Chappaquiddick's shifting sands follow a natural pattern
Photo courtesy of NOAA
On the wind-swept southeast corner of Chappaquiddick this week, a team of engineers, contractors, and builders was at work preparing to move a luxurious seasonal home and adjacent buildings back from an eroding bank and the encroaching waves.
International Chimney Corporation of Williamsville, New York, a company that specializes in building relocation, plans to move the 8,313-square-foot, seven-bedroom, seasonal home of Richard and Jennifer Schifter of Washington, D.C., including its foundation, basement bowling alley, and massive two-story chimney, — back from the brink to an adjoining lot 275 feet away.
The assembled team of Island builders, engineers, and mainland experts is in a race against natural forces in which the finish line is far from certain.
The Woods Hole Group (WHG), an international environmental, scientific, and engineering consulting organization headquartered in Falmouth, prepared an eight-page analysis of the historical shoreline changes and coastal geomorphology for the south-facing shoreline of Chappaquiddick for Mr. Schifter. Dated December 11, 2012, the report was sent to George Sourati of Sourati Engineering Group of Vineyard Haven, a member of the house team, and provided to the Edgartown conservation commission. It described the situation on Wasque Point.
"The wide beach and dune resources that once existed have completely eroded, and recent erosion since August 2011 has caused a loss of 120 to 150 feet of coastal bank. When the Schifter residence was initially completed in 2006 the top of the coastal bank was approximately 200 feet away from the building, however recent erosion has brought the bank to within 73 feet of the residence," WHG coastal geologist Leslie Fields wrote.
In the conclusion to her report, Ms. Fields described possible timelines for when the Wasque Point bank might stabilize, based on the three-stage natural history of periodic breaks in Norton Point beach, the two-mile long barrier beach that separates Katama Bay from the Atlantic Ocean.
"The magnitude of further bank erosion can be estimated from the previous cycles of evolution. For example, during the 1953 to 1970 cycle the bank along the southeast side of Chappaquiddick Island eroded at an average rate of -27 ft/yr. During the current period, which started in 2007 when the inlet breached, the average rate of bank erosion has been approximately -31 ft/yr. ... Using the more recent average, it is estimated that the coastal bank could retreat another 124 ft by the end of 2016. If the late phase of Stage 2 does not occur until 2023, the bank could retreat approximately 310 ft beyond its current location."
The current cycle of erosion began in April 2007, when a one-two punch of storm-driven ocean waves and powerful spring tides knocked open a cut in Norton Point Beach. The result was two long narrow spits of sand stretching east and west toward one another. Over the course of the past six years the cut has continued to migrate eastward to Wasque Point.
For those familiar with the natural history of Chappaquiddick, the breach and subsequent erosion came as no surprise. The only question now is how long it will take before a natural cycle recorded many times in the last four centuries plays out, and the cut eventually disappears into Wasque Point.
Ms. Fields describes a three-stage pattern of geomorphologic evolution, or the science of land and undersea changes, all of which are inextricably linked to the larger coastal system.
In the first stage, ocean waves and tidal levels combine to punch a hole in vulnerable spots in the barrier beach. "Inlet breaches typically form near the center of Katama Bay, although slight variations east or west have occurred. Over the past 75 years storm-induced breaches have developed into semi-permanent inlets on three separate occasions: 1938, 1953, and 2007. Other cuts through the barrier have also occurred; however these have been short lived."
Spit grows, channel lengthens
During stage 2, the inlet begins to migrate east towards Chappaquiddick and the dominant easterly flowing shoreline current causes the Norton Point spit to grow. As that spit extends to the east, the barrier beach on the Chappy side of the inlet tends to shorten and erode. Often the eastern barrier will also rotate north into Katama Bay as incoming tides push sediment into the Bay. During the early phase of stage 2 the shoreline along the south side of Chappaquiddick is relatively stable, with little or no erosion. "This is true as long as the eastern barrier spit is intact and can supply sand to the south side of the Island," according to the report.
"At times, the beach/dune system at Wasque Point has also contained an elongated pond, known as a 'cat-eye pond.' The pond is located at the landward edge of the beach/dune deposits near the base of the coastal bank. The site of this pond has migrated landward over the years as the shoreline has retreated; an indicator of the dynamic nature of the Chappaquiddick shoreline.
"The process of easterly inlet migration and barrier spit growth occurs until the eastern barrier is completely eroded and Norton Point begins to overlap the southwest corner of Chappaquiddick. During this middle phase of stage 2, the absence of a sediment source from the west in combination with tidal currents directed against Chappaquiddick, cause rapid erosion of the south facing shoreline. Initial erosion removes the modern day beach/dune deposits and the cat-eye pond, until the upland portions of Chappaquiddick are exposed as an eroding coastal bank. Continued erosion causes retreat of the bank and loss of upland."
It is that stage that is occurring now, according to the report.
"Since August 2011, the bank at the Schifter property has retreated between 120 and 150 ft. Until the system reaches the end of the stage 2 evolutionary cycle, continued erosion of the bank can be expected."
In stage 3, the tidal channel that connects Katama Bay to the Atlantic Ocean eventually closes as tidal currents are not strong enough to flush sediment from the opening. Waves gradually push the Norton Point barrier spit to the north and the beach eventually welds onto Chappaquiddick.
The site of the previous tidal channel forms a new cat-eye pond as the barrier spit moves onto the island. This process results in a relatively quick and dramatic accretion along the south facing shoreline of Chappaquiddick as the beach/dune and cat-eye pond deposits weld onto the coastline.
Finally, during the last part of stage 3 the beach/dune system begins to retreat as ocean waves, tides, and currents cause erosion. The process continues until a new breach in the Katama Bay barrier forms and then the cycle starts over with stage 1.
"Although the natural evolution of the coastline causes the beach/dune to come and go depending on the stage of evolution," Ms. Fields said, "the location of the coastal bank is always retreating."
In conclusion, she said, "While all information in this report is presented to the best of our understanding, there is no crystal ball that can be used to predict future shoreline and bank locations with any greater degree of accuracy."