"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.