Storm surges to get bigger in south, smaller in north

As the sea level rises, it will take less bad luck to cause the same amount of flooding. When truly bad luck strikes, coastal floods will be worse than anything we experience today. On the bright side, storm surges themselves aren't going to change much - a happy surprise for climate researchers.

Severe storm surges will get bigger around parts of the South Island and smaller around the North Island as the climate changes, two studies have now predicted.

The good news is that the changes won’t be huge for either island – about 10cm bigger or smaller on average, depending where in New Zealand you live.

Unfortunately, this comparatively bright spot of climate news does not take away the rising danger of coastal flooding, which will still get worse because of climate change, despite storm surges not changing much.

A storm surge is what happens when the wind and atmospheric pressure associated with a storm push seawater up and inland, flooding coastal areas.

The worst coastal floods in New Zealand memory – such as the one that devastated Kaiaua and the Thames coast on January 5, 2018 – have happened when an extremely big storm surge has coincided with a king tide.

The worst flooding the town of Kaiaua has ever experienced (according to the town’s civil defence chief) happened because an extraordinarily big storm arrived on the same day as one of the highest tides of the year.

It was a particularly bad stroke of luck, which allowed the king tide and the storm to combine to send salt water gushing further inland than either event could have achieved alone.  

But in future, this kind of extreme flooding won't be reserved for super unlucky times. 

That’s because the baseline, sunny-day sea level is gradually rising, as the icy poles melt and the ocean gets warmer and expands. By 2100, based on current emissions rates, global average sea levels may be around a metre higher than pre-industrial levels – though the actual level could be lower or higher depending how people and ice sheets behave.

The rising baseline sea level is going to give every storm surge a leg-up, meaning that even a comparatively modest storm in the future could cause the same amount of damage as a very bad storm today.

The visualisation tool shows what happens to places like Tamaki Drive, Wynyard quarter and the CBD after various combinations of sea level, tides and storm surge. 

While nobody knows when the next big flood will happen, climate scientists can say for certain that, with each passing year, storm surges are launching from a higher starting point. As the seas rise, in other words, they'll allow a smaller storm to cause a bigger amount of flooding. Since smaller storms happen more frequently than big ones, today's extreme levels of flooding will happen more frequently. 

And when a truly big storm hits, the added boost from the already-higher seas will create floods unlike anything we see today. “The really bad luck events are going to be worse than what they are now,” says NIWA’s Scott Stephens.

New Zealand researchers have been trying to develop a more nuanced picture of the changes. For example, researchers in Wellington have been factoring in the impact of our land moving up and down with seismic shifts in various coastal regions.

But there has also been a major question about storm surges: Would they get bigger or smaller with climate change, and what would that mean for New Zealand’s overall flood risk – and the risks to specific towns and cities?

Two research projects have now modelled the estimated impact of climate change on storms surges around New Zealand. Both found there’d be reasonably modest changes, providing a rare moment of reassurance amid increasingly dire climate projections.

The first project – called Wave and Storm Surge Projections (WASP) – was led by NIWA in 2012. Its findings were backed up by a second study published in August 2019, in the International Journal of Climatology.

“What is really interesting is that both these studies show kind of the same thing, what they suggested was that wave heights and size of storm surges are going to be bigger in the south and west of New Zealand and they are going to be smaller in the north and east of New Zealand,” says Stephens, who was involved in both studies. “But the actual changes are not large and quite subtle, so in fact ... we are not going to see a massive difference,” he says.

Laura Cagigal, the lead author of the most recent study, said the storm surge changes were only about 10cm bigger or smaller than today. They were calculated for storms of a severity that happens about once every 50 years. However, there would be isolated surges around the country that would be significantly bigger than the ones seen today, said Cagigal, even in areas where severe surges on the whole would get a little less frequent. 

To make the estimates, Cagigal, a PhD candidate at Auckland University’s School of Environment, worked with other researchers in Spain and New Zealand to design a statistical model of how storm surges will change. The model takes into account the changing wind and sea level pressure conditions expected from storms as the climate changes, and makes projections for two different IPCC emissions scenarios – a high-emissions path, and mid-range emissions. The high track is the path the world is on now.

To test whether their model worked, the researchers compared their results to real tidal gauge data from the past - entering the conditions that created real, past storm surges to see if their model would estimate flood levels accurately (it did).

Looking out to 2100, they discovered that storm surges would get bigger on average in the South island (mainly the lower west coast) and smaller on average in the North, but with isolated, big events in both islands that would be outstrip today’s large surges. Cagigal said the team was surprised to see that there weren’t going to be bigger shifts before 2100 - they had expected the picture to look worse. 

Unfortunately, the comparatively benign news about storm surges does not mean the flood risk will stay the same for people living near the coast.

Laura Cagigal, a PhD student at the University of Auckland’s School of Environment, demonstrates the university’s virtual reality sea level rise simulator for Auckland. Photo: Supplied

Even in the North Island, where the models predict fewer extreme storm surges, flooding will not grow less frequent, because of the rising baseline effect. For regions in the South, where the surges may be bigger than now, the effect is a double-whammy. Christchurch and Dunedin are already among the cities projected to be hardest-hit by sea level rise, because of the high number of people living close to sea level.

Cagigal and Stephens agree that the main factor driving increased coastal flooding around New Zealand will be sea level rise. “The key take-home message is that sea level rise is going to be the main driver of the changing frequency of coastal events, and their changing impact,” says Stephens. “We have problems now, every now and again we get a big storm that combines usually with a very high tide ... at our highest high tides that’s when we’re most exposed, so that randomness and luck of things combining is really important,” he says.

“But that’s going to become less important in the future with sea level rise, because these impacts we see now, which only occur very occasionally, when we get all the right things lining up ... those thresholds will be reached increasingly frequently. The frequency is actually going to change really quickly [because] 20cm, 30cm, 40cm of sea level rise is going to make a really big difference to the frequency of places being flooded,” says Stephens. 

There is another potential factor that is still being explored - whether heavier rainfall might coincide with storm surges to overwhelm stormwater drains and flood people from inland, at the same time they are being inundated by the sea. 

“We are looking now at the likelihood of extreme rainfall occurring at the same time as large surges," says Stephens. "We are trying to see how likely these things are to occur together, because obviously if you get large rainfall and it runs off and meets a high tide and a surge, and the sea is rising, it’s a compound event," he says. "You’ve got these multiple things occurring at the same time."

The rain modelling hasn't been finished yet. 

But, in the meantime, Cagigal and her collaborators are helping people visualise what a bad luck combination might look like. 

Data visualisation specialists at the University of Auckland have created virtual reality experience that allows visitors to pick a point in the future, pick an emissions scenario, and add an extreme storm surge on top of a big tide. Visitors can don a headset and use controls to view the results on a huge 3D screen - zooming in to see what happens to places like Tamaki Drive, Wynyard quarter and the CBD after various combinations of sea level, tides and storm surge. People can try the flooding visualisation tool at home, too, though the researchers warn their estimates are not yet fully refined.

Cagigal said the VR studio had already received visits from staff at Auckland Council.

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