Flooding at high tide on Sunday morning, September 20, in Charleston, South Carolina. (Image credit: Eric Dunphy)

Significant coastal flooding has been affecting much of the Southeast and mid-Atlantic U.S. coast since September 15, during the high “king tide” period associated with the New Moon of September 17. The king tides have been exacerbated by big swells from Hurricane Teddy, high runoff from the heavy rains from Hurricane Sally the previous week, and powerful northeast winds associated with a strong area of high-pressure positioned over New England.

Moderate coastal flooding at high tide has been widespread during the past week from Florida to North Carolina, with a few regions including Charleston, South Carolina, experiencing major flooding during multiple high tide cycles. NOAA indicated that Charleston experienced major flooding during high tides on September 15, 19, and 20, with these events ranking in the top-25 water levels of all-time. The high water did major damage to a $50 million beach renourishment project at nearby Folly Beach, washing much of it away. The tidal flooding has also forced the closure on multiple days of the only highway connecting North Carolina’s Outer Banks to the mainland, NC-12.

Figure 1. Highway NC-12 at the Canal Zone on the Outer Banks of North Carolina, as seen at 10:52 a.m. EDT September 21 from an NCDOT traffic camera. NC-12 was closed on Monday in both directions from just south of the Ocracoke Ferry dock to just north of the Pony Pen beach area, and from the town of Rodanthe to the Bonner Bridge, in light of standing water in the roadway.

How climate change turns minor tidal flooding into a big deal

Disruptive tidal flooding that now affects the U.S. Gulf and Atlantic coastlines on three to six days per year will strike as often as 80 to 180 days a year by the 2040s, according to a 2018 report from NOAA’s National Ocean Service, “Patterns and Projections of High Tide Flooding along the U.S. Coastline Using A Common Impact Threshold” (see PDF). That report built upon projections of global and regional sea-level rise that were released in a separate NOAA report earlier in 2018 (see PDF). The first sentence of the earlier report makes no bones about the situation: “Long-term sea level rise driven by global climate change presents clear and highly consequential risks to the United States over the coming decades and centuries.”

Though it’s often called “nuisance” flooding because it poses little threat to life or limb, high-tide flooding is a fast-growing threat to the economies and the built environment of coastal areas, especially along the Gulf and Atlantic shores. In South Florida alone, hundreds of millions of dollars are being spent to combat both long-term sea level rise and also routine “king tides” that are getting worse. High tide flooding is now accelerating at 75 percent of NOAA tide gauge locations along the East and Gulf Coasts, with nearly all other locations rising, but not yet accelerating. Already, the U.S. annual high tide flooding frequency is more than twice that in the year 2000 as a result of rising relative sea levels.

Figure 2. The number of days per year with at least minor coastal flooding in Charleston, South Carolina, shows a worrisome trend. (Image credit: Charleston National Weather Service)

High-tide flooding is distinct from extreme storm surges related to tropical cyclones and nor’easters, although they can overlap. By definition, high-tide floods happen at predictable points in the tidal cycle, such as the period from late summer into autumn when astronomical tides are at their highest. They can be enhanced by seemingly innocuous weather features, such as strong high pressure offshore that pushes high water toward the coast under sunny skies.

Also see: Flooding is the new normal in Miami

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Posted on September 21, 2020 (3:39pm EDT)

Jeff Masters

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...

Bob Henson

Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance...

62 replies on “Swells from Hurricane Teddy drive major king tide coastal flooding”

  1. “$50 million beach renourishment project” Now here’s the ample road to Corruption Hell.
    Affluent coastal communities have had themselves built piers as anti-erosion “sand catchments” which does’nt work because refraction from wave action carries the sand away. So they bitch to their congress hacks that property values are affected so please have COE renourish their (private?) beach which turns out to be BILLIONS handed out to COE so COE passes $$$ to their favored contractor after taking their cut (budgetary tool). This is repeated yearly of course.
    😉

  2. I wonder if things have reached the threshold where insurers of residential and commercial properties are starting to take a hard look at future coverage. A 30 year mortgage takes us to 2050.

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