Gaining Ground on the East Coast’s Intracoastal Waterway
On a sunny April day in 2019, in a quiet corner of Georgia, a boat pumps sediment out of Jekyll Creek through a snaking black pipeline. Mud fans out the end, settling in a narrow band over a nearby marsh.
A handful of scientists from the U.S. Army Corps of Engineers looks on, along with other experts, in the hopes that placing a thin layer of dredged sediment over the top of the marsh will help them gain ground, literally, in the fight to save the saltwater marshes that flank the Intracoastal Waterway. The 3,000-mile water passage runs along the East and Gulf Coast, and it’s one of the busiest inland waterways in the United States.
“It would be a benefit for both worlds,” said Army Corps of Engineers biologist Erica Janocha, who helped manage the project. She was referring to the two-fold opportunity: find a use for sediment dredged from the increasingly shallow Intracoastal Waterway to ensure it’s deep enough to navigate, and build up the drowning marshlands surrounding it.
Salt marshes like the one along Jekyll Creek serve as buffers between water and land, putting them on the frontlines of climate change; over the last century, salt marsh habitats lost half their global coverage, due in part to rising seas. And in Georgia, the state’s low shoreline geology means some marsh areas may soon get overtaken by the sea.
The Army Corps’ Savannah District wants to raise the Jekyll Creek marsh by 15 to 20 centimeters across the site. A pencil’s length of a distance, yet one that makes a meaningful impact to the marsh’s chances of survival — and helps deepen the Intracoastal Waterway so that boats can safely pass through.
But laying the sediment that April day — in what’s known as a thin layer placement, or TLP, project — was only the beginning. In the five years since, a team of scientists at Georgia Southern University and the University of South Carolina has sampled and evaluated this marshland, trying to figure out if TLP projects could be implemented over larger areas in other regions without damaging local flora and fauna.
Salt marshes like the one along Jekyll Creek serve as buffers between water and land, putting them on the frontlines of climate change.
Dredging itself can exacerbate the erosion of marsh edges, since it encourages faster and more extreme tidal changes. However, the biggest challenge with dredging is where to put all the resulting sediment. As government funding has increased for the waterway— the Corps has devoted roughly $150 million to operating and maintaining the southern portion since 2022, according to the Atlantic Intracoastal Waterway Association — so has the amount of sediment.
If that sediment can successfully raise marshes in Georgia, experts say there’s a high likelihood the technique can translate to other parts of the East Coast and become a bulwark against climate change’s onslaught of sea water.
However, Janocha warned, “It’s definitely a slow process.”
The Intracoastal Waterway appears in many places a pastoral river, but the protected waterway is not just a playground for the pleasure craft of recreational boaters and snowbirds. Commercial ships also rely on the route to carry goods to inland ports, notably fuel as well as heavy construction materials like rock and gravel.
Federal law requires most of the waterway to have a depth of 12 feet. But many parts are now in violation of that; sometimes as shallow as 5 feet in the Atlantic Intracoastal Waterway, or AIWW, which shadows the serpentine curve of the lower East Coast through Virginia, North Carolina, South Carolina, Georgia, and Florida.
This is due to years of inadequate funding, resulting in a backlog of maintenance projects. And the number of areas that need to be dredged often increases after hurricanes and flooding introduce more sediment into the channel, explained Brad Pickel, executive director of the Atlantic Intracoastal Waterway Association, a nonprofit that secures support for AIWW maintenance.
Increased federal funding in 2019 allowed the Corps to start to catch up: That year, it began a cycle of critical channel deepening projects, like at Jekyll Creek, the shallowest point in Georgia’s more than 160 miles of the AIWW.
Dredging is not a one and done thing, more like cutting the grass than pouring a foundation. And with the toll of increasingly extreme weather — though it’s not certain, Category 4 and 5 hurricanes are expected to become more frequent in the Atlantic — many parts of the AIWW will need multiple dredging cycles, as often as once a year, to get the waterway back to its authorized depth. Dredging doesn’t appear to be slowing down. For fiscal year 2024, the government has appropriated more than $48 million for operations and maintenance, including dredging, a more than 20 percent increase from the previous year.
The majority of the brackish AIWW that is being dredged is fringed by fans of salt marsh, a low topographic terrain prone to edge erosion. Yet, marshes can also prevent shoreline erosion by mitigating floods and filtering water runoff. The benefits don’t just extend to humans but also animals, like the gulls and terns that use the marshes as refuge during high tide to breed.
Accelerated sea level rise poses a significant threat to tidal marshes nationwide. A 2022 study in the journal Ecological Engineering found that sea level rise in one marsh system in North Carolina outpaced marsh elevation growth by roughly 4 millimeters annually over an eight-year period.
And it may only get worse, said Clark Alexander, director of the University of Georgia Skidaway Institute of Oceanography: “If they are not able to accumulate enough sediment, then the marshes will be inundated more and more frequently.”
Hard data on the effectiveness of thin layer placement is limited, even though there has been interest in the effects of adding sediment to marshes for almost half a century. As early as 1978, the Savannah District published results of an experiment on the effects on cordgrass, a perennial grass that covers nearly all of Georgia’s 400,000 acres of salt marsh, after the dispersal of dredged sand and clay. The cordgrass was able to penetrate up to 23 centimeters of placed sediment. The study team still cautioned that, “the technique cannot be considered proven and must be approached with care.”
The Jekyll Creek pilot project is unprecedented in scale in the southeast. Jekyll Creek’s marshes received a runny mixture of fine-grain sediment called pluff mud. “A lot of the other studies that have been done, they haven’t been done in that large of an area and they’ve been done with much sandier soil,” said Christine Hladik, an associate professor of geography at Georgia Southern University involved in the Jekyll Creek monitoring.
Historically, dredged material was often stored in confined disposal sites, removing vital sediment from the water system. TLP can help reverse this. Still, not all sediment is the same. Grain size is particularly important, as coarse grain sediment drains well and supports plant growth. Yet finer particles and siltier mixes — sediments that aren’t as beneficial for most types of reuse projects — are more common in the waterway. “A lot of clays; a lot of silt. Not a lot of sand,” said Janocha.
“If they are not able to accumulate enough sediment, then the marshes will be inundated more and more frequently.”
The concept of laying sediment over marshes also has negative associations: in the early 20th century, companies would dump mounds of material from dredging and drilling in the nearest available area (usually a marsh), with no thought to the environmental effects such as smothering plant life.
While the “thin” in thin layer placement can refer to a layer of sediment up to 50 centimeters, the target thickness of the deposited material rarely exceeds 20 centimeters. This low elevation helps preserve existing vegetation, said Alexander. “If you build it too high it won’t be inundated by the tides and then it will turn into a salt pan.”
Other recent TLP projects up and down the AIWW have had mixed results. The Philadelphia District of the Army Corps of Engineers spearheaded modern TLP pilot projects, beginning with a collaboration with government, nonprofit and corporate partners at three cordgrass marsh sites along the New Jersey Intracoastal Waterway from 2014 to 2017, with a goal of increasing surface elevation. By 2019, two sites had maintained at least some of their elevation gains. However, years later, vegetation at all three sites was still below the original baseline, according to a 2023 assessment.
But members of the Corps and other scientists learned crucial lessons about the tradeoffs between elevation gain and vegetation regrowth, and in 2017, National Oceanic and Atmospheric Administration scientists spread an average of 6 centimeters of sediment on low-lying marshes along the AIWW in North Carolina, and by 2021 determined there had been a doubling of flora. Vegetation can stabilize placed sediment and change sediment characteristics that can allow for a moderation of water currents.
There were similar findings for recent projects in New England. The Narragansett Bay National Estuarine Research Reserve is at the forefront of TLP experiments in the northeastern United States, and collaborated on two large-scale projects in Rhode Island since 2015. When asked about thin layer placement, Kenny Raposa, the reserve’s research coordinator, laughed: “We don’t actually call it that anymore in Rhode Island, our marshes are so low in the tidal waters. We’ve started calling it ‘elevation enhancement and sediment addition.’”
In the years since the sediment was added, most signs are promising. Crab burrows proliferate in the marsh. Plugs of marsh plantings placed by volunteers improved drainage and plant life came back quickly.
For the Jekyll Creek pilot project, the Army Corps of Engineers handled dredging and laying the actual sediment, but the researchers — Hladik and Risa Cohen at Georgia Southern University and James Morris at the University of South Carolina — have led the subsequent monitoring, much of it done via airborne remote sensing, that wraps up next year.
“I tell everyone every chance I get that what we need is research directed toward this issue. That’s the missing piece to really make this a long-term, routine, viable technology.”
The research team has been surveying six plots along with a control site since 2019. Mounted cameras capture images multiple times a day. On the ground, the team collects sediment cores to assess the presence of microbenthic algae and below-ground biomass, a sign of sediment recovery and stabilization and a measure of the marsh’s health. The pluff mud wasn’t distributed as evenly across the marsh surface as desired, in part because pumping it in created a low point in the riverbed that led to pooling.
The team also regularly measures plant height and density for each plot. “The vegetation is showing that it is coming back,” said Hladik, who led the geospatial aspect of the monitoring. As of April 2024, the placement site’s study plots had “from 70 to 100 percent cover vegetation.”
These encouraging results, which came five years after the original sediment placement, took longer than expected, she said: “We really thought it would be done in two or three years. We thought we’d see much more regrowth.”
The careful application that gives thin layer placement projects their name may be difficult to reproduce on a wider scale, experts say. At least until technology advances enough to allow for more cost-effective ways to spread sediment uniformly across larger swathes of marsh. Currently, the relatively small volume of sediment used for TLP can lead to a relatively high overall project cost since the unused dredged material still needs to be moved and stored elsewhere. Placement of less than 8 centimeters of sediment may not be cost effective, according to a New Jersey Department of Environmental Protection report that assessed past TLP projects.
“I think technology is what’s holding us back in using this right now,” said Alexander, who is a member of several working groups discussing TLP and other material reuse projects with the Corps. “I tell everyone every chance I get that what we need is research directed toward this issue. That’s the missing piece to really make this a long-term, routine, viable technology.”
“The test is going to be 10 years, 20 years on, to see how this area of marsh compares to the marsh around it.”
There’s still much more to learn about salt marshes as well as how to save them. This includes whether it is better to design future projects to be elevated for future sea rise or for current conditions and subsequent re-applications.
Marshes are also highly efficient at capturing carbon dioxide to lessen the world’s overall carbon footprint, and building up marshland through TLP may help preserve carbon storage in marshes, researchers say. Though the full value will take years to determine, Raposa hopes to examine how TLP affects carbon sequestration in an upcoming project.
The Savannah District doesn’t have any more TLP projects on the docket; though Janocha was quick to add that the Corps could implement thin layer placement in other areas, it wants to wait until the Jekyll Creek monitoring concludes.
Hladik echoes the Corps’ caution: “The test is going to be 10 years, 20 years on, to see how this area of marsh compares to the marsh around it.”
Mac Carey is a freelance writer from Virginia interested in science and subcultures. Her work has appeared in Texas Monthly, Washingtonian, and Mental Floss, among others.