For Erosion Protection And Dredging, Tern Island May Be The Key

By: Alan Pollock

Topics: Erosion , Dredging

The flats extending north (right) from Tern Island (center left) might be nourished to extend the island, providing bird habitat and shoreline protection. SPENCER KENNARD PHOTO

CHATHAM When it comes to controlling shorefront erosion and maintaining navigation, Chatham Harbor poses some problems. The solution, or at least part of it, might be found on Tern Island.

On Wednesday, the state announced that Chatham has received more than $50,000 from the office of Coastal Zone Management to support a study of the island. Officials want to know whether adding sand to the island from nearby dredge projects might protect the mainland shoreline from some erosion, while providing more habitat for protected shorebirds.

“It’s still very much in its early conceptual stages,” Chatham Coastal Resources Director Ted Keon said this week.

Since 1936, the roughly three-and-a-half-acre island has been owned by the Massachusetts Audubon Society, held for conservation and wildlife habitat. Originally the remnant of a sand spit a century ago, Tern Island is larger today than in the past.

“It had been repeatedly utilized in the past as a dredge material disposal area,” Keon said. And in informal conversations with the town, Mass Audubon officials have expressed some interest in renewing that practice. Notwithstanding the current debate over dredging in the harbor (see related story), it’s assumed that there will be an ongoing need to deepen channels in the area to allow access to the fish pier by commercial fishermen and other mariners.

“We will continue to need new disposal areas,” Keon said. While there are a couple locations on the mainland shore where dredge spoils can be deposited, those areas have a limited capacity to receive sand.

Meanwhile, a recent coastal resiliency study predicts continued erosion along parts of the mainland shore of Chatham Harbor in the decades ahead, owing to the southward migration of the North Cut in the barrier beach. “I think we all recognize, as the inlet progresses, there will be continued exposure of the eastern shoreline to wave and storm activity,” Keon said.

Earlier this summer, Applied Coastal Research and Engineering issued recommendations about how town officials might mitigate that erosion in various parts of the waterfront, and engineer John Ramsey offered two potential approaches for the shoreline just west of Tern Island. The town might consider installing a series of around seven temporary timber groins designed to capture sand and allow the beach to widen in this vulnerable area, he said. While the structures would provide some protection to waterfront properties, that protection would be limited during severe storms, and could starve other areas of sand if not properly maintained. Such a plan would also require the town to negotiate with many individual property owners.

Another idea proposed by Ramsey is to use dredged sand to extend the northern end of Tern Island about 2,700 feet, creating a spit around 150 feet wide and six feet high. Doing so would require around 150,000 cubic yards of sand, Ramsey estimated. While the spit would still be washed over during severe storms, it would block a portion of the shoreline from ocean waves coming through the North Cut.

In addition, the spit could provide prime nesting habitat for protected and endangered shorebirds like piping plovers, which prefer low beaches that are relatively free from obstructions.

Building on its previous work, Applied Coastal Research and Engineering has proposed conducting additional research focused specifically on Tern Island. On Sept. 4, the Baker-Polito Administration awarded the town a $51,666 Coastal Zone Management grant for the study; the town will pay the remaining $17,500 to fully fund the new research, which is estimated to cost just under $70,000.

“This study will analyze physical data and computer models to evaluate alternatives for reducing erosion along the mainland shoreline while assisting threatened and endangered shorebirds,” Health and Natural Resources Director Robert Duncanson wrote in the announcement of the grant.

The idea of elongating Tern Island is promising, but it is far from a certainty, Keon said. For one thing, it remains to be seen what kind of permitting might be needed for such a project, he said.

“It will not be without considerable discussion, and possibly resource issues and concerns, given its historical viability as shellfish habitat,” he said. The flats north of the island have been productive shellfishing grounds in the past, but it’s hard to know how long that might last.

“Those flats are undergoing a lot of change right now,” he said. With changes in currents and new shoals related to the inlet migration, “It’s going to be very dynamic over there,” Keon said.