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Your questions answered on proposed $1.75 billion Charleston, SC, flood wall

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A $1.75 billion, 8-mile-long flood wall around the Charleston peninsula is proposed by the Army Corps of Engineers. Here, The Battery is seen on Monday, April 20, 2020. Lauren Petracca/Staff

A $1.75 billion, 8-mile-long flood wall around the Charleston peninsula proposed by the Army Corps of Engineers could send a  lifeline to a low-lying city feeling the threat of sea level rise.

The project would be the most expensive and extensive flood-control work in the city's history, and readers had plenty of questions about it.

Here are our answers to some key queries:

What's proposed?

The Corps proposes a wall around much of the Charleston Peninsula to keep out storm surge. Five new pump stations would move rainfall outside of the perimeter of the wall and a 4,000-foot-long breakwater would slow down waves at the southern tip of the peninsula, below the current High Battery. 

The Corps suggests "nonstructural measures" for some neighborhoods north of the wall, including Rosemont and Bridgeview Village, which may include buyouts, floodproofing, building elevations or relocations. The Corps says the elevation of these areas is generally higher and that it would be more cost-effective to use measures other than a permanent wall.

What will it look like?

It's not clear what the wall would look like. City officials say those details will be worked out as public comments come in and the plan is refined over the next year. Any improvements beyond the Corps' base design would be paid for by Charleston. 

The wall's height will vary based on elevation, but would be about 3 feet higher than the High Battery. 

How will we pay for the plan?

The federal government would pay 65 percent of the cost, leaving Charleston with the remaining 35 percent, or about $600 million. That whole sum doesn't have to come out of the city budget, however. There are state assets and private businesses behind the proposed wall and those groups may be convinced to share the cost.

The city of Charleston would not have to commit to its financial portion until fall 2021, but wouldn't start paying money until the work begins.

The project isn't a sure thing, however, unless Congress agrees to fund the federal portion after that date.

Will a wall downtown flood out West Ashley, James Island, Mount Pleasant or other areas?

Unclear. That potential effect (and any other negative consequences) will be studied by the Corps over the next 12 months.

The Corps' initial analysis shows the wave-deflecting breakwater south of the wall would not make surge worse in surrounding communities.

Will a wall create a 'bathtub effect' and trap rainfall inside the city?

The wall would include several gates to let water run off the peninsula during normal storms. If tides in the Charleston Harbor are predicted to reach 8 feet or more, the gates would be closed to keep those tides out. 

When the stormwater gates are closed, the Corps says that the five proposed pump stations would help move rainfall outside of the wall so that flooding in the interior isn't worse. But its draft plan says that the engineering group will continue to study the potential for interior flooding as it refines the proposal. 

How severe a storm would the wall protect against?

The proposed surge wall would have easily deflected the highest tides from recent tropical cyclones, including Matthew in 2016, Irma in 2017 and Dorian in 2019. 

It's also designed at about 2½ feet higher than Hurricane Hugo's surge in downtown Charleston. Having said that, downtown Charleston did not see the most severe surge from Hugo — that was measured near McClellanville.

If Hugo's peak surge hit the peninsula at high tide, it would overtop the wall by 5½ feet, according to the Corps' calculations. 

Could the wall be higher or added to after it's built?

The Corps' plan says the height of the wall — 8 feet above where tides start to flood the city — is as high as it can be without ballooning costs. 

Anything higher than that would require either lifting or putting a gate across the U.S. 17 Ashley River bridges. At 3 feet higher than proposed, it could also require modifications to Interstate 26.

The type of walls the Corps has selected for the project will have deep piles driven underground, making it possible to add onto them after they're built, a Corps spokeswoman said.

Would a surge wall decrease flood insurance for people behind it?

Flood insurance policyholders inside the wall could see changes, but the wall would need to be in place before new rate maps are drawn, said Stephen Julka, Charleston's floodplain manager.

If there's not a redrawing in the cards when construction finishes, Charleston could ask the Federal Emergency Management Agency for new maps. But the city would have to pay for it.

"It's not going to be a flip of the switch that the wall goes up and insurance rates go down," Julka said. 

Would the wall follow the High Battery and Low Battery around the perimeter of the city?

The wall would follow the path of the Low Battery and High Battery seawalls. From there, the path varies. It could change with more study over the next year.

Along the Ashley River, the wall would protect most waterfront structures, with the exception of the Charleston City Marina and a small street that juts out from the Wagener Terrace neighborhood, Lowndes Pointe Drive. The wall would end at Interstate 26, just north of Wagener Terrace. 

On the Cooper River side, there's more landmarks outside the path of the wall, including S.C. Ports Authority facilities, the S.C. Aquarium, the site of the future International African American Museum and several gravesites, including Magnolia Cemetery. The wall would end at Mount Pleasant Street on that side.

Some of those landmarks, like the aquarium and museum, already stand higher than the wall would be. 

Would you be able to walk on top of it? Would there be no view of the water except for from the top of the wall?

City officials say their goal is to expand access to the waterfront with the wall, by creating pathways similar to the current High Battery along a larger portion of the city. In some areas, however, the path of the wall would be broken by gates to let car, pedestrian or boat traffic through existing transportation routes, or to let stormwater drain. 

In some places, like low-lying Lockwood Drive, the wall would inevitably obstruct visibility to the water. But in places with higher elevations, the wall might only be a few feet tall, and thus, far less noticeable.

Reach Chloe Johnson at 843-735-9985. Follow her on Twitter @_ChloeAJ.

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