Building sunken breakwaters off SC coast to halt beach erosion has unclear future

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Posted March 11, 2020

A submerged breakwater system proposed to lessen beach erosion in South Carolina won’t pop up offshore anytime soon.

No one is sure of their exact design or where they would have to go to be effective. Or what such devices would do to the environment up and down the coast.

They don’t know how deep to sink them or how many feet, yards or miles the breakwaters would have to stretch.

The people who have studied the potential for South Carolina haven’t guessed at the cost.

It is clear that the concept is one of the more novel solutions floated as the state considers ways to protect its nearly 200-mile coast, the main draw for a $24 billion tourism industry. Threats to the shore are compounding with global-warming-driven sea level rise, and have been emphasized with several severe storms in the past five years.

The idea — building an underwater barrier to slow down waves, and thus slow the flow of escaping sand — surfaced in the work of Gov. Henry McMaster’s S.C. Floodwater Commission. That group worked for more than a year investigating the idea, but it’s still more concept than reality.

Coastal experts worry about disrupting the coastal flow of sand, nesting sea turtles and worsening water quality on the beach.

Even the name the commission attached to the idea — “artificial reef” — isn’t quite clear. That’s what the state calls sunken ships in deep water meant to attract fish, which is an entirely different proposition than something engineered to slow down waves.

South Carolina won’t get any more clarity until the state hires a chief resilience officer. The position, tasked with preparing the state for the effects of climate change, is a new job McMaster has asked the S.C. Legislature to fund.

That still-hypothetical officer will prioritize the breakwater and other ideas from the flooding task force, said Tom Mullikin, the volunteer chair of the group. Mullikin said none of the suggestions have been taken off the table.

Deploying such a fix would be unprecedented in South Carolina, which bans some beach infrastructure, like seawalls, and where environmental groups frequently mount challenges to others, like sand-trapping groins.

The idea has attracted some criticism already, and has its limits: experts agreed they would do little to stave off storm surge from hurricanes, for example.

It’s also exposed a frequent clash about the vulnerable coast over whether to build complex protective structures to stave off flooding and erosion, or whether to mitigate harm by protecting the natural landscape and move people away from the threats.

Fred Holland, retired director of the Hollings Marine Laboratory, said, “I’m not sure we can engineer our way out of the flooding problem and the surge problem and the beach erosion problem.”

Mullikin countered that work on flooding threats needs to proceed “holistically” and include a focus on how to protect vulnerable people and properties.

Some have worried about the scale of such a breakwater, but the commission was never suggesting that the entire coast be buttressed with a barrier, said Will Ambrose, of Coastal Carolina University’s School of the Coastal Environment. He helped author the Floodwater Commission’s report on breakwaters.

The report suggested piloting one or two of the reefs on the coast. The authors did not estimate a cost for that work.

“We didn’t advocate the wholesale deployment of artificial reefs all over the place,” Ambrose said. “We state quite clearly that these could possibly be beneficial in some areas.”

River of sand

Though there are few such wave barriers deployed around the United States, breakwaters are common in other countries, particularly around Europe, said Siddharth Narayan, an assistant professor of coastal engineering at East Carolina University.

In all cases, Narayan said, engineers use them to trap sand.

As ocean waves hit beaches at an angle, they create “littoral drift,” or a river of sand, flowing approximately parallel to the coastline. In South Carolina, that river runs generally from the state’s northeast edge to its southwest point.

When a breakwater is constructed parallel to the shore, Narayan said, it stops waves from scraping sand off the beach and into that river. That means sand drifting to the protected section of beach will start to collect behind the breakwater, potentially filling in all the way back to land.

Ultimately, coastal engineers in other countries are moving away from breakwaters these days, Narayan said. There are sometimes unintended consequences to disrupting the natural flow of sand.

Damming that flow in South Carolina is a problem already, Holland said. That, combined with sea level rise, is fueling erosion in most places around the state.

In one example, Folly Beach’s rapid erosion rate has been blamed on jetties at the mouth of Charleston Harbor. On the opposite side of those jetties, Sullivan’s Island is one of the few places in the state where sand is piling up.

Projects far inland have the capacity to affect beach sand, as well, because the sediment that ends up on the beach is transported here though the state’s rivers. Lakes Marion and Moultrie, which were created by damming rivers, have steeply depleted the flow of sand from the Santee River.

“The natural, long-term movement of sediment is disrupted pretty badly along the entire east coast of the Southeast,” Holland said.

Testing the concept

Testing a breakwater thoroughly would involve modeling — putting together mathematical equations to predict how sand movement will change, and potentially also building a scale model of the device in a wave pool to test it, Holland said.

The floodwater commission’s report did not specify a location where a breakwater should be tested, but Ambrose said it needs to be an area where long-term data on sand movement is already available. An undeveloped barrier island north of Myrtle Beach, Waties Island, might be an option; one of Ambrose’s colleagues has been studying the area at Coastal Carolina, he said.

There will be more to test than sand movement.

Depending on the size of the barrier, it might limit water mixing. In Long Beach, Calif., a more than two-mile breakwater constructed in the 1940s to protect Navy operations there is often blamed for poor water quality as runoff can’t mix with ocean water, the Los Angeles Times reported. A recent study on what to do about the barrier concluded it had to stay in place for those Pentagon operations to continue.

Additionally, little is understood about breakwaters’ impact on marine and coastal life, Narayan said.

One project off the southern coast of England didn’t make swell break into large, surfing waves as intended but did prove to attract some marine life, like the reefs already off coast of South Carolina, he said.

Other coastal engineering in the Palmetto State — such as “wave dissipation” systems constructed on Isle of Palms (and later removed following a lawsuit) — have been found to dissuade sea turtles from nesting.

David Whitaker, retired assistant director of S.C. Department of Natural Resources’ Marine Division, worried that such a barrier would scare off species that rely on a sandy beach.

Whitaker said constructing a near-shore barrier could mean “inhibiting movement of fish and sea turtles and other animals that come up to the beach to feed and lay eggs.”

“There could be all sorts of problems,” he said.

Source: postandcourier.com