14 May 2020

Little bit of sea-level rise = lots more coastal flooding

From Our Changing World, 9:06 pm on 14 May 2020

Scientists warn that a small amount of sea-level rise could have big consequences for some low-lying parts of New Zealand.

Sea-level around New Zealand has risen by 20 centimetres in the past century, and is forecast to continue to rise.

Sea-level around New Zealand has risen by 20 centimetres in the past century, and is forecast to continue to rise. Photo: Dave Allen / NIWA

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NIWA coastal scientists Dr Scott Stephens and Dr Rob Bell are co-authors on a recent paper which concludes that small increases in sea level rise are likely to drive huge increases in the frequency of coastal flooding in the next 20–30 years.

The researchers analysed more than a century’s worth of extreme sea-level and storm surge events to look for patterns in the timing and occurrence of damaging coastal flooding and found that the worst events happened when a number of factors coincided.

Sea-level 101

There are a number of things that have an effect on sea-level over different time frames.

Sea-level changes over the course of year due to temperature - it is slightly higher in summer as warmer water expands.

“It’s not a big fluctuation, just 10 centimetres or so, and yet surprisingly it has quite a strong control on when we observe our highest sea-levels,” says Scott.

A king tide floods boardwalk in Wellington's Clyde Quay marina.

A king tide floods boardwalk in Wellington's Clyde Quay marina. Photo: RNZ / Alison Ballance

Every day, sea-level changes with the tides, rising and falling up to four metres around Nelson and less in other parts of New Zealand.

But not all tides are equal; tide height varies with the lunar cycle, and every fortnight spring tides, which coincide with the full and new moons, have a wider range than the intervening neap tides.

Extra-high tides known as king tides occur every seven-or-so months. These are the result of the moon’s 29-day elliptical orbit around the earth. These are predictable and NIWA publishes an annual red-alert tide calendar for king tides.

On their own, king tides can cause sunny day flooding in low-lying areas. Nelson’s Wakatu Square is susceptible to this nuisance flooding as seawater encroaches up stormwater drains during a king tide and floods the carpark.

“This nuisance flooding is what we’re going to see a lot more of due to sea-level rise,” says Scott.

However, trouble really arrives in the form a storm surge coinciding with a spring or king tide.

“When we get stormy weather systems with strong winds, the winds push the water up against the land surface, causing … a storm surge,” says Scott.

Scott and Rob’s analysis of past coastal flooding events showed that the most damage was usually the consequence of an extreme high tide coinciding with a moderate storm surge.

The sea is going up

There is, however, an extra confounding factor: long-term sea-level rise, caused by global warming.

“We’ve got a changing mean sea-level base which is rising,” says Scott.

“In New Zealand, we’ve had an average of about 20-centimetres of sea-level rise over the last century – and that is actually making a difference,” says Scott.

“So the tides and storm surges are going to run up on top of that base, and increasingly they’re going to cause more frequent flooding, and they’re going to reach higher and higher.”

He gives the example of Little Shoal Bay in Auckland. “The water only used to flow across the culvert in the very highest sea-levels. Now it flows across there quite rapidly and is starting to encroach onto the cricket fields and kill the grass.”

The surprising finding in the study, says Scott, is that the small annual sea-level cycle made quite a big difference.

A king tide and storm surge causing flooding along Auckland's Tamaki Drive.

A king tide and storm surge causing flooding along Auckland's Tamaki Drive. Photo: Stu Mackay / NIWA

Scott and Rob say the situation will get worse in the coming decades and centuries.

“In the next 30 years," says Rob, "it’s pretty certain we’re going to get somewhere between 25-35 centimetres of sea-level rise.”

Adaptation

Rob works with councils and local communities in low-lying coastal areas to help them develop strategies to deal with rising sea-levels.

“Coastal flooding is the really big emerging risk ... compared with coastal erosion, which gets a lot of press,” says Rob.

He points out that coastal hazards vary by location. He says the large waves in mid-April that flooded roads and houses on Wellington’s south coast were an example of wave overtopping on a high tide.

A bridge near Ōwhiro Bay is damaged after massive swells overtook the area. April 2020.

A bridge near Ōwhiro Bay is damaged after massive swells overtook the area. April 2020. Photo: Supplied / Grant Maiden

The Hauraki Plains and south Dunedin on the other hand have rising groundwater which makes them more susceptible to flooding.

“We’re seeing these compound hazards, like a ‘flood sandwich’ between storm surges at the seaward end, and more intense rainfall, river floods and rising groundwater at the other end,” says Rob.

Rob says coastal flooding will increase in frequency and severity as sea level rises, but that it is unrealistic and far too expensive to protect coastal housing and infrastructure against every possible scenario.

“We’re trying to provide guidance that gives councils a bit more of a realistic look at how these things combine,” he says.

To find out more about this research and how scientists are working with local communities to adapt to rising sea-levels, listen to the podcast.

The Deep South National Science Challenge and the Resilience to Nature’s Challenges National Science Challenge do a lot of research in the area of hazards, especially coastal hazards.

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