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