Red alert high tides: Coastal flooding risk amid February king tides

Climate change and sea-level rise are making coastal flooding events more frequent, an Earth Sciences NZ modeller cautions. Photo / Alex Robertson

Monday’s full moon may have been hard to see because of cloud cover over Tāmaki Makaurau, but its effects can still be seen with higher tides across the motu.

Earth Sciences New Zealand (formerly Niwa) have the high tides between February 1 and 4 marked as red alert dates with increased coastal inundation potential.

Glen Reeve, Earth Sciences NZ coastal and environmental modeller, says the tidal range experienced along the coasts of Aotearoa New Zealand is driven primarily by the gravitational forces of the moon and, to a lesser extent, the sun, combined with the earth’s rotation.

“The moon provides the dominant forcing, while the sun modulates tidal range through spring (high tide) and neap (low tide) cycles,” Reeve said.

Tides don’t all rise and fall together around the country due to our location between the large tidal nodes (areas of minimal tidal range) in the Tasman Sea and the South Pacific which, when added to the coriolis affect of a spinning planet, results in an anti-clockwise rotation.

The tidal wave moves from the West Coast, around the south of the country, then pushes along the East Coast with local bathymetry (submarine features) and coastal shape further affecting the timing of tides.

February is an uncommon month this year, and not just because of the number of red alert days.

In addition to the red alert days of the full moon, a further three days between the 19th and 21st are also ones to watch.

Reeve explains the additional red alert days are a warning of unusually high tides due to the alignment of the sun, moon and earth (around the new moon phase) creating stronger gravitational pull on the oceans.

When added to the higher declination of the moon as it wobbles in orbit around the earth and the increased draw of the sun in our summer, higher tides are expected, with alerts for coastal areas.

These high tides, also known as king tides, “greatly increase the risk of coastal flooding, especially when combined with waves, storm surge, or heavy rainfall”, Reeve said.

And, this week, a low moving in from the Tasman brings the possibility of all three adding to the tidal phenomenon.

“Because they represent the highest natural tides of the year, king tides also amplify the impact of even minor storms or onshore winds,” Reeve warned.

“A storm that might otherwise cause little flooding could produce significant inundation.”

The outlook for tides to impact our lives is only increasing with climate change and sea level rise, cautions Reeve.

“Sea-level rise is causing tides that once only caused nuisance flooding to become more frequent,” he said.

“This leads to changes in coastal flooding regimes ... where previously rare coastal flooding events are expected to occur more regularly.”

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