Reflections on Erosion
Plum Island Point
October 29, 2016
On October 29, I drove back to Plum Island with a few citizen scientists to take hand measurements of the amount of erosion we had experienced in the first month of the erosion season. It was high tide and large waves were breaking over the jetties and running far up on the shore. I hadn’t been on the beach for close to a month and it was hard to get my bearings.
During the summer I had led groups out here every week at both the high and low tides. So I had known about what to expect. A beach is different every day but if you visit it often enough you get a feel for what is happening. But we had just entered the stormy Northeaster season so the waves were high and getting even higher as they rode on top of the ten-foot king tides.
The combined forces had eroded 31 feet off the dunes on the north end of the beach, and 6 feet off the dunes in the center of the beach. This tracked closely with what had happened the year before when the erosion started on the north end of the beach, then moved south during the winter months. We thought this was being influenced by the seasonal shifting of a so-called wing bar of sand that projected into the river.
Sandy Tilton had also taken a photograph of four-foot deep sinkholes beside the jetty so I was eager to get out there before high tide. But we were disappointed; sand was only flowing through the jetty near the ocean. It turned out that Sandy’s photographs of the sinkholes had been from the landward end of the jetty where sand had blown in from an adjacent dune. This was something we hadn’t seen so dramatically in 2016.
But the far end of the jetty didn’t totally disappoint. In addition to a few waves breaking over the jetty from the oceanside, other waves were washing under the top of the jetty from the riverside. We could see water from the waves rushing through the boulders four feet below us.
We could also see the results. The waves had washed rocks and sand from under the large boulders causing them to settle and crack. In the beginning of the summer you could just fit your hand in some of the cracks in the boulders, now they were three inches wide.
So the beach erosion had started in earnest, as had the settling of the South Jetty. Now we were in a race to see which would happen first; waves reaching the houses on Northern Reservation Terrace, or waves settling the jetty enough so the beach could start growing again.
If it were a calm year the erosion would probably win but if it was a stormy year settling could be the victor. This is what had happened so dramatically during the Blizzard of 1978.
Several of our citizen scientists had posted photographs of the rapid erosion brought on by the month’s storms and king tides. Some were ephemeral and quite beautiful like the one of a dune whose eroding face looked like a row of symmetrical hamlets clinging to the edge of a limestone pinnacle in Guizhou, China.
On the way home we discussed how we felt about such erosion. It was certainly frightening if it was advancing toward your home and it was certainly sad if you felt it was destroying what had been your favorite childhood beach.
But I had to admit I also found it fascinating. There are very few places on our planet where you could witness such rapid geological change. The southern end of Plum Island grows almost a quarter of a mile longer every year. An astronomer sitting on Mars could see such change. The northern end of Plum Island wants to grow just as fast but it is locked up in a million cubic yard prism of sand behind the newly repaired jetty.
We had seen the beach above the jetty grow as much as 100 feet toward the ocean as long period waves had welded offshore sandbars onto the beach during a single tidal cycle. It was more difficult to see such growth than to see waves eroding away great chunks of dunes. Growth was more subtle but in ways infinitely more awe-inspiring.
It was equally fascinating to realize that we had just witnessed one split second in the earth’s timeless cycle of geology. One day it would mold this sand into sedimentary sandstone, heat it into quartzite then raise it into a towering new mountain range of schist that would already be eroding sand back toward a new beach — half a billion years in the future.
William Sargent’s latest book, Plum Island; 2016, is available in local bookstores and at http://www.ingramcontent.com.