The city of Isle of Palms might have to come up with more than an average of $6 million a year to maintain the integrity of its sandy shore, according to a study that was presented to IOP's Beach Preservation Advisory Committee Sept. 2.
Chris Creed, a senior coastal engineer with Foth Olsen, which was hired by the city earlier this year to conduct a study of IOP's beach restoration strategy, told committee members that it would cost between $5,799,800 and $6,614,200 annually if the city chose to supplement the beach with sand dredged from the Intracoastal Waterway by the U.S. Army Corps of Engineers, counteract erosion by managing the shape of the Dewees Inlet shoal on the north end of the island and build a terminal groin at the south end.
A terminal groin, which would come with an estimated cost of $4.5 million to $5.5 million, is a structure perpendicular to the shoreline to trap sand and slow beach erosion. Creed said the 1,200-foot-long groin would be built with around 15,000 tons of rock.
“The groin does not alleviate the need for sand, but it will reduce the demand of sand from offshore borrow sites," he explained.
He cautioned that the figures he presented Sept. 2 were preliminary “planning level” numbers and could change based on a variety of factors, including whether the beach erodes at a low rate or at a high rate – which it did between July 2021 and September 2024.
Creed presented other options, all of them aimed at maintaining a baseline condition established by Foth Olsen, a professional coastal engineering consultant headquartered in Jacksonville, Florida. They included:
- sand from the Intracoastal and Dewees Inlet shoal management, at a cost between $5,303,000 and $7,385,800 a year;
- scheduled renourishment of the beach, ranging from $6,404,400 to $8,745,300 annually;
- renourishment supplemented with sand from the Intracoastal and Dewees Inlet shoal management, which would cost the city an annual average between $5,303,000 and $7,385,800;
- renourishment supplemented with sand from the Intracoastal, Dewees shoal management and a terminal groin at the south end of the island, at an average annual cost between $5,543,200 and $6,614,200;
- triggered renourishment, implementing a project when the baseline somewhere along the island has been jeopardized, at an average annual cost between $7,046,900 and $11,580,500.
Creed told committee members that trigger renourishment is “not that feasible … and really not a cost we want to talk about.”
At the Sept. 2 meeting, Isle of Palms City Administrator Douglas Kerr said a cost of $6 million to $9 million a year “jumped off the page at me,” noting that “we lose a couple of hundred thousand cubic yards of sand a year. If I applied that rate of Marinex coming by and replacing what’s been lost, I come up with less than $2 million a year.”
"We look at the gross loss," Creed responded. "We looked at how much is being eroded away but don’t take credit for sand that accumulates in adjacent areas. It’s not off of the island. It’s just not where it’s supposed to be anymore."
“That’s the way we look at the problem. This is a data-driven analysis,” he added. “We’ve used the data that’s available to us. I think these are solid numbers here.”
Creed pointed out that the city can look at several alternatives: Do nothing and see how long it takes to get to the baseline condition; count on beach renourishment, either regularly scheduled every five, seven or 10 years or taking the triggered approach; managing the shoal on the north end of the island; building a groin on the south end; or a combination of two or more of these options.
He said if the city takes no action, the baseline probably will be violated at the south end of the island and near the middle of the island, but “for the most part, we’re still OK after five years.”
“By seven years,” he added. “Things are really falling apart,” even if the erosion rate slows down.
According to Creed, the Corps of Engineers most likely will be able to provide the city with 150,000 cubic yards of sand from the Intracoastal every three years, which would help alleviate the erosion problem near Breach Inlet. However, he also noted that this sand might not be sufficient to maintain baseline conditions on the south end of the island.
Creed said he favors the shoal management approach before the city spends a lot of money trying to fix the erosion problem on the north end of the island.
“There’s not a lot of agreement on whether you want structures on the beach there or not,” he said. “Those structures on the north end of the island are the last line of defense and not intended to reduce the sand-loss rates, particularly right after the project is built because they will be buried and not affecting the sand-loss rate."
Members of the Beach Preservation Advisory Committee include Chair Dieter Ostermann, Wendi Pasterik, Alice Bova, Dan Slotchiver and John Shillling.