Charleston could see as many as seven days of significant disruptive tidal flooding in the next year, the National Oceanic and Atmospheric Administration projected in a report released Wednesday.
That flooding would be enough to hinder traffic or close roads without any rain or storm surge.
Authors of the report emphasized in a conference call with reporters that the flooding is already affecting daily life in coastal cities around the country and will only increase in frequency in the coming decades as sea level rise escalates.
"We cannot wait to act," said Nicole LeBoeuf, acting director of NOAA's National Ocean Service. "This issue gets only more urgent and complex with each passing day.”
The estimate does not include flooding from tropical systems or sudden cloudbursts — other frequent sources of flooding in coastal South Carolina and elsewhere.
NOAA's standard used to declare a Charleston "flood day" in the report is also half a foot higher than the water level at which the National Weather Service warns coastal communities about flooding, meaning it may not capture every instance of flooding in the Lowcountry.
For example, NOAA noted about five flood days in Charleston from May 2018 to April 2019, but the harbor's tidal gauge reached or exceeded 7 feet on 44 days over that period. That's about the point at which minor coastal flooding begins.
"We’re tracking a slightly deeper and more severe flood than (local communities) witnessed on the ground," said William Sweet, an oceanographer with NOAA.
The report, he added, uses a different standard because it's comparing 98 disparate tide gauge locations across the country.
Twelve of those locations broke or tied their previous records for flooding days in 2018, with some of the most frequent tidal flooding reaching Washington, D.C., and other cities near the Chesapeake Bay.
Charleston, by contrast, has had drainage issues since its founding. But the pace of tidal flooding is accelerating to unprecedented levels.
The city flooded four days a year about 50 years ago, but that average is now closer to 40. Projections used in Charleston's recent sea level rise report say the flooding could happen as often as every other day by 2045.
The city has been working to alleviate the flooding with built solutions, such as check valves that stop tidal waters from seeping upward through drainage pipes and a new berm along Morrison Drive, a flood prone corridor that's one of the city's latest hot spots for growth. City staff also use portable pumps during flooding to try and drain some of the most water-logged areas.
Some bigger public projects have hit hurdles. For example, a massive underground pipe and pump system to drain water of the Septima P. Clark Parkway, also referred to as the Crosstown, is $43 million over budget. The system will drain one of the city's busiest thoroughfares, but it won't be completely finished until 2024.
Still, Sweet said, "Without adaptation, impacts are likely to become chronic sooner rather than later."