Motorway flood chaos warning

The cycleway flooded early this month. Photo / NZTA

High tides and bad weather could flood Auckland's Northwestern Motorway in coming months, stranding thousands of commuters and causing gridlock.

The New Zealand Transport Agency has warned motorists and cyclists of unusually high "king tides" that could swamp the Northwestern Motorway, one of Auckland's major arterial routes.

The motorway, which passes over reclaimed land on the edge of Waitemata Harbour, has been gradually sinking for the past 50 years.

More than 87,000 vehicles use the motorway every day.

Waitakere mayor Bob Harvey said a flooded motorway would "bring Auckland to a standstill".

"If the Northwestern [Motorway] is stopped, the traffic goes right back to Newmarket, right back to the city, and everything stops."

The next danger period is between February 27 and March 5, when another king tide is expected. The days on either side of the tide are included in the flood warning.

The agency has warned cyclists to avoid using the cycleway alongside the motorway 90 minutes either side of high tide on those days.

The agency's Auckland state highways manager Tommy Parker said having a major highway so vulnerable to tides was unique.

"I'm not sure if anywhere elsewhere in the world has the same problems, but obviously it's not a good look and something we're looking forward to rectifying."

The causeway between Waterview and the Rosebank interchange is thought to have sunk about half a metre since it was built in the 1950s. It will be raised by 1.5m as part of improvement work, which the agency aims to start by mid-2011. Fill removed from the Waterview tunnel could be used to build up the causeway.

But Parker said there was no way to prevent flooding in the meantime. "If it did flood it would only be for a couple of hours so it's not worth spending tens of millions when we're fixing it in a year or so anyway."

Parker said the agency was monitoring water levels.

"It [flooding] is a possibility and if that happens we'll put in temporary traffic management to make sure there's no risk."

The first danger period passed without major incident earlier this month, although the cycleway alongside the motorway was flooded, with water lapping at the edge of the motorway's bus lane.

Expert warns flooding risk could rise

Coastal oceanographer Dr Rob Bell says this year's flooding risk will be increased if storms or winds push the water higher.

Perigean high tides, or king tides, might only be 10cm to 20cm higher than average, but the real danger came when winds or storms raised water levels and increased the risk of flooding.

Bell, who works for the National Institute of Water and Atmospheric Research (NIWA), said a tropical cyclone or a low pressure system could "push the water level up to half a metre on top of those king tides, so it's more about a vulnerability to weather systems."

The unusually high tides occur when a spring tide coincides with a perigee tide, when the moon was closest to the earth. The very high tides peak one to two days after a new or full moon when the moon is in its perigree.