If you were inland on Waitangi Day, you would have missed some of the largest waves to visit central New Zealand in over 25 years. Blue skies and just a stiff breeze gave no hint of the giant waves battering Wellington’s south coast.

8 April 2002

While an undeniable headache for ferry passengers, yacht owners, and workers who had to repair coastal damage, the Waitangi event was a spectacular example of wave power formed by storms far from New Zealand. A similar storm, albeit of lesser magnitude, occurred within the last week. It again disrupted ferry traffic, affecting the end of the Easter weekend, and scattered debris across Wellington roads. This latest storm also featured severe waves after local winds had died away.

Distant storms, local waves

Oceanographers have long known that waves formed by large storms can travel great distances. In a classic experiment in 1965, a group of wave experts, involving the eminent Wellington scientist Norman Barber, set up several wave recording sites that extended across the Pacific Ocean from New Zealand to Alaska. For two and a half months of the Austral winter, the scientists tracked the passage of storm waves, formed around Antarctica, to New Zealand, Samoa, Hawaii, California, and finally to Alaska – a total journey of over 14 000 km. Thus, in one of Nature’s quirks, storm waves formed in an Antarctic winter became the summer waves that powered the surfers off California. This simple but elegant experiment captured the hearts of the locals. The New Zealand monitoring site was Cape Palliser where the lighthouse operator helped moor the instruments in understated “trying conditions”. In Samoa, the High Chief Satele not only provided accommodation, but also tended the instruments for the last two weeks of the experiment.

Waitangi Day Storm

Around midday on 5 February 2002, a recording buoy off Baring Head lulled in seas with waves about 1 m high. Just 15 hours later, on Waitangi Day morning, waves reached 13 m, and fluctuated between 10 and 12 m for the rest of the day. Thereafter, waves declined only slowly. It took a full 4 days to return to their pre-storm height. The culprit was an intense depression – centred 800 km southeast of Wellington. This wind and wave generator moved only slowly eastward, all the while creating large waves that travelled straight to Cook Strait. As waves approached Wellington, they began to stir sand on the seabed in water depths down to 150 m. Marine organisms living in the sand were caught up in the turbulence. But as marine biologists have learnt, these organisms soon recolonise their shaky habitats. About 5 km from Island Bay, the waves began to break and turn the bay into a gigantic foam bath. As the tides rose, waves encroached further inshore to scour unprotected sectors of the coast. Rubble and seaweed covered coastal roads.

Nothing is new

The Wellington region has experienced large waves from distant storms before and will do so again in the future. One of the largest in recent years was the Kapiti storm of 11–13 September, 1976. For 3 days, a large stationary depression near Stewart Island pumped out wind and waves that traveled over 1000 km to crash into the west coast of the North Island. On the Taranaki–Kapiti coast, winds and changes in barometric pressure caused a storm surge of 0.7 m. This extra rise of sea level allowed the waves to break further up the beach. And the effects were devastating. Over 6 m of coast was eroded at Waikanae, while Raumati coast lost up to 11 m of beach front, including some houses. Because the piled-up water had to go somewhere, powerful rip and along-shore currents formed to shift the eroded sand south towards Cook Strait.

Waitangi vs Wahine

The violent storm that sank the interisland ferry Wahine, on 10 April 1968 has become the benchmark by which Wellingtonians judge all other storms. So how does the Waitangi event compare? Certainly, the circumstances of the waves were different. Waitangi waves formed far out in the Southwest Pacific, whereas Wahine seas blew up in the southern approaches to Cook Strait. Average Waitangi winds were under 55 km/hour, but Wahine winds exceeded 110 km/hour for 6 hours and peaked at 200 km/hour, well above hurricane force. This unassailable wind, coupled with a sharp drop in barometric pressure, forced Wellington Harbour waters to rise nearly 1 m above the high tide. Waitangi had no comparable surge.

Although weather and tides were measured during the Wahine storm, waves were not. However, we can estimate wave height using a combination of wind speed, duration, and fetch – the extent of open ocean affected by the wind. Using computer models to simulate the storm conditions, we estimate that Wahine waves had heights similar to their Waitangi counterparts.

When is the next “Big Wave” event?

The short answer to this question is “we don’t know”. Wahine occurred in 1968, Kapiti in 1976, an unnamed storm with 12 m waves in Wellington in 1989, and Waitangi in 2002. Just four events spaced 8–13 years apart – hardly enough to make reliable predictions. However, weather satellites, together with computer models of weather and waves, sea-level measurements, and local wave measurements, such as those that have been collected off Baring Head for the past 6 years, will allow us to be better prepared for the next big surf.

For further information contact:

Dr Lionel Carter
Phone +64-4-386 0371
l.carter@niwa.co.nz

Dr Andrew Laing
Phone +64-4-386 0333
a.laing@niwa.co.nz