, Salem, MA

March 23, 2008

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

By Stephen Tait

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.

"You need a massive stone to resist the wave energies," O'Donnell said. "If they break apart they are not very useful."

The structures, which are more than 100 years old, were built based on basically the same theories that a stone jetty would be based on today, O'Donnell said. The one thing that has changed, however, is that emerging technology makes it easier to predict the exact toll the ocean waves will take on the jetties.

"Nothing has changed a whole lot," he said. "We have better computer models to know what the wave lengths are ... but it hasn't really changed a whole heck of a lot."

Another purpose of the jetties is to keep river sand moving toward the ocean to help keep the navigation channel in the river deep enough for boats. He said the jetties allow for the "high velocity" river current to continue moving at a fast rate instead of slowing down as it reaches the ocean.

"It is like a hose, almost," he said. "That is the theory behind it."

Jetties also redirect the sand that moves along the coastline, usually forcing it to move far offshore. The federal National Oceanic and Atmospheric Administration says jetty location can contribute to beach erosion.

The south jetty, which is on the northern point of Plum Island, is in need of repair. It is a project that has been at the heart of a continuing debate among leaders in Newburyport and Newbury to get the federal government to dedicate funds to the project, which is expected to cost about $2.5 million.

O'Donnell said the south jetty is "kind of porous" and that "fixing it would provide some temporary relief." To fix it, he said, would be to "basically pull it apart and rebuild it" by adding more stone and getting rid of the smaller, ineffective stones.

A particular problem for the jetties is funding.

"Projects like Newburyport, which is mainly recreational boating, don't fare well," O'Donnell said, adding that the federal government tends to favor channels that carry mainly commercial vessels. "We count on the local delegation to add funds to our budget to help support our work here."

The groins

The movement of sand on any barrier island comes primarily from the ocean's waves that constantly smash against their shores.

Rebecca Haney, a coastal geologist for Coastal Zone Management who studies the change of the state's coastline, suggested thinking about sand movement this way: When a person throws a ball against a wall, the ball bounces off the wall at a specific angle directly related to the angle at which it hit the wall.

The ocean, and the movement of sand along the beaches, works the same way, she said.

In other words, on Plum Island, if waves hit the shore from the north, the sand there will move to the south along with the water.

Complicating movement on Plum Island is the fact that waves hit the shoreline in both directions, making the sand along the shoreline move both north and south.

The movement of sand "depends on the direction of the waves approaching the shoreline and which way the wind is blowing," she said. "If you think about the waves approaching the beach, they don't come at the beach straight on — they come at an angle."

Further complicating the movement of sand are storms, Haney said.

The amount of energy in the waves determines how much sediment is transferred. Haney said that during high-energy storms, more sediment transfer happens, and since most of the large storms in this area come from the Northeast, the quickest sedimentary transfer goes from north to south.

That is where the groins come into play: The rock piles are designed to stop the movement of sand along the coast.

Haney said the typical way sand moves along shore is that waves wash up and there is a movement of the sand in one direction. But a groin is designed "to help slow sediment transfer."

The only way for the sand to move past a groin is for the sand to build up so much that it moves over or beyond the structure.

"If there is a rock structure there, the sand can't move past that rock structure," she said. "It interrupts longshore sediment transfer."

But groin fields are becoming less and less useful, Haney said.

"By trapping sand in one location you can have effects on someone else, basically causing erosion," she said.

That is easily noticeable on Plum Island, especially at center beach where a groin is located. Nowhere along the island's coast is there such a stark example of how a stone structure can impact the flow of sand along the beach.

For years it seemed that the groin was working. It was completely buried by sand, and the beach and dune extended far out in front of the cottages.

But now, it's apparent that the groin is interacting badly with other natural forces — tides and currents primarily — and has greatly impacted erosion in the center. The south side of the groin is plush with sand, which creates a wide beach that, come summertime, will play host to many beachgoers. If you were to step off the south side of the groin, you would step down a couple feet in most spots.

But on the north side of that groin, the sand continues to disappear. Some argue the prevailing south-flowing currents have been hitting the groin and creating a funnel effect that has accelerated erosion. A bay has been gouged out, and the dunes are crumbling, endangering several homes. If you were to step off the groin on this side, you would plunge about 10 feet.