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

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

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

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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.

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…

Go to this episode on rnz.co.nz for more details