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 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.