The past few hurricane seasons have left some of us complacent. We have been lucky to dodge bullets in the past years, but what impact will a hurricane have on our area? A wonderful article was published in the Post and Courier on Wednesday, March 5 of 2008. Tony Bartelme wrote the following article outlining the various categories of hurricane strength and how each will effect Charleston.
“A Category 2 hurricane would turn much of downtown Charleston and Mount Pleasant into a soggy flooded mess, according to special computer-generated maps used by emergency officials.
A Category 3 storm surge, meanwhile, would flood all but a few patches of higher ground east of the Cooper and turn Savannah Highway into Savannah Causeway.
And a Category 5? Break out the snorkels.
Wet and Wetter
View the map of which areas would flood in different hurricane-intensity scenarios.
The National Hurricane Center generates these storm surge maps using the little-known but well-named computer model called SLOSH, which stands for Sea, Lake and Overland Surges from Hurricanes.
Government agencies then use the SLOSH data to decide which areas to evacuate and when.
A poster of an interactive version of the model is being displayed at the federal Interdepartmental Hurricane Conference in Charleston this week. The version allows users to single out individual landmarks and get a vertical look at how deep the water would be at that landmark. The model is in development.
Hurricanes are notoriously fickle and can arrive in an infinite number of directions, speeds and tidal levels. This makes predictions of their impacts just as tricky.
Still, a SLOSH map obtained by The Post and Courier and generated for a hypothetical direct hit on Charleston provides an intriguing snapshot of how the city might fare in certain hurricane scenarios. It’s also a bit of a reality check for Hurricane Hugo veterans.
When Hugo spun into South Carolina, the storm’s sustained winds in downtown Charleston were 87 mph with a 108-mph gust, and a 10.4-foot storm surge in the harbor.
By most measures, Hugo put downtown Charleston in Category 1 and 2 conditions. It was a different story in the Francis Marion National Forest, 30 miles north of downtown. There, winds were at Category 4 levels — 130 mph and higher. The storm surge in McClellanville hit 19 feet, so high that shelter seekers in Lincoln High School nearly drowned.
Partly because of Hugo and the Lincoln High School incident, the National Hurricane Center created SLOSH to simulate storm surges for different hurricane intensities and directions, said Doug Marcy, a scientist at National Oceanic Atmospheric Administration’s Coastal Services Center in Charleston.
The newspaper obtained SLOSH calculations for a hurricane with an eye coming in south of Kiawah Island. This reflects more of a worst-case scenario for the city because surges are higher in a storm’s northeastern quadrant.
In a Category 1 storm, a surge of 7 to 10 feet would have a relatively minor effect on the area, the calculations show.
Floodwaters would fill the area’s marshes and low-lying areas on the peninsula and the sea islands. But parts of the Isle of Palms and other barrier islands would still remain above water.
That changes dramatically in a Category 2 storm with a surge of 12 feet. In this scenario, the barrier islands all but disappear and most of downtown Charleston and James Island flood.
In Mount Pleasant, the only dry ground would be along Johnnie Dodds Boulevard, which is built on an ancient sand ridge. In this Category 2 scenario, floodwaters would begin to march into North Charleston toward Park Circle. And for the first time since it was developed, most of Daniel Island would temporarily lose its island status and become a seabed.
In Category 3 surge, most of West Ashley goes under water, along with a few specks of downtown Charleston.
In a Category 4 scenario, a 15- to 20-foot surge inundates much of Johns Island, while floodwaters on the Charleston Neck are neck deep.
A Category 5 scenario? Some parts of Charleston would be under more water than the low areas of New Orleans after Katrina.
Forecasters caution that SLOSH maps aren’t perfect. They say the calculations have a 20 percent margin of error, and they also don’t take into account tides, which could add or subtract a few feet of surge depending on a hurricane’s timing.
Still, they offer an important lesson: For much of the immediate Charleston metro area, Hugo wasn’t a worst-case scenario.
“Hugo was an awful storm but had it moved south and come in over Kiawah, we could have had 18 to 20 feet of storm surge, instead of the 10 feet we got on its backside,” Marcy said.
Reach Tony Bartelme at firstname.lastname@example.org or 937-5554.”