Swollen tides, wild waves to become more common, scientist warns

Wednesday, 13 October 2021

Big waves around Wellington's south coast on Wednesday ripped up tarmac near Ōwhiro Bay and threw a tree stump across the road.

A king tide that mixed with big waves with enough power to rip up tarmac and hurl a large tree across a road is a peek into the future, a scientist warns.

The king tide – a much higher-than-usual tide that saw the sea reach the bottom of boat sheds in Wellington Harbour at high tide about 11.30am – was caused by the moon and sun being in alignment, with both of their gravity pulling on the oceans.

Added to this, the moon was at the lowest point in its elliptical course around the world, meaning its pull was stronger.

Gale force winds and swells of up to six metres hit Wellington’s south coast with the Wellington City Council closing a section of the coastal road where rocks had washed up near the airport, and a section where large bits of debris washed up between Ōwhiro and Island bays.

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The big waves and high tide forced the closure of parts of Wellington’s south coast on Wednesday.

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Tarmac was ripped up by waves between Ōwhiro and Island bays on Wellington’s south coast.

A tree trunk was washed on to the coastal road.

It was here that waves tore up sections of tarmac, washed a large tree trunk over the road, and a parked car had its rear end caved in by debris in the surf. The road to Eastbourne in Lower Hutt was also closed for a period.

Land Information New Zealand data shows Wednesday’s 11.30am high tide was 1.8m.

National Institute of Water and Atmospheric Research (Niwa) coast and estuaries chief scientist Scott Stephens said the scenario was going to become more common with climate change-induced sea level rises.

A raising of the sea’s average height meant, when there were big waves, they could reach land and property much easier, he said.

“Sea levels in New Zealand have risen by almost 20 cm over the last 100 years.”

Wellington’s climate change-induced “relative” sea level rise was faster than the rest of the country because the land was actually dropping due to tectonic movement under the ground.

The sea was creeping up on New Zealand at a rate of 1.81mm per year but in Wellington the rate was 2.3mm.

Niwa had a calendar of “red alert” times when all the factors combined. Wellington’s red alert this time around began on Sunday and goes through to Saturday.

“On these red alert dates it takes only a moderate storm to combine with the very high tides to cause flooding,” said Stephens.

”It is a sign of what’s to come in the future, in more places and with increasing frequency.

“Future sea-level rise will cause sunny-day nuisance flooding to incur increasingly often.”

Councillor Fleur Fitzsimons, who has been involved in helping Ōwhiro Bay residents mitigate wave risk, said it was crucial agencies such as MetService and Niwa worked together to make sure residents in coastal communities were warned when big waves were coming.

“We will see more extreme weather events in the coming years, it’s critical that the public agencies working on this have the best modelling and analysis in order to protect people and property.”