, Salem, MA

Local News

March 23, 2008

Plum Island: Man's walls can't hold back the sea Some say erosion controls help, others say they hurt

PLUM ISLAND — Against the humbling power of nature, there's not a lot man can do to stop the ocean from destroying and reshaping the beach.

But just about every major manmade anti-erosion technology and technique to slow the process is employed at the mouth of the Merrimack River and along Plum Island. Even with those pieces in place, there are problems.

The structures include two jetties — one each at the north and south sides of the river's mouth — that extend into the Atlantic Ocean. The north jetty, which is in Salisbury, juts 4,118 feet into the ocean, and the south jetty, off Plum Island, juts out 2,445 feet.

Meanwhile, five stone barriers that for years were covered by sand but have recently become exposed due to erosion dot the beaches of Plum Island. Those "groins" — called a groin field — run perpendicular to the ocean and are placed where engineers believed erosion-stopping walls were needed.

While the structures continue to age, and at least one falls apart, the jetties and groins still provide a practical purpose. Experts say that without them the shape of the island and the location of the mouth of the Merrimack would look much different than they do today.

The jetties

Ed O'Donnell, chief of the navigation section for New England District of the Army Corps of Engineers, the organization that in the 1880s built the jetties, said the structures stabilize the inlet of the river. Their primary function is to let ships pass into the river by creating a "funnel effect."

"The inlet meandered about," O'Donnell said, "probably cutting through Plum Island at times and probably north into Salisbury at times. So what the jetty does is fix the mouth in place."

The jetties are constructed with a wide base on the ocean floor that narrows to the top. The structures are made from large boulders that can withstand the power of the ocean. The Corps made repairs to the jetties in the 1950s and from 1968 to 1970.

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