Alone
in a wild ocean, New
waves
Zealand gets smashed by
some of the world's biggest
4 August, 2019
Far from any other piece of land, and sitting among some of the wildest waters on the planet, New Zealand has some of the world's biggest waves.
Massive seas hitting the south and west of the country this weekendshow what can happen when conditions are right. And as is most often the case, it's the south and southwest of the country that's taking the biggest pounding, followed by the west coast.
And there's plenty of coastline to batter. Land Information NZ puts the length at 20,500km, with 6000km around the South Island, 8500km around the North Island and 700km for Stewart Island. Outer islands account for the rest.
Ed Atkin, oceanographer at marine and freshwater consultancy eCoast, said the 8-11m inshore waves forecast for Friday and the weekend were "very substantial" in the grand scheme of things.
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* Massive wave south of NZ believed to be a Southern Hemisphere record
* Eating the shore: New Zealand's shrinking coastline
* King tide flooding leaves Auckland residents cleaning up 'heartbreaking' mess
Because New Zealand had sections of coast facing all directions of the compass, different parts of the country experienced distinctly different wave climates, he said.
Much of New Zealand's wave climate was dominated by storm tracks around the globe in the Southern Ocean. "These storms generally generate the largest waves on the planet. Our west, south and southeastern extremities of New Zealand are right in the path of these storms and the waves they produce." Waves as big as 11 metres had been recorded around Southland, Atkin said.
GLENN JEFFREY/STUFF
Far from any other piece of land, and sitting among some of the wildest waters on the planet, New Zealand has some of the world's biggest waves.
The northeastern parts of New Zealand tended to have a smaller wave climate on average, but could experience extreme wave and weather events when cyclones made their way down form the tropics.
Why are the waves around New Zealand so big?
A 1979 paper by RA Pickrill and JS Mitchell from the NZ Oceanographic Institute explains this neatly.
The country is wholly within a belt between latitudes 30deg and 70deg where westerly winds blow around the Southern Hemisphere virtually unimpeded by any large land masses. "Waves dominated by these westerlies dominate the wave climate on most of New Zealand's coastline," the paper said.
New Zealand Defence Force offshore patrol vessel HMNZS OTAGO battered by huge seas in the Southern Ocean.
"New Zealand is isolated from other large bodies of land. In almost every direction fetch (the area of ocean over which the wind blows in an essentially constant direction) conditions for wave generation are unlimited; Australia is the closest neighbour, 2000km to the northwest, and for all but extremely rare events this represents unlimited fetch conditions for wave generation."
Niwa coastal and estuarine physical processes scientist Dr Richard Gorman spells it out: "As the wind blows across the sea, it transfers some of the energy to the water surface. The longer it blows and the stronger it is, the larger the waves."
Where are the biggest waves in New Zealand?
IAIN MCGREGOR/STUFF
Penny Madden's home in Hector, north of Westport, was hit by a huge sea swell during a storm in early 2018.
The south and west coasts of both islands get the highest energy waves, driven by the predominant westerly flows, Niwa said. The energy grading was highest in the southern South Island and lessened further north.
On the northeast coast of New Zealand, from North Cape to East Cape, the landmass provided shelter from the Southern Ocean, and waves were driven by weather systems in the Tasman Sea and ex-tropical cyclones. "Generally waves are smaller there, but occasionally you can get big waves in cyclone conditions," Niwa coastal scientist Dr Scott Stephens said.
And on the coast from Gisborne to Otago, waves usually arrived from the south and east. Although Cook Strait could experience wave conditions choppy enough to cancel ferry crossings, it was sheltered from large ocean swells and more affected by local winds.
So how big?
ALEX CROUCHER/YOUTUBE
Offshore patrol vessel HMNZS Otago was battered by a heavy storm during an operation in the Southern Ocean in late 2015.
There isn't a comprehensive record of measurements from around the coastline, but it's possible to get some idea of how big the seas can get from what is available.
Starting with the biggest first. MetService subsidiary MetOcean Solutions measured a 23.8m giant at a buoy in the Southern Ocean during a storm in May 2018. It was thought to be the biggest wave ever recorded in the Southern Hemisphere, even though it was unlikely to have been the biggest wave during the storm. Individual waves higher than 25m were possible, MetOcean said.
A Niwa report on renewable energy resources said the wave energy resource to the south of New Zealand was "exceptionally high" by world standards. Higher resource areas were also most likely to experience the most extreme, damaging wave conditions. For example, a one-in-100 year wave could have a 35m trough to peak height.
Niwa reported that during a storm in June 2013, 15m waves were recorded at a buoy 2km off Wellington's Baring Head. They were the largest recorded at the buoy since measurements started in 1995, and Niwa thought they were likely to be the largest at that point in 50 years.
During a big storm on Waitangi Day 2002, the biggest waves were 13m off Baring Head. The significant wave height during the 2002 storm was above 8m.
Niwa's Gorman said Fiordland had some of the country's highest waves. Wave heights averaged 3m on a typical day, but reached over 10ms at times. Wave heights in the North Island averaged around 2m on much of the west coast, and 1m on the east coast.
Anecdotal information recorded the wave height at the time of the 1968 Wahine disaster at 12-14m.
What about further back in time?
JED FINDLAY
Waves smashing into SH6 at Punakaiki were so big concrete was "thrown across" the road, in a storm in early 2018.
The biggest storm to hit New Zealand in the 20th century is thought to have happened in February 1936. The Wahine storm of April 1968is thought to have been the second biggest. Niwa said the 1936 storm was thought to be an ex-tropical cyclone, which "swept with cyclonic fury" through the North Island. "Phenomenal rainfall and extreme winds caused widespread damage and destruction of property throughout the North Island."
Te Ara The Encyclopedia of New Zealand records 40 boats were sunk or driven ashore in the Waitematā Harbour, and chillingly in hindsight, the interisland ferry Rangatira steamed onto rocks on Wellington's south coast. Unlike the Wahine in 1968, the Rangatira avoided disaster, managing to reverse off the rocks after 20 minutes, then turning and backing slowly up the harbour.
During the 1968 Wahine storm, a woman was injured at Waihi when waves washed through a beach cottage. In Whakatāne, huge seas were driven inland, flooding many hectares of coastal farmland. Also at Whakatane, huge waves smashed a concrete retaining wall and washed logs across the road.
And then there was the six-day storm of February 1868, which is also thought to have been an ex-tropical cyclone. Niwa said there were 12 shipwrecks during the period, and at Waikouaiti in east Otago, waves washed away or levelled several buildings storing goods on a sandspit.
What's the general picture?
SVEN MARTIN
Waves pound the Boulder Bank at Glenduan north of Nelson in early 2018.
The Pickrill and Mitchell paper said the country could be divided into four broad regional groupings on the basis of wave characteristics.
In Southern New Zealand the wave environment was "extremely high energy", with the prevailing deep water wave being 3.5-4.5m high. Rarely did wave heights drop below 2m, and waves up to 11m had been recorded.
In Western New Zealand, off the west coast of central New Zealand, prevailing deep water waves were 1-3m high, changing to 0.5-1.5m when seen from the beach. While this grouping was also exposed to the prevailing winds, the westerlies were not as strong as those further south, and that was reflected in the lower wave height.
The Eastern New Zealand area, which was sheltered from the prevailing winds, had prevailing deep water waves 0.5-2m high. Onshore that was transformed into a 0.5-1.5m wave. Storm waves on the east coast were rarely more than 3m high, except on the south Wellington coast where southerly wave trains arrived at the shore unrefracted and storm waves may be higher than 4m.
In Northern New Zealand, the area between North and East capes - had a 0.5-1.5m deep water wave, which was probably transformed into a 0.4-0.8m wave at the beach. Storm waves at the shore were probably only occasionally more than 2.5m high.
What have king tides got to do with all this? In fact, what is a king tide?
CATRIN OWEN/STUFF
A king tide coinciding with a storm closed Tamaki Drive in Auckland in early 2018.
According to the US National Oceanic and Atmospheric Administration,"king tide" is a non-scientific term often used to describe exceptionally high tides. Higher than normal tides typically happen during a new or full moon, and when the Moon is at the closest point to the Earth during its orbit of roughly 28 days.
That closest point is perigee, and when it happens the gravitational pull of the Moon on the Earth is at its strongest, and during those periods there is a slight increase in the average range of tides.
Also during full or new moons - when the Earth, Sun and Moon are nearly in alignment - average tidal ranges are also slightly larger. These happen once a fortnight.
About three or four times a year, when the new or full moon coincides closely with perigee, the tidal range is even bigger, although not by much. But at these times coastal flooding from a storm could be significantly worse than it otherwise would have been.
Horowhenua
mayor Michael Feyen said locals had grown increasingly concerned
following the Fox River disaster, with contractors from the area
offering their services to reinforce the dunes.
Mr
Feyen said one local contractor took him there so he could see how
wet it was getting with his own eyes.
"Over
the years people will say the water has been this high before with
flooding and spring tides, but he said to me I've never seen it like
this before.
"The
ground is permanently wet up here."
Locals
had seen the river change course multiple times and suspected its
next change would cut north - the direction of the landfill, Mr Feyen
said.
"We
are really concerned that the water is only really got that sand bank
to go and it's into the dump. We don't want another Fox River do we?"
Rising
water levels could not be disputed by anyone, and sand did not take
much to be washed away, he said.
However,
a Horowhenua District Council spokesperson said the sand dune which
protected the dump site from the river was safe for now.
The
spokesperson said the sand bank was "stable at present" but
it was working to ensure the land stayed that way.
The
council was creating plans to educate locals around how to protect
the area, such as by driving responsibly when using vehicles on the
beach.
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