Lowcountry in some pretty deep water

Chris Alexander gives Melissa Kucin and her dog a boat ride from their flooded home at the end of Two Loch Place to high ground after Shadowmoss Plantation was hit hard by flooding during a record rainfall in late August.

The flooding last month that swamped a mobile home park in North Charleston and impacted thousand of people’s commute from the Sea Islands is likely to happen again — and soon, experts predict.

Nuisance flooding is on the rise in the Lowcountry as development increases, filling in wetlands and creating more impervious surface, such as blacktops and roofs. It’s a problem felt up and down the rapidly growing East Coast.

The Shadowmoss Plantation flooding was in a low-lying area near Church Creek, along rapidly developing Bees Ferry Road. The Charlestowne Village Mobile Home Park flooding in North Charleston took place in a low spot across Dorchester Road from the newly built Jerry Zucker Middle School and in the vicinity of development around Charleston International Airport.

And the Main Road flooding below the Limehouse Bridge to Johns Island took place at a low point of the road where it crosses wetlands, near the newly built McLernon Trace townhouse subdivision and commercial development along U.S. Highway 17.

Overall, the Aug. 31 flooding was bad enough that Gov. Nikki Haley declared five coastal counties a disaster and the federal Small Business Administration is making available low-interest loans to help with recovery.

At a public meeting last week with state and county transportation staffers, residents weren’t mollified when the Main Road flooding was characterized as a “freak” weather occurrence of heavy rains and high tides.

“If you’ve got all that going on, all those houses, you’re always going to have flooding there,” said Edisto Island resident McKinley Washington, a former state senator, at the meeting.

“If you build up one side of the road and not the other, the water has to run down somewhere,” said the Rev. Bernard Brown to murmurs of assent.

The S.C. Department of Transportation operations chief engineer did not disagree. “Absolutely,” operations chief Andrew Leaphart said after the meeting, confirming that nearby development makes it harder to maintain roads not only because of more cars but because of difficulties draining them. “The geographical features of an area impact it,” he said.

The numbers back them up. A 2014 National Oceanic and Atmospheric Administration study concluded that Charleston floods four times more often today than it did in the 1960s, and it’s going to get worse. A number of factors were cited, including the loss of natural barriers, such as wetlands, as well as sea-level rise and land subsidence.

An ongoing College of Charleston study has found that the National Weather Service issued coastal tidal flooding advisories 10 times more in the past 10 years than it did a half-century ago. Researchers haven’t pinned down causes, said Lee Linder, atmospheric studies professor.

“There is no question the loss of wetlands affects it, but we don’t know how much it influences it. The wetlands (act like) a sponge,” he said. “The more you pave over an area, the worse flooding becomes. Definitely in the suburban areas, we’ve seen much more of that.”

As Leaphart noted during the meeting, “We can’t always build or construct our way out of these traffic jams.”

The problem is impervious surface — asphalt, concrete, roofs or similar areas that don’t absorb rain. Study after study, as well as common sense, has indicated that the fewer places water has to drain, the more it pools.

“Wetlands within and upstream of urban areas are particularly valuable for flood protection. The impervious surface in urban areas greatly increases the rate and volume of runoff, thereby increasing the risk of flood damage,” the North Carolina State University water quality program said on its website.

“In a developed watershed, much more water arrives into a stream much more quickly, resulting in an increased likelihood of more frequent and more severe flooding,” reported the U.S. Geological Survey on its website in August.

The fixes are expensive. Main Road improvements to relieve traffic congestion at the U.S. 17 intersection nearby have been estimated to cost at least $3.5 million. Residents want an overpass that would raise at least part of the flooding area. That would cost an estimated $50 million.

And the Lowcountry is rapidly developing into an urban area. A decade ago, a breakthrough study at the federal Hollings Marine Laboratory on Fort Johnson indicated that 14 percent of Charleston County’s land was covered by impervious surface. The lower portions of Dorchester and Berkeley County weren’t that far behind.

No more recent study has been done, but more than 60,00 people have moved to the tri-county area in the past five years, along with a number of large “footprint” businesses.

A residential and commercial development boom that started in the 1990s slowed with the 2008 Great Recession but is back underway. In 2013, that three-county area around Charleston was the 12th-fastest-growing metropolitan area in the nation, according to the U.S. Census Bureau.

Berkeley and Dorchester counties made a top-100 list of fastest-growing U.S. counties.

Reach Bo Petersen at 937-5744, @bopete on twitter or Bo Petersen Reporting on Facebook.