Pounding waves — are they threatening our coastline?
New Zealand has some of the highest average wave heights in the world. Michael Daly reports.
The sea. Many of us want to live as close to it as possible, and for those who are wealthy enough that can mean right next to it, even amid warnings that as sea levels rise sections of the coast will eventually be swamped.
Storms that frequently batter the New Zealand coastline show clearly just how damaging the sea can be, like when huge swells that smashed into coastal properties in Haumoana, south of Napier in September 2018.
But while the base level of the sea is rising, what does it mean for our waves?
New Zealand has some of the highest average wave heights in the world, although many of the biggest are in the southwest corner of the country, where there are few people.
The very largest are even further south. MetService subsidiary MetOcean Solutions measured a 23.8m giant at a buoy at Campbell Island deep 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 even been the biggest wave during that storm.
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 measured there since measurements started in 1995.
Anecdotal information recorded the wave height at the time of the 1968 Wahine disaster at 12-14m.
But average wave heights from trough to crest around most of the New Zealand coastline aren’t getting bigger.
Among those closely monitoring New Zealand’s long coastline and turbulent seas is Niwa coastal and estuarine physical processes scientist Dr Richard Gorman.
Measurements of wave height by satellite had been under way long enough to indicate whether wave heights were changing, he said.
‘‘We find that down in the Southern Ocean, so south of New Zealand, there is a definite trend to increasing wave heights over the time . . . in the past 20 to 30 years.
‘‘Whereas, further north, around New Zealand latitudes or a bit north of New Zealand there are some bits where wave heights may be decreasing a little bit,’’ Gorman said.
Wave heights were increasing in the very southwest corner of the country.
Work to try to assess what will happen in the future indicated wave heights would continue to increase in the Southern Ocean, specially the most extreme storm conditions. But more uncertain was which parts of the New Zealand coast would have increases, decreases or not much change.
New Zealand seemed to be ‘‘in a bit of a neutral point’’, with fairly subtle differences depending on the assumptions made, including future greenhouse gas emissions.
‘‘It’s hard to get a big definite trend for what’s happening just around New Zealand,’’ Gorman said.
While there would definitely be a sea level rise effect, ‘‘anything on top of it from waves is probably fairly subtle and minor’’. The possibility there could be more – or fewer – big storms also needed to be considered.
While he did not work directly on such issues as the possibility of an organised retreat from the coast in some places, sea levels were rising and coastal communities were going to have to consider how they responded, Gorman said.
‘‘Ultimately in most places it’s possible, if you spend enough, and you build enough, you can protect a particular asset . . . but it will become the balance of how much of the coast can you spend money doing that to.
‘‘And you might protect one place, but then, if you stop your seawall a kilometre down the coast you might be creating a worse problem down there at the end of the seawall where there’s no more protection in place. So that all needs to be considered.’’
Understanding how waves behaved was important for building along the coast.
That included taking into account sea level, including variation due to tides and the extra rise resulting from low air pressure during a storm. On top of that was wave action. The aim was to assess statistics for when high sea levels combined with big waves.
‘‘Generally speaking waves get generated by winds and once they are created in a storm they can travel across the ocean,’’ Gorman said.
‘‘Down in the Southern Ocean you get strong persistent westerlies a lot of the time . . . They’ve got quite a long distance over which to act, building up the energy in the waves,’’ Gorman said.
‘‘Further north, in the Pacific, from time to time big storms come through but not quite so consistently.’’ The northeast coast did not get the big swells coming up from the south.
The biggest storm in a year in many places might have significant wave heights – a kind of average – of 6 metres, and maybe 8 metres further south.
Looking at a map of average wave heights around New Zealand, there was a trend of gradually decreasing size on the open coast, going north from the southwest, up either the east or west coasts, Gorman said. Then the ‘‘bit of a shadow’’ in the northeast had on average the lowest waves.
Gorman runs numerical models that estimate how much energy winds flowing across the ocean are putting into building up waves.
‘‘We run those models, both to do forecasts, to look the next few days ahead at what conditions we’re likely to see,’’ he said.
‘‘But we also look back over a historic period, so that’s what we call a hindcast, looking backward in time to simulate what happened over say the last 20, 30, 50 years.’’
That long record was needed for estimates of such things as the probability of coastal infrastructure getting inundated. To check the accuracy of the models, the simulation could be compared to what had been recorded by wave buoys in several locations around the country, and worldwide.
‘‘You can get a model to predict what it thinks happened there, and just compare the two, and see if the models are getting it right . . . You can also look at available satellite data. We do use those measurements we do have, to check the models give good answers,’’ Gorman said.
The models tended to show 8-metre waves in the biggest storms. In some cases waves could get up to 10 metres, mostly around the southwest corner of the country. Satellites measured wave height over the larger expanse of the ocean. A satellite passing overhead sent down a radar beam, which bounced off the surface of the sea back to the satellite. The height of the waves could be measured based on how spread out in time the returning radar pulse was.
But the satellite might only pass over any given spot about once a week, so it was not the way to capture what happened during a storm event, Gorman said.
‘‘We’ve probably missed actually measuring the biggest waves, there aren’t that many buoys out there.’’
Understanding how waves behaved was important for building along the coast. Niwa scientist Dr Richard Gorman