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Norton Point Beach - the anatomy of a breach
At the edges of our lives, the sea pounds on the shore, and the shore stands guard against the sea's assault. Or does it? The Vineyard littoral has changed repeatedly and significantly over time. It is changing still. The Times asked Jo-Ann Taylor, a coastal planner, to describe and illustrate the changing Island shores, in occasional illustrated installments, of which this is the latest.
Norton Point Beach reaches across the mouth of Katama Bay from Edgartown proper to Chappaquiddick, forming a barrier between the quiet bay on the north side and the much more boisterous Atlantic Ocean waves on the south side. Many Vineyard residents and visitors use the two-mile long beach for recreation and travel, often lining the ocean side with SUV's, coolers, beach towels, and fishing rods, and the trails from end to end are the province of four-wheel-drive vehicles traveling between Edgartown and Chappy.
Evidence of random vehicle traffic in 1991. Photo by JoAnne Taylor
On a typical day, the surf pounding on the Atlantic side can cause even the most casual observer to sit up and take notice, while the north side is troubled by no more than the gentle tugging of the bay tides. However, the days are not all typical. Halcyon days are inevitably interrupted by stormy weather, and the disproportion of turbulence between the Atlantic Ocean and Katama Bay becomes infinitely more pronounced. On a stormy day, the ocean fairly boils with confused, white-capped waves. On such a day, and maybe on such a beach, Wordsworth was inspired to write of "the wind that sang of trees uptorn and vessels tost."
A friend who frequents the beach recently told me that the beach is quite thin and overwashed in places at the highest tides. He wondered if the beach is likely to breach, opening a cut through from the ocean to the bay. Norton Point has been breached during a number of storms, including the gale of January 1886, the 1938 hurricane, Hurricane Edna in 1954, and Hurricane Bob in 1991. At first glance, it would seem that the raging tumult on the Atlantic side surely must be responsible for cutting an opening through the beach. In fact, other forces are at work, and are revealed by close observation of a breach.
A 2003 aerial photo of Norton Point Beach reveals tidal deltas (sand fans) left from previous breaches. Image MassMIS
The Vineyard's own J. Gordon (Pete) Ogden III witnessed the breach wrought by Hurricane Edna in 1954 and wrote a detailed account in the journal Quaternary Research in 1974. The storm tide, he wrote, was receding from the ocean side but still flooding in the bay, so that the water level on the bay side was higher than on the ocean side. He saw a small "fault cliff" in the sand, stepped onto the lower part, and sank in up to his knees.
The breach created during Hurricane Bob begins to fill in four days after the August 1991 storm. Photo by JoAnne Taylor
The breach was made from the bay side, not from the ocean. Storm waves may overtop the beach, but the hydraulic head, or difference in height between the higher water level in Katama Bay and the lower water level on the ocean side, is what dumps millions of gallons of water out of the bay, tearing a wider and wider gap as it pours out. So, for all the thundering surf on the ocean side, only the noiseless power of gravity can tear open a barrier beach.
A chart dated 1894 shows the 1886 breach that connected Katama Bay to the Atlantic Ocean. Image NOAA
The breach that occurred during Hurricane Bob in 1991 was, thanks to a fortunate tide, minor and healed itself within a few days. That breach happened in an area of the beach that had become entirely devoid of dune grass, due most likely to the traffic pattern of the day, which was no pattern at all. Instead of the trails that are managed today, vehicular travel on the beach was a free-for-all, with the result that dune grass was eliminated from large areas of Norton Point.
Norton Point Beach is located on the southeast end of Martha's Vineyard.
So, we humble humans do have our part in the breach story, in spite of the awe-inspiring forces of nature at work. It is our choice to practice good stewardship of the beach, keeping the dunes in good shape. And it is our good fortune to be able to recreate the breach to maintain optimum conditions in some of the south shore ponds. The same principles of hydraulic head apply to the artificial breaches used to open the south shore ponds such as Edgartown Great Pond and Tisbury Great Pond. They can only be opened when there is sufficient volume of water built up on the pond side. Until the pond is full enough, all the oxen or backhoes on the Vineyard can't make a decent breach.
Jo-Ann Taylor studied geology at Smith College and Boston University, earning BA and MA degrees. Her background includes planning, oceanographic research (marine geophysics), small engineering projects, and government administration. For the past 16 years, she has served as the coastal planner for the Martha's Vineyard Commission.