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Spreading sand to save a salt marsh
On a humid August morning, the Webhannet salt marsh in Wells was a sea of tousled, neon-green grass. But at the marsh’s northern edge, one spot stood out — about two acres of glittering white sand with only a few plants breaking through. Late last winter, excavators spread sand dredged from the nearby harbor there, in a bid to save the sinking marsh and protect the birds that rely on it.
The marsh is a test site for a type of restoration never before tried in Maine, known as the “beneficial use of dredged materials.” The method involves spreading a thin layer of sand and sediment to raise the marsh surface, helping it better withstand sea level rise and support birds like the endangered saltmarsh sparrow and black ducks. At this site, crews used 1,000 cubic yards of sand to raise the elevation by 3 to 6 inches.
At this time last year, this part of the marsh was much wetter. It flooded often and a large deep pool was expanding every year.
“When it’s completely waterlogged all the time the peat starts to break apart,” said Bri Benvenuti, a regional biologist at Ducks Unlimited, which led the project in cooperation with a group of local and federal agencies and university researchers. “It gets really soggy and mucky and decomposes, essentially.”
The Webhannet marsh, like many others in Maine and along the East Coast, is drowning as sea levels rise. Marshes act like sponges, absorbing flood waters and storm surges, and buffering communities from storm damage, while providing habitat to many species.
One of the reasons marshes are struggling to keep pace with climate change is because they are starved of sediment-like sand and mud. In an unaltered system, storms would wash up over barrier beaches and dunes, bringing sand onto the marsh.
This constant accumulation of sediment counteracts erosion, helping the marsh remain stable — which is even more critical as sea levels rise. But development — like roads, culverts and the strip of beachfront homes on Drakes Island beach has disrupted the process.
“Those dunes are largely non-existent now because they’re houses,” Benvenuti explained. “So the sand isn’t moving over those dunes at all.”
To mimic these processes, some scientists and conservationists in some parts of the country, particularly the Gulf Coast, have turned to the sand and sediment dredged from harbors and channels. While much of the material is dumped offshore or spread on beaches, it could be put to better use, Benvenuti said. “If we have this sediment that’s going to be pumped onto a beach or disposed of offshore, why can’t we put it onto a marsh instead?”
The idea isn’t quite as simple as it sounds. Getting the necessary permits took years, since this was the first project of its type in Maine. After dredging, the sand had to be tested for contaminants, and screened to remove debris.
University of New Hampshire researchers and Ducks Unlimited measured the site’s topography using LIDAR to determine how much sand to spread in different areas. To improve drainage to the site, a berm was removed and small channels were dug. And, in order to protect the marsh from damage, the crews had to wait until the ground was frozen and, even then, used special mats to move across the surface. |
