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Dredged sediment is key for restoring Chesapeake islands

Swan Island, MD, part of the Martin National Wildlife Refuge in the Chesapeake Bay, has thriving grasses on its eastern acreage where once there was only sand. The island is shown here in August 2022, with Smith Island visible in the background.

Posted on May 22, 2023

It’s August 2019 and heat radiates from  the expanse of sunbaked “mud” that covers two-thirds of Swan Island, a once half-drowned 25-acre island at the southern edge of Martin National Wildlife Refuge in the Chesapeake Bay.

The western third of the island, which is part of the archipelago that includes Maryland’s Smith Island, is a forested hummock known as the “rookery.” The newly placed mud (sediment, strictly speaking, which is a mixture of mud and sand) is about a foot deep and slopes gradually from high to low elevation, west to east. Marsh plants, some 200,000 of them, small and struggling in the summer heat, reach across the otherwise barren landscape in parallel rows.

Months earlier, this sediment was at the bottom of a nearby shipping channel — then dredged by the U.S. Army Corps of Engineers and spread on the island in an effort to restore it. It was enough dredge material to fill 18 Olympic size swimming pools.

In the past, sediment was considered an unwanted byproduct of routine dredging — scraped from the bottom of channels to keep them deep enough for ships. Until fairly recently, the common practice was to dispose of the dredge material upland and offshore, effectively removing sediment from the system. Now sediment is a prized resource in efforts to restore drowning Bay islands.

Given what we are up against, with climate change multiplying threats, we need to think beyond concrete and rebar, beyond even single-habitat restoration. We need to adopt a “multiple lines of defense” strategy to take advantage of the resilience potential of nature. Multiple habitats like islands, wetlands and reefs provide more protective benefits from storm and non-storm conditions than a single habitat type.

This image from August 2019 shows Swan Island, MD, soon after a layer of dredged sediment was laid down on its eastern acreage.

Sediment is the key to sustainability for these low-lying ecosystems. Plants have a natural “baffling” effect — slowing the flow of water — which allows sediment to drop out of suspension and settle on the bottom, where the plants trap and stabilize it. This is how natural systems like islands, mangroves, and marshes build elevation. But this can’t happen without enough sediment to outpace sea level rise. Plants drown when water rises and there is not enough sediment for them to build elevation.

Islands are the mainland’s first line of defense from the Bay’s waves, reducing erosion and flood risks to communities and shorelines. But those defenses are wearing thin, quite literally. Erosion and land subsidence (sinking land) have done their work for millennia, and rising seas have sped up the process.

Poplar (before restoration), Barren, James, Tangier and other islands have dissolved to slivers of their former selves. In a 15-year period alone (early 1990s to mid-2000s), islands in Tangier Sound lost 21% of their land to erosion from repeated dune and vegetation washovers. This has caused precipitous declines in waterbird nesting.

Islands are a lifeline for struggling bird populations. “Loss of habitat is driving waterbird declines throughout the Bay,” said Matt Whitbeck, a wildlife biologist with Chesapeake Marshlands National Wildlife Refuge Complex. “Islands are especially important for birds because they can provide predator-free nesting habitat.”

By August 2022, the once mud-caked eastern third of Swan Island had become a lush waist-high meadow of high marsh sloping toward lower elevations, where patches of 6-foot-high marsh grass push outward into a mosaic of intertidal pools and mud flats — now home to skulking egrets and grandstanding fiddler crabs brandishing claws. Sediment trapped by robust low-marsh thickets forms elevated micro-dunes where dune grasses begin to take root. Semipalmated plovers and sanderlings scurry, heads low, across the pockmarked mud flat and through shallow pools in search of their next meal.

While the “elevation capital” has increased and bought the island more time, the erosional trend is clear. Without action — i.e., some kind of intervention to slow erosion, facilitate accretion or add sediment — Swan Island’s days are numbered.

This story is playing out all over the Chesapeake. Starting in 1984 with Hart-Miller Island near Baltimore, the Corps of Engineers has used dredged sediment for the restoration of Battery Island, near Havre de Grace in the upper Bay; Poplar Island, near Tilghman; and parts of Barren Island, off upper Hooper Island in the middle Bay. After Poplar’s completion, Barren and nearby James Island will receive sediment from the Port of Baltimore.

Danielle Szimanski, a project manager with the Corps’ Baltimore District, has been helping to build islands for the last seven years. “The beneficial use of sediment from routine channel maintenance can slow island loss, keep sediment in the system and provide multipurpose benefits,” she said. “And, given all the remnants out there, I hope to continue restoring islands.”

Fortunately, momentum is building for beneficial-use and island restoration. With this strategy we can capitalize on the resilience power of nature and go beyond the single-minded use of conventional reinforced concrete. With its Engineering With Nature initiative, the Corps of Engineers has embraced natural approaches and a multiple-lines-of-defense strategy.

We need to embrace those ideas, too, and develop a new vision of what coastal resilience looks like.


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