such as wave swell and tides. Parts of the harbour were surveyed by the crew of HMS Acheron in 1849, but the sixth and latest survey was the first to map the entire harbour.

The survey results will help the council decide how to fix the silt problem, possibly by using some of the silt to make beaches or by shifting it to deeper parts of the harbour.

Next week, rubbish will be fished from the Porirua Stream delta a section of the harbour near the city centre. It will be photographed and weighed as part of the survey.

- © Fairfax NZ News

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