NOAA: 30 days of ‘sunny day’ flooding in Charleston yearly by 2020

NOAA: 30 days of ‘sunny day’ flooding in Charleston yearly by 2020

Charleston Post and Courier
December 25, 2014

By David Slade

Nuisance flooding in Charleston has increased fourfold since the 1960s, and it’s going to get worse as sea levels continue to rise, according to NOAA.

The National Oceanographic and Atmospheric Administration says Charleston is among coastal cities that will have at least 30 days of “sunny day” flooding yearly by 2020.

Essentially, the gap between the highest average tides and the water level that will cause flooding is getting smaller, as sea levels rise.

The new estimates follow an NOAA report earlier this year that said Charleston was among 10 U.S. cities with the largest increases in nuisance flooding since 1960. According to that report, Charleston used to average fewer than five nuisance flooding days yearly, from 1957 through 1963, but averaged more than 23 from 2007 through 2013.

Within six years, the average number of such flooding days in the Charleston area is expected to be six times higher than in 1960.

A handful of cities have already hit the “tipping point” defined in the NOAA study as 30 or more annual flooding days; Wilmington, N.C.; Annapolis, M.D.; and Washington, D.C.

Flooding in downtown Charleston is a well-known problem – city leaders were looking for solutions in the early 1800s, and an extensive tunnel-and-pump system is being created – but tidal flooding impacts a broader area. On Monday morning at high tide, for example, the Mount Pleasant Police Department warned of water on a section of busy Long Point Road that passes across the marsh at Boone Hall Creek.

The new NOAA study, presented at the American Geophysical Union’s fall meeting and published in the group’s peer-reviewed journal Earth’s Future, raises questions about when “nuisance flooding” becomes a serious hazard due to its frequency.

“Coastal communities are beginning to experience sunny-day nuisance or urban flooding, much more so than in decades past,” said NOAA oceanographer and study co-author William Sweet. “This is due to sea level rise.”

“Unfortunately, once impacts are noticed, they will become commonplace rather quickly,” he said. “We find that in 30 to 40 years, even modest projections of global sea level rise – 1.5 feet by the year 2100 – will increase instances of daily high tide flooding to a point requiring an active, and potentially costly response, and by the end of this century, our projections show that there will be near-daily nuisance flooding in most of the locations that we reviewed.”