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NOAA predicts fewer days of severe tidal flooding in Charleston this fall vs. last

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Tuesday flooding_3.jpg (copy)

Charleston logged a record 89 flood events in 2019, including this flooding in December 2019 from a high tide and rain. File/Staff

High-tide flooding in Charleston is expected to lessen from last year, the National Oceanic and Atmospheric Administration projects, but the Holy City and the rest of the county are still on an increased trajectory for sea level rise. 

In an annual report of coastal flooding tied to the rising seas, NOAA predicts Charleston will see between four and seven days of severe coastal flooding for the period of May 2020 through April 2021.

Typically, the worst tidal flooding for Charleston and the Southeast comes in the fall when wind direction and the lunar calendar team up to spur higher oceans.

That's in contrast to a record-breaking 2013, when Charleston's tidal gauge measured severe flooding levels for a record-breaking 13 days.

The agency uses a higher benchmark for flooding that would close roads or disrupt traffic throughout the area without any rain.

It's a higher flooding threshold than used by the National Weather Service, which starts counting minor flood events at a 7-foot reading.

That level is unnoticeable in many parts of the region but is when the most minor effects from the rising ocean start. By that measure, Charleston logged 89 flood events in 2019, also a record. 

Across the country, "the underlying trend is one that's accelerating," said William Sweet, an oceanographer for NOAA. Flooding events are particularly severe in the Southeast, he said, because the region's shallow continental shelf makes it easy for winds blowing in the right direction to pile water onto land. 

Generally, sea levels are rising because of climate change, or a warming planet that's being insulated by fossil fuel emissions. That translates in two ways: Melting arctic ice adds more liquid to the globe's oceans and, to a lesser degree, the oceans themselves take up more space when water temperatures are higher.

Scientists refer to that second effect as "thermal expansion."

In some cases, the land itself is also sinking, which makes the local effect of sea level rise even worse.

In all, NOAA predicts that Charleston could see between 35 and 90 days of disruptive flooding annually by 2050. The trend is clearly getting worse, but it's not easy to guess in any given year how it will play out.

Last year was an especially bad anomaly. Ocean levels ran consistently high along the Gulf and East coasts last fall, and tide forecasters didn't have many clear explanations for why

One culprit could be ocean circulation, like the Gulf Stream, a highway of warm water along the Atlantic Coast that's been linked to rising seas when it slows down. Another might be hidden cases of thermal expansion, Sweet said. 

All ocean water doesn't mix evenly; it moves in currents or may collect in large "blobs," Sweet said, that are pushed around by winds and other factors. It is possible that pockets of warmer water as many as a thousand meters deep could be expanding and contributing to flooding problems, he said.

"It's not always obvious at the surface that the water is warmer," he said.

Charleston's tidal gauge, which has been measuring the height of the Cooper River at the mouth of Charleston Harbor for more than a century, has logged one of the longer records of sea change in NOAA's network.

"This network supports safe shipping and tracks dangerous storm surge, and it also tracks the steady creep of sea level rise," said Nicole LeBoeuf, acting director of NOAA's National Ocean Service.

It wasn't until 1979 that the gauge has logged 13 days of disruptive flooding over its history to that point, NOAA said in its report, a more than 50-year period that produced the same amount of flooding as 2019 alone. 

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Reach Chloe Johnson at 843-735-9985. Follow her on Twitter @_ChloeAJ.

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