Secrets in the Sand
January 10, 2016
On January 10, 2016, Sandy Tilton took a photograph of the sharply scarped dune on Plum Island’s North Point. She was one of a casual cadre of citizen scientists who were monitoring Plum Island’s erosion on a weekly basis.
But this photograph was different. It showed a piece of rough concrete jutting out of the base of the eroding dune. It lay beside an old telephone cable and the remains of probably an old engine hoist and what looked like the engine from an ancient truck.
I wondered how old all these artifacts could be, so I checked into the history of the twin lighthouses on Plum Island and found some suggestive passages.
In 1871 the federal lighthouse board expounded on the importance of Plum Island’s wooden lighthouses but recommended that a new cast iron one be built on a concrete base extending below the waterline. They figured that if the lighthouse had to be moved they would only have to pay for constructing another concrete base in a new location.
The twin towers did have to be moved in 1874, along with the lighthouse keeper’s house and several outbuildings. This was seven years before the Army Corps of Engineers started to build the 2,445-foot south jetty that you can still see abutting Northern Reservation Terrace.
The Department of Justice also noted that “trespassers” had built a railway between the two lights as well as “a considerable number of dwellings in the immediate vicinity.” Then in 1898 a telephone line was buried in the dunes between the lighthouse and the Plum Island Lifesaving Station.
This meant that the piece of concrete had to be at least 200 years old. It was probably the foundation of the old lighthouse that had been out here before the jetty was completed and after the Merrimack River stopped flowing into the ocean through what we now call the basin. What we now call the spur of the jetty was built to protect the lighthouse from erosion before it was moved in the Seventies. So what we are starting to see are artifacts from several eras.
But for residents of Northern Reservation Terrace, what was most interesting was that the artifacts were lower than the sand on the south side of the jetty. How did this come about?
Well if you look at the old charts you would see that North Point was at its widest and longest just before the original jetty was completed in 1900.
The natural beach was covered with dunes and growing from the steady supply of sand flowing north along the ocean side of Plum Island, but because this natural beach was healthy and in equilibrium it was also several feet lower.
But all that changed when the jetty was completed. Sand started building up on the south side of the jetty but North Point started to rapidly erode, because the jetty was starving it of sand. This was the same situation the residents of Northern Reservation Terrace had found themselves in early 2016.
It also meant that the ocean that was herniating through the dunes because of the jetty might be stopped by the buried remains of the 1800’s jetty, but nothing would prevent it from shearing off the entire north end of the point along with the present lighthouse, the state’s shellfish depuration lab and about 235 homes, worth between $115 million and $235 million in private property and public infrastructure.
However, the artifacts also indicated how to reverse the erosion. The same rapid erosion had occurred when the Corps repaired the jetty in the 1970’s. The repaired jetty had also cut off the natural flow of sand leading to such severe erosion that the old lighthouse had to be moved into town and it looked like erosion would continue to shear off the rest of the point.
But then the Blizzard of 1978 struck, and it was so powerful that it caused several of the boulders on the landward end of the jetty to settle. This allowed sand to flow through the jetty once again, eventually building up almost 400 feet of sand dunes between Northern Reservation Terrace and the ocean.
These were the same sand dunes that were eroding in 2016, at the rate of 150 feet a year. If this erosion continued the houses on Northern Reservation Terrace houses would be in imminent danger in two short years.
But hopefully, this time the residents of Northern Reservation Terrace wouldn’t have to wait for another storm. The Army Corps of Engineers was studying whether they could install a weir jetty or use pumps to return the system to a semblance of its former equilibrium. It was a winnable proposition. Science, nature and human intervention would all be pulling in the same direction, unlike in Newbury, where they were pulling in opposite directions.
Read more in William Sargent’s new book, Plum Island; 4,000
Years on a Barrier Beach is available in local bookstores and through Amazon.com.