NewburyportNews.com, Newburyport, MA

February 22, 2013

Startling deja vu

Plum Island withstood threat of erosion once before

John Macone
Newburyport Daily News

---- — Editor’s note: This is the first of two parts looking at current erosion on Plum Island and its parallels to erosion crises in the 1970s.

The clang of huge rocks dropping out of dumptrucks can be heard all over the north end of Plum Island these days.

It’s a welcome sound for many. It means gaps in the island’s 2,445-foot-long stone jetty are finally being fixed. And many believe that will help solve the erosion problems that have been sweeping away wide swaths of the beach some 2 miles to the south.

The same sound was heard a little more than four decades ago, when the jetty underwent a similar repair. But within a few years, an unexpected crisis occurred. Large chunks of Plum Island Point started to wash away, literally clawed out to sea by the powerful scouring action that the repaired jetties created in the mouth of the Merrimack River.

It reached its crisis point in late 1974. At a public meeting, authorities told horrified islanders that up to 80 homes at the northern tip could be lost. Desperate measures such as building massive sandbag walls and berms had failed; they simply washed away. After that meeting, Newburyport’s City Council took emergency actions, installing a wall of massive concrete blocks all along Reservation Terrace, a last line of defense for the homes on the other side of the street.

Today, the concrete blocks are still there, but the threatening and clawing sea isn’t lapping against them anymore. here’s a vast, healthy sand dune, perhaps 200 or more yards wide, between those blocks and the sea. Over time, Plum Island Point healed.

But as the rocks are gradually refitted into the jetty today and the holes are blocked up, islanders who have been watching the progress have been noticing some familiar signs. The shore of Plum Island that presses against the rivermouth is starting to erode again. The Army Corps of Engineers is watching, but thus far, it’s unclear whether the kind of extensive erosion the Point endured 40 years ago will happen again.

Jetty & Bar

When they were built a century ago, the twin stone jetties at the mouth of the Merrimack River served two main functions — they were intended to hold the location of the Merrimack River in place and create a funnel that would harness the enormous force of water rushing down the Merrimack River and into the sea. That funnel was designed to scour out the mouth of the river, rip away the sand shoals and make it safe for large ships to sail in and out of Newburyport harbor. They would solve problems that had plagued sailors trying to get through the rivermouth for centuries.

But they didn’t work quite as intended. Sand built up into an enormous sandbar, commonly called “The Bar,” just beyond the jetties. Jerry Klima, a Salisbury selectman who owns a summer home on Plum Island and has studied the jetties, noted the jetties were extended a few hundred feet out to sea in an attempt to defeat the massive sandbar that formed at their seaward end. The extensions ended up moving The Bar a few hundred yards further offshore, where it still causes navigational hazards and must be dredged on a regular basis so that large boats can get over it.

The funnel action of the jetties had a scouring effect along the northern tip of Plum Island, at the very mouth of the Merrimack River. The river’s current flows hard along through this area, ripping away at the island’s tip. Old-timers recall that what is now a parking lot at the northern tip of the island used to be open water.

But as the years went on, the jetties deteriorated, greatly impacting the flow of sand and the shape of the island, including the tip. Even though the jetties are built from rocks that can range up to several tons, they take a tremendous pounding from the sea that can knock the stones off, creating gaps. The gaps create holes in the funnel, allowing water and sand to seep through. They also generate a current that rips along the front of the beach, causing erosion.

Crisis years

The repairs to the jetties in the late ‘60s were a reaction to severe erosion near the northern part of the island that was wiping out cottages. Former Newburyport Mayor Byron Matthews recalls the Army Corps of Engineers had a keen understanding of the island’s dynamics, and had identified that erosion along parts of the shore was directly tied to gaps that had formed in the jetties. The jetties hadn’t been repaired since their construction some 60-plus years earlier.

In living memory, the 1950s and 1960s stick in many people’s minds as a crisis point. Widespread damage occurred all along the cottage-dotted Plum Island coast, sometimes wiping out a half-dozen cottages in a single storm.

Matthews remembers those years, particularly a period in the mid ‘60s when homes around his inlaws on 47th Street succumbed. The dune near the cottage had been 150 feet wide; it shrank to 18 feet.

Not long after he became mayor in 1968, Matthews worked with federal lawmakers to get the Army Corps of Engineers to repair both the Plum Island and Salisbury Beach jetties. The work was completed in 1970.

“The following year we started to see the beach come back. That really was the solution to what was happening there,” he said.

But in the mouth of the river, something else was happening. A scouring action caused by the repaired jetty funnel system was tearing at the shoreline of Plum Island Point again.

The dunes continued to retreat, despite literal walls of defense put in the path of the scouring waters. A 3,000-foot-long sand berm was built with a bulldozer and washed away. Dozens of volunteers stacked thousands of sandbags, which also washed away.

“All the dunes were leveled,” Matthews recalled. “It seemed like there wasn’t anything we could do to stop it.”

By late 1974, the dire message went out: the houses on northern tip appeared to be in danger. In January 1975, a 1,300-foot-long concrete wall was built on the seaward side of Reservation Terrace, just a few yards from dozens of homes. It was a last line of defense.

“That seemed to help a lot in preventing the water from getting through,” Matthews recalled. That wall is still there today, but now it is 200 or more yards from the edge of the water. “

Over time, the beach built out again. The homes on Reservation Terrace were saved.

There’s any number of explanations and opinions on what happened. Possible explanations include changes in the amount of sand that washed down the river, dredging by the Army Corps that changed the underwater topography, natural changes in the structure of The Bar and sand seepage through the jetties. As some islanders joke, there’s 1,200 homeowners on the island — and 1,300 opinions on erosion.

The Army Corps of Engineers has been considering the impact that the repaired jetties may have on the northern tip of Plum Island. Newbury Conservation Agent Doug Packer, a local expert on the island, said the scouring action is a concern, but it is unclear how it will manifest itself this time.

Packer pointed out that there are many variables to consider, any of which can change what will happen. For instance, in 1970, both sides of the jetty were repaired. This time, only the Plum Island side is getting fixed thus far. It’s unclear what impact that will have on the funnel action of the jetties.

It’s also unclear whether the sand that has built up in the river mouth, and along Plum Island’s northern tip, will change the funnel dynamic. Packer and others said that the depth of the waters off the northwest tip of Plum Island, in the river mouth, have changed over the past few years — shallower in some places, deeper in others. What impact this change in depth will have is hard to predict.