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Coastal county’s new flood map might already be outdated

October 7, 2018

CHARLESTON, S.C. (AP) — A South Carolina county is getting new flood maps for the first time in 14 years, but it’s possible they’re already out of date.

The Post and Courier reports Charleston County’s new flood maps won’t include data from floods of 2015, 2016 or 2017, meaning some areas that went underwater during those events won’t be designated as high risk.

The maps mainly are based on what areas would flood in a Category 3 hurricane, County Building Services Director Carl Simmons said, meaning they don’t forecast floods like those from heavy rains in 2015 or if storm surge and heavy rain combine, as with Tropical Storm Irma in 2017.

Simmons says the Federal Emergency Management Agency doesn’t model a double punch of storm surge and heavy rainfall, as seen in last month’s Hurricane Florence.

“I don’t think we have done a good job of finding out where a flood from the Atlantic Ocean and stormwater meet. I don’t think we know where that line is,” Simmons said. “I think we’re going to have to work on locating that line.”

FEMA also doesn’t consider sea-level rise projections, which could extend the reach of storm surge.

Overall, the new Charleston County maps will remove 13,000 properties from flood zones, while putting 3,000 others into flood zones.

The maps are the basis for building and land-use regulations in flood-prone areas, and affect insurance decisions. About 38 percent of properties in Charleston County have a policy through the National Flood Insurance Program, according to the Insurance Information Institute.

“We have a huge protection gap when it comes to flood insurance,” said Lynne Yeates McChristian, executive director of the Center for Risk Management Education & Research at Florida State University.

She said many homeowners wrongly assume if they don’t have to buy flood insurance, they aren’t at risk. But more than a third of federal disaster assistance goes to flood victims outside flood hazard areas.

The most protective measures, such as elevating building, typically only apply to high-risk zones, even though floodwaters rarely stay neatly within those boundaries.

Charleston County requires all new structures in unincorporated areas to be built 2 feet above FEMA’s standard level. The city of Charleston requires buildings in the highest-risk zones to be elevated 1 foot above FEMA’s standard.

There are also questions of increasing development and sea rise. In Charleston County, developed area has increased 18 percent between 1996 and 2010, and impervious surfaces such as pavement have increased 20 percent, contributing to flash flooding.

One sea rise projection released this year by the Union of Concerned Scientists estimated that more than 16,000 homes in South Carolina could flood dozens of times a year by 2045. That would displace the roughly 24,000 people living there today, sink property values and possibly harm the region’s economy.

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