King tide 'lifted' higher by low pressure

Looking west up Tamaki Drive. Photo / Ken Batkin

The tide which spilled over Tamaki Drive was a rare combination of weather phenomena which made flooding all but inevitable.

Sunday morning's king tide was "lifted" by a low pressure system, and pushed by northeasterly winds into north-facing coastlines. In addition, the mean sea level around New Zealand was already higher due to the La Nina weather cycle.

Any of the four factors (tide, atmosphere, wind, climate) alone may not have a significant effect. But combined, they were the reason for flooding in downtown Auckland, eastern suburbs and the North Shore. A perigean spring tide, or king tide, is an especially high tide caused by the fuller moon and the close orbit of the moon to earth.

On Sunday, it was predicted the king tide would measure 3.5 metres in Auckland.

However, the low pressure system bearing in on the city may have raised the tide even further. The lower atmospheric pressure "sucks" the water level upwards.

For every millibar that the atmosphere drops, the water level rises another centimetre. So on Sunday, the king tide may have been raised by up to 12 centimetres.

While 12cm may not sound like much, when that higher water level is pushed by wind, it acts like loose sand, skimming across the surface and creating large waves.

The effect of the wind on sea level is variable and depends largely on the shape of the coast. But in general a strong wind blowing onshore will pile up the water and cause the sea level to be higher than predicted.

By 10am on Sunday that wind pushed the tide over Tamaki Drive's seawall, submerging the road.

In some places, MetService predicted that tide levels were 300mm higher than expected.

Last February, the New Zealand Transport Agency warned that king tides would swamp the Northwestern Motorway. But because the king tide came on a windless day, the high water lapped on to the cycleway but went no further.

On Sunday, the wind pushed the king tide on to the Northwestern, closing four lanes.

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

Niwa coastal hazards expert Rob Bell said most seawalls were built with the tidal extremes in mind. However, many of Auckland's protective barriers were constructed before comprehensive data on tides was available.