Breakthrough: The Barrier Beach Process

Weather Forecast

Nature's Design: The Barrier Beach Cycle

From "Breakthrough: The Story of Chatham's North Beach"

[Part 2]

At the same time, other, less visible forces are at work on the beach. Coastal submergence is the term used to describe the increase in sea level relative to the mainland, which is actually sinking due to gravity and other pressures. Submergence shapes the contours of the shoreline — both on the inner and outer beaches — and even a slight increase in the rate of the ocean's rise can have a drastic effect on the coast. In his report, Dr. Giese cited a sea level increase of between .005 and .01 feet per year, resulting in one-quarter to one-eighth of an acre of upland being lost to the ocean for every linear mile of coast.

Estimates of the rate of sea level rise have been revised upwards in the years since Dr. Giese wrote his report. The Greenhouse Effect — the warming of the Earth's temperature due to the introduction of synthetic compounds, man-made gases and chemicals into the atmosphere — is considered the major culprit; the compounds trap the planet's radiant heat, hastening the melting of our old friends, the glaciers, now relegated to the extreme north and south poles. Although the Greenhouse Effect theory remains controversial and is not universally accepted, estimates of the sea's increasing levels range from .45 feet to 1.57 feet per year; the latter figure, a "worst case" scenario presented in a Commonwealth of Massachusetts report in 1987, would consume 3.11 percent of Chatham's upland annually, cause massive erosion and drastic changes in marine habitats. In real terms, sea level rise has yet to make much of an impact on New England, yet the results of the 1987 breakthrough in Chatham, which created higher tides and more opportunity for storm damage, provided a glimpse of a potentially devastating future.

The triad of factors controlling North Beach's shape, then, are coastal submergence, sea level rise, and wave and tidal action. Now we know what the beach is, how it got here, and what forces are at work on it; how do these factors mesh to create a regular cycle of beach growth, breaching, disintegration and regrowth?

At both the beginning and end of the cycle, North Beach is one continuous stretch of sand, extending southerly from Orleans, enveloping Pleasant Bay and Chatham, as it was before the 1987 breach, and as historical maps show it was in 1846. Once the southern end of the beach begins to overlap Monomoy, the tidal flow in and out of Chatham Harbor and Pleasant Bay becomes constricted; it's like four lanes of traffic suddenly narrowing to one. Anyone who has drive down Route 6 in Dennis at the height of the summer season knows that means gridlock. The same thing happens to tidal flow. The estuary's resources wane and water stagnates as the tidal flow ebbs before an adequate amount of water can be exchanged between ocean and bay. Tidal ranges between the inner and outer beaches draw farther apart; the ocean side stands at about six to seven feet, the harbor and bay nearly three and a half. There is about a two-hour lag between high and low tide on either side of the barrier beach. Treacherous shoals and bars, infamous off Chatham for centuries, develop at the mouth of the harbor, known as the South Way in the latter stages of the cycle.

With the stage set thus, the time is right for a breach. Now, as Professor Mitchell noted, "the too confined waters of Pleasant Bay force a more direct outlet again, and the march of the beach from above has recommenced."

It’s like squeezing a tube of toothpaste with the cap still on. The material under pressure has to go someplace. In the case of North Beach, a new inlet is created in the place of least resistance: historically, this has been opposite Minister's (Allen's) Point. Factors must be right for a permanent break to occur: usually a high pressure weather system, which drains the bay and harbor to lower-than-normal levels, a strong northeast storm and higher-than-normal tides. All of these were present when the beach broke in January 1987.

The location of the most recent break was somewhat unexpected; inlets had formed east of Minister's Point in the 17th, 18th and 19th centuries, according to a study by geologist Charles E. McClennen. Even Dr. Giese, in his 1978 report, projected a late 20th century breakthrough opposite Minister's Point. Nature, defying predictions as always, stole south in 1987 when nobody was looking and punched a hole through the beach directly opposite the Chatham Lighthouse.

When a breach is wide and deep and permanent enough, the differences in the hydrological pressure between the harbor and the ocean speed the tidal exchange through the new inlet, scour it and keep it open, all the time battling the southern littoral drift which naturally wants to plug the hole up again. The velocity of the current instead sweeps sediment into the harbor, plugging that up, much to the consternation of fishermen and recreational boaters. Literally starved of sand, the southern portion of the beach (South Beach in local parlance), now an island off and on connected to the mainland, gradually over a period of decades is eroded and overwashed westward, filling up the inner harbor with shoals and flats and cutting off the old southern inlet. Eventually South Beach disappears, the sand migrated onto the mainland, Morris Island, and the northern end of Monomoy. North Beach's southern tip then slowly begins to grow southward.

Then the cycle starts all over again.

No doubt this has been happening for thousands of years, but reliable maps and records documenting the process go back only two centuries. A permanent break in the making was first viewed by eyewitnesses in 1846; previous to that the harbor had become so choked and constricted with shoals that it threatened to ruin Chatham's seafaring economy. In 1841, Edward Hitchcock noted that 20 years earlier, Chatham's harbor was excellent; but two decades later it had filled in. "Nothing can save it from complete destruction," he wrote, "but forming a new entrance."

The U.S. Army Corps of Engineers would use almost the same language in a 1968 report on the feasibility of cutting a new inlet in North Beach. "Daily tidal delays [have] become common to the commercial fishing boats as well as a threat to life," the report stated, concluding that "if no navigational improvements are made in the area, it is expected that the existing Chatham Harbor inlet will in the not too distant future close completely." (The corps' report is explored in depth in Chapter Two.)

The area was first mapped by English navigator Bartholomew Gosnold in 1602. From descriptions of his sightings in W. Sears Nickerson's Land Ho! and elsewhere, it appears that at the time, North Beach had been breached some years before and was well into the inlet migration cycle. The land he named Gilbert Point was attached to the mainland at approximately the present day location of Minister's Point, and curved south and east to a point opposite the present day lighthouse. In 1606, Samuel De Champlain produced a map describing a spit that terminated more to the south. Records of Sutcliffe's Inlet of 1619, which opened into Pleasant Bay east of Strong Island, and Governor William Bradford's 1622 entrance north of that seem to indicate that the beach was in the process of breaking up, with washovers and inlets forming up and down its length.

Since 1620, Goldsmith noted, North Beach has undergone "at least two, probably three full cycles of the large scale inlet migration." From the maps and descriptions extant, it seems likely that a major breach occurred sometime prior to Gosnold's visit in 1602, probably opposite Minister's Point. The severed southern portion of the broken beach attached itself to North Chatham, as if the mainland had grown an arm, and gradually welded onto the shoreline. Des Barres put the entrance to "Old Harbor" just north of Minister's Point in 1740; the beach built steadily south from there until the next major breach, in 1846. The 1740 entrance probably marked the spot of another major breakthrough.

Will this process continue for thousands of years to come? Dr. Giese doesn't think so.

He noted in his report that like the cliffs to the north, Nauset/North Beach is also moving westward year by year, an important factor when it comes to restricting the flow of water in and out of Pleasant Bay, Sand is moved from the east side of the barrier beach to the west by winds and storm washovers, and it is also deposited along the inner beach as flood tidal deltas. According to Giese, the entire beach shifts west at a rate of between five and 10 feet a year.

Perhaps the most dramatic demonstration of the beach's constant desire to "go west" is the case of the Sparrow Hawk. The ship either piled up on shoals or was wrecked in the night, according to William Bradford, on the inner shore of Nauset Beach just south of Hog Island in greater Pleasant Bay in 1626. The wreck was discovered 237 years later when erosion on the outer shore uncovered the wooden remains. During the intervening years, the beach had moved west a distance approximately equal to its width, about 1,000 feet, right over the wrecked ship.

Giese theorizes that after several more cycles, the northernmost portion of North Beach will have migrated so far west that it will "no longer overlap the mainland of Chatham. Rather, the barrier beach system will stop at North Chatham, the coast from North Chatham to Morris Island will be exposed to ocean waves and an ever-widening barrier spit will extend southward from Morris Island."

Which means that, about 500 years from today, the open-ocean exposure and tremendous erosion Chatham's eastern shoreline has experienced in recent times could be status quo. That probably wouldn’t have surprised our forefathers. They might not have had the scientific understanding of the barrier beach cycle we have today, but when devastation came, when lighthouses tumbled into the sea and waves lapped at doorsteps, they didn’t complain or attempt to fight back against nature. They took it in stride, built new lighthouses away from the sea cliffs and moved their homes inland. In some innate way, born, probably, of their close association with the sea, they understood and accepted the powerful, unseen hands that constantly mold the beach. That appreciation seems lost today.

* * *

In April 1988, after more than a year of observations, Dr. Giese came up with a few revised theories about the North Beach barrier beach system. There appeared to be three separate and distinct phases to the process: first, the switch in emphasis from the southern inlet to the new cut, which happened much quicker than expected; after just over a year, Dr. Giese estimated 90 percent of the tidal exchange between the Chatham Harbor/Pleasant Bay system and the ocean flowed through the break.

We are now in the midst of the second phase, the breaking up of South Beach. For the past several years, a bridge of sand has connected the beach to the mainland just south of Lighthouse Beach. In a recent study, scientists predicted that in a matter of years, all of the sand between South Beach and the mainland will be gone, washed onto the mainland or lost to the deep water. Starved of sand, South Beach's dunes are getting lower and lower. In a slow process that could take up to 50 years, overwashes will lower the beach even more, and it will begin to break up, the sand drifting southwesterly and possibly melding onto the mainland from the Morris Island dike south. A large salt marsh would be lost, but ironically the sand could protect dozens of homes lying between Little Beach and Morris Island Road, and Morris Island itself, from devastating erosion. Chatham’s eastern shoreline would then look much as it did some 50 years ago, with a clear path lying ahead of North Beach as it begins the third and final phase of the process: building south once more toward the next inevitable breakthrough.

[Part 1]