‘Sunny day’ high tide floods are on the rise along U.S. coasts

As sea levels continue to rise, many coastal U.S. cities will see an increasing number of days each year that streets flood during high tides, according to the U.S. National Oceanographic and Atmospheric Administration. For many parts of the country, particularly along the U.S. East Coast, that increase has already ramped up over the last two decades.

From 2000 to 2019, these “sunny-day flooding” events jumped by 190 percent in the Southeast, and by 140 percent in the Northeast, according to a report by NOAA released July 10. Such events can devastate coastal infrastructure — for example by disrupting traffic, inundating septic systems and salting farmlands.  

In its fifth annual high tide report, NOAA details flood risks faced by different U.S. regions using tide gauge data collected at 210 stations around the country from May 2018 to April 2019. Officials’ definition of a “flood” can vary, depending on factors including the shape of the land, urban development and storm-proofing. But across all U.S. coastal areas, tidal flooding occurred an average of five days during the study period — repeating a record set in 2015, the report says.

Still, some regions saw tidal flooding far more frequently than the national average. The Chesapeake Bay region set new records in the last year, with 22 days of high tide floods for Washington, D.C., and 12 days each in the Maryland cities of Annapolis and Baltimore.

“It’s primarily an issue in the East Coast and Gulf Coast at the moment,” said NOAA oceanographer William Sweet, who led the study, during a July 10 news conference. Flooding in the densely populated Northeast, in particular, is influenced by “a very energetic system” offshore involving winds and ocean currents along with sea level rise. The land surface in the Chesapeake Bay region also is slowly sinking, part of a delayed readjustment to the retreat of the great ice sheets that covered North America thousands of years ago (SN Online: 8/15/18). 

Looking ahead to the “meteorological year” from May 2019 through April 2020, there will be an average eight days of tidal flooding in the region stretching from Virginia to Maine, the report found. Cities set for more flooding than the national average include Boston, which will likely get 12 to 19 days of tidal floods, and New York, getting eight to 13 days.

The U.S. Southeast, from Florida to North Carolina, is expected to see an average of five days of tidal flooding through the meteorological year. The western Gulf of Mexico is set to record an average of six days in tidal flood events per year, a 130 percent jump relative to two decades ago.

The report also projects future high tide flood days for each station in 2030 and 2050. For those projections, the researchers used two different greenhouse gas emissions scenarios: a business-as-usual scenario in which emissions continue on their current trend and an intermediate emissions scenario that assumes some reduction in current emissions trends.

By 2050, what are currently the worst flood days in some cities will basically become a new normal, the results suggest. The southeastern Virginia city of Norfolk, for example, will have about 170 days of high tide flooding each year by 2050.