Shoreline change is a dynamic process, especially at Wasque, the southern part of Chappaquiddick, owned and managed by The Trustees of Reservations. This shoreline is very dynamic, sometimes accreting rapidly and sometimes eroding rapidly. Wasque is now eroding rapidly — parking lots have eroded away this winter, leaving an almost unrecognizable beach. What is going on? Why is it eroding so much now? There are several factors that explain this change.
First, there is an annual cycle of erosion and accretion. The wide summer beach — a good thing for our recreational activities — starts getting narrower in the late summer (perhaps at the start of hurricane season) when larger storm waves from the south remove sand from the beach. This eventually produces the narrow winter beach, which starts widening sometime during the winter, when smaller waves tend to move offshore sand onto the beach. This produces the wide summer beach and the cycle repeats itself.
Erosion rates vary on any beach. On the 2.5-mile long Norton Point Beach, stretching from Katama to Chappaquiddick, erosion is very localized; most of the beach tends to be fairly stable, but erosion may be severe on a quarter to half-mile stretch of beach. The next winter’s erosion is likely to be severe on a different portion of the beach. Such localized erosion is due to natural variations in the size of the offshore sandbar, which lies where waves are visibly breaking offshore.
The behavior of the breach that occurs regularly in Norton Point Beach is yet another confounding factor affecting the rates of erosion and accretion at Wasque. Historically, this breach migrates eastward along the barrier beach, passing the southern shoreline of Wasque before closing at the southeastern corner. As the breach migrates past Wasque, the shoreline accretes dramatically as a new barrier beach is built. This last happened in the 1960s, when a wide beach was deposited.
The beach has been eroding since that last breach closed around 1970. And this winter the erosion has been very dramatic. I wanted to see it for myself, and so on Feb. 23 Wendy Culbert and I visited Wasque. The former parking lot and tire-deflation lot on the southwestern corner of Wasque — the last upland before accessing Norton Point Beach — was gone! There is a six to eight-foot tall coastal bank, and the former tire-deflation lot is now ocean beach. Further east, only the eastern half of the Swan Pond remains; the western half is now either ocean or sandy beach.
Google Earth (a free download from google.com/earth/index.html) has a series of aerial photographs, which for Chappaquiddick date back to 1991. These photographs show the remarkable erosion that has changed Wasque Point. I used their most recent aerial photo, from July 2008, and, based on my visit, I drew the approximate locations of the current shoreline (black line) and inland edge of the beach (white line).
Making measurements from that map, the westernmost part of Wasque has eroded about 690 feet since 2008. And the majority of that erosion started last summer, when some of the uplands fell away, creating the coastal bank mentioned above that prevents vehicular access to Norton Point Beach (it is hard to drive up a vertical cliff). But erosion is not consistent across all of Wasque (Remember that offshore sandbar?) Based on my map, erosion was as little as 400 feet, especially near the western end of what used to be the Swan Pond.
Erosion on this scale has happened before at Wasque. The 1991 aerial photograph from Google Earth shows the former incredibly wide beach at Wasque Point. By 2001, that beach was narrower by 1,000 feet. Then, for seven years the beach was fairly stable; most of the changes may be attributed to the annual cycles of erosion and accretion. Since 2008, however, erosion took over again and another 250 feet of beach has eroded away. A beach almost one quarter of a mile wide has eroded away since 1991. Again, erosion is not consistent across all of Wasque; the western end has lost about one fifth of a mile, while along the middle stretch of this beach only about one eighth of a mile has eroded away.
Fortunately for Wasque, these high rates of erosion are not consistent over time. A Massachusetts Coastal Zone Management study of shorelines mapped between the late 1800s and 1994 documents an average erosion rate of seven feet per year. This larger time scale (100 years) is important, since it includes several cycles of extreme accretion and erosion due to the breaches in Norton Point Beach. The observations in this article focus on the past 20 years, which is only the dramatic erosion part of the cycle. That these numbers contrast with the 100-year average illustrates just how dynamic the interplay between the ocean and the land can be.
Can we predict what will happen next at Wasque? Geologists generally agree that the current Norton Point Beach breach will migrate across Wasque, once again creating (accreting) a large barrier beach. But there is one factor that may have a huge impact on the eastward migration of the breach. Norton Point Beach now ends at the western end of Wasque, while it used to extend all the way across Wasque, enclosing an arm of Katama Bay that reached all the way to where the Swan Pond is now. The Vineyard Conservation Society has aerial photographs from 1938 and 1952, both showing the larger Katama Bay and Norton Point. Some time after the last breach closed around 1970, erosion filled in the narrow channel that connected the Swan Pond to Katama Bay.
So the current breach is the first one that will encounter land as it migrates towards Wasque. Will the breach continue to migrate past Wasque anyway, creating a new channel and a new barrier beach? Or will the breach close once it reaches Wasque, leaving the currently narrow beach as it is? Stay tuned, we will find out in a couple of years!
(The CZM Shoreline Change map is published at mass.gov/czm/hazards/shoreline_change/shorelinechangeproject.htm).
Robert Culbert leads guided birding tours and is an ecological consultant living in Vineyard Haven.