‘King tides,’ summer swells and high sea levels could imperil coastal areas

A rare convergence of rising sea levels with some of the highest tides of the year could cause episodes of serious coastal flooding in Hawaii over the next few months, scientists are warning.

And if a sizable southern swell slams into the islands at the same time as the extreme “king tides,” look out.

The flooding could offer coastal communities a snapshot of what the future might look like in a world of escalating climate change and rising sea levels, said Mark Merrifield, director of the University of Hawaii Sea Level Center.

The next king tides will occur Wednesday and Thursday, followed by June 23-24 and July 21-23.

A sample of what might happen occurred April 28, when ocean waters were recorded 9 inches above what was predicted and ended up swamping beaches, boat ramps and some coastal roads in vulnerable areas across the island, including Mapunapuna and Hawaii Kai.

At Waikiki the Hilton Hawaiian Village canceled its weekly fireworks show after water covered the beach where a contractor normally launches its pyrotechnics.

Meanwhile the Honolulu Harbor tide gauge recorded the highest daily mean water level since such measurements began 112 years ago.

Merrifield, an oceanography professor who is also director of UH’s Center for Coastal and Climate Science and Resilience, said a number of factors are contributing to the extreme tides and coastal flooding threats.

The ocean around the islands has been experiencing higher sea levels over the last couple of years as cyclical global influences have pushed a vast stretch of higher seas across the Pacific, he said.

Over the last 20 years, sea levels were rising faster in the western Pacific under the influence of strong tradewinds and other factors, Merrifield said. Now the trades are in a relaxing mode, and that glob of high water has switched back to the east.

“We’re in the middle of something that is going to take a while to resolve,” Merrifield said. “The sea level will go down eventually, but right now we’re in this high period.”

Merrifield’s Sea Level Center is predicting that elevated water levels are likely to linger at least through the summer.

In addition, large bulging ocean eddies have been slowly swirling in and around the islands, boosting sea levels even more at times, he said.

During these periods of higher-than-average sea levels, the normal high tides influenced by the gravitational pull of the moon become even more elevated.

If those high tides are the king tides — the highest tides that occur around the winter and summer solstices — sea levels that run up on our shores are that much higher.

“It’s a stacking problem,” he said. “When you’ve got high sea level and you add the eddy and then add the king tide, that’s when you start to get to a super elevation, a flooding elevation.”

But that’s not all. Summer is the season for sizable southern swells, and if one of those hit the islands at the same time as a super-size king tide, there could be extra flooding, damage and other hazards.

“South-facing shores, like Waikiki, are the areas we are worried about,” he said. “Even without the waves, groundwater flooding will be an issue. With the waves, now we’re talking about overtopping of berms, coastal flooding and erosion.”

Will the stars align to bring a full-on watery assault on our southern shores?

Merrifield said it’s hard to say. There’s even a chance no one will notice if a swirling eddy moves away or results in a temporary drop in sea levels.

“We don’t know exactly how all of these components are going to line up,” he said. “Eddies are like the weather — you don’t know where they’re going to be from week to week.”

Even if extreme conditions fail to develop, the latest sea level forecasts call for flooding Wednesday and Thursday in Mapunapuna and other low-lying coastal regions as part of the king tides.

People and entities should be prepared with sandbags, sump pumps or whatever else is needed to protect assets during these periods of coastal flooding, the scientist said.

Coastal property owners and residents should anticipate impacts like the ones that occurred during the high tides of late April. Boaters, paddlers and fishermen might also face unusual water levels, currents and coastal hazards.

Chip Fletcher, associate dean of the UH School of Ocean and Earth Science and Technology, told the UH System News that water levels could reach more than 1 foot above typical high tide.

Fletcher said these events will be a preview of things to come as climate models indicate sea level rise accelerating.

“Within a few decades this will be the new normal. Hawaii should consider this a practice run and re-evaluate policies and development practices accordingly,” he said.

As the decades pass, Hawaii and other coastal areas will experience a growing amount of groundwater flooding, waves sweeping over the shoreline, land eroding away and storms and tsunamis penetrating increasingly inland over time, he said.

To help illustrate the growing problem, the university has created the Hawai‘i and Pacific Islands King Tides Project, a citizen science program aimed at documenting the high water levels.

“The king tides are a good indicator of what future sea level rise might look like,” said Maya Walton, program leader with the UH Sea Grant College Program.

The program is recruiting citizen scientists to take photos of the king tides for use in scientific research.

Training for those who want to get involved will be held June 1 from 6:30 to 7:30 p.m. For those who can’t be there in person, the session can be viewed online at youtube.com/hanaumatalks.

For further information on the king tides program, go to ccsr.seagrant.soest.hawaii.edu/king-tides.

The latest six-day sea level forecast for Hawaii and some other Pacific islands is found here: www.pacioos.hawaii.edu/shoreline-category/ highsea.