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News / Northwest

King tides have greater potential to flood Puget Sound

Sea levels around Seattle have risen 8 inches in 100 years

By Conrad Swanson, The Seattle Times
Published: December 18, 2023, 6:00am

The odds are changing in Puget Sound.

Last winter, when the king tide rolled in with heavy rains and an extreme low-pressure system, water from the Duwamish River rushed into Seattle’s South Park neighborhood, flooding homes, sparking broader evacuations and catching the city off guard.

Decades ago, that flood would have been thought unlikely, perhaps even statistically impossible, said Ian Miller, coastal hazards specialist at the UW’s Washington Sea Grant program.

But no more.

Climate change caused by burning fossil fuels is warming the atmosphere, melting polar ice caps and dumping freshwater into the oceans, which raises the overall sea level around the world. The warmer conditions also cause water to expand, raising the sea level even more.

Those rising waters mean that king tides now carry greater potential to damage or destroy homes, farms and infrastructure across Puget Sound, Miller said. Should those high tides pair with bouts of torrential rain and low-pressure weather systems, they could reach previously unrecorded heights.

The conditions almost collided last week, just days after the November king tides ebbed, when an atmospheric river dumped great quantities of rain on the region, flooding rivers, major roadways and neighborhoods across Western Washington.

“Sea level rise is weighting the dice,” Miller said.

What are king tides?

Gravitational fluctuations from the moon and the sun cause the ocean’s daily high and low tides. Those daily highs and lows also fluctuate throughout the year as Earth’s orientation changes.

King tides are the colloquial term for the peak of a given tidal cycle, the strongest of which arrive each year in November, December, January and February, Miller said. Another set of king tides arrives each summer, though they’re not as strong as the winter highs.

But the sun and moon aren’t the only variables influencing high and low tides, Miller said. Sea level is a substantial factor, and the average sea level is rising.

Over the past century, sea levels around Seattle have risen about 8 inches, Miller said.

Others estimate the rise in Seattle might be closer to 9 inches and climbing as climate change melts glaciers near Antarctica and Greenland.

By 2050, the rise already recorded around Puget Sound could more than double, according to the latest federal climate assessment, published last month. By the end of the century, those levels could rise further to 3 feet.

The king tides will continue throughout it all, stacking high tides on top of the rising waters, Miller said. Storms and low-pressure systems also amplify tides. (High-pressure systems compress them.)

Seattle saw the culmination of all those variables late last year when a king tide paired with heavy rains from an atmospheric river and an extremely low-pressure system, said Reid Wolcott, warning coordination meteorologist with the National Oceanographic and Atmospheric Administration.

The tide on the morning of Dec. 27 rose 2 to 3 feet higher than normal, and water from the Duwamish River spilled into at least 13 South Park homes, forcing additional evacuations in the area.

As climate change warms the Earth, atmospheric rivers will grow in size, last longer and carry more moisture, bringing with them heavier rainfall and faster winds.

“We have every reason to expect that we will see that more and more frequently in the coming years and decades,” Miller said.

Areas at increased risk

Facing the future, Miller explained three basic options for communities throughout Puget Sound and for coastal regions across the world. They can adapt to the new conditions, try to block them from happening, or move homes and buildings out of the hazard zones entirely.

King County is opting for a mix of the first two options.

As local officials pinpoint places at an increased risk, they can rely on emergency patches to guard against spot flooding and other issues, said Lara Whitely Binder, King County’s climate-preparedness program manager.

But those are short-term solutions.

“You can’t rely on sandbags as the fix. You have to start looking at more effective solutions,” Whitely Binder said.

New or expanded levees can block out rising sea waters and high tides, Whitely Binder said. Some buildings can be retrofitted for the new climate. New flap gates at wastewater pump stations can keep out increased water pressure, she said.

Or buildings can be designed with added flexibility so local officials can easily upgrade them as the climate changes further, she said.

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Some upgrades are already underway or finished. This year, county officials partnered with Seattle Public Utilities to install a new water pump station on the Duwamish River area that flooded last December. They also installed new drainage pipes and grinder pumps in homes to prevent sewer backups.

Wolcott said officials with NOAA used last year’s event to develop a new threshold for when the agency issues a flood warning, taking into consideration the ever-changing conditions.

But other areas need more work.

Whitely Binder added that low-lying places on Vashon and Maury islands — covering potentially hundreds of homes — will be at risk of rising seas and increasing tides.

That area will undergo a vulnerability assessment to determine the best course of action, Whitely Binder said.

The Jamestown S’Klallam Tribe is similarly taking stock of its position. The tribe already has decommissioned one of its buildings after sea levels in Sequim Bay rose too high, and now tidal waters are lapping at a second, said Hansi Hals, director of the tribe’s Natural Resources Department.

With the latest king tides, Hals said she must keep an especially close eye on the weather and the building that houses the department’s lab. Lately, high tides have come within half an inch of inundating the building, she said.

In the next year or two, the building will have to be decommissioned and the lab relocated, Hals said.

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