Originally published in the Historic Nantucket, Vol 38 no. 1 (Spring 1990), p. 4-6
Sesachacha & Sankaty: Pond opening and erosion on Nantucket's eastern shore
by Wesley N. Tiffney, Jr and Clinton Andrews
History of Pond Opening on Nantucket Island
Settlers on Nantucket, and elsewhere in coastal New England, may well have learned the practice of opening shoreline ponds to the sea from local Indians. This practice promoted runs of saltwater fish (eels, alewives or herring, and white perch) into and out of the ponds, depending on the habits of the individual species. While in the ponds, or running to or from them, fish could be caught by weirs, nets or hooks-and-lines in the summer and (in the case of eels) by spears wielded through the ice in the winter. The brackish ponds thus created frequently supported oyster beds and low salinities helped control oyster predators. These fish and shellfish provided a handy year-round supply of bait and food.
Pond opening has been practiced on Nantucket for over three centuries until the present decade. In 1665, settlers and Indians cooperated to dig a permanent ditch from Long Pond to the nearby salt water at Hither Creek. A May 1876, issue of the Inquirer and Mirror contains the first account of opening a pond by breaching a barrier beach. From settlement to the present, it is likely that the other six major coastal ponds on Nantucket (Squam, Sesachacha, Tom Nevers, Miacomet, Hummock and Capaum) have all been opened to the sea on a more-or-less regular basis. Until 1933, pond opening was an informal process, accomplished by groups of fishermen working together to dig ditches when pond levels were high enough to provide sufficient "head" for eroding substantial channels. From 1933 until 1981, the Town of Nantucket paid for heavy equipment to open ponds each year.
From 1874, when fish packed in ice were first shipped from Nantucket to city markets, until 1960 when excessive transportation costs made shipment unprofitable, sale offish from the ponds was a lucrative seasonal industry. After 1960, eels, alewives and perch were in less demand for food, perhaps due to changing tastes. Nantucket's offshore fin-fishery declined and mainland-based fishing vessels provided their own bait. Oysters were imported or grown in farms. In 1981, pond opening ceased under provisions of the Massachusetts Wetlands Protection Act. Today, the seventeenth-century ditch connecting Long Pond with salt water is still open, and fish still run to and from the pond; but only a few people use these resources.
Pond Opening Controversy
Even after the harvest and sale of fish were no longer commercially profitable, ponds continued to be opened. Proponents of the opening claimed that the practice was effective in controlling mosquitoes, setting free trapped aquatic life, and generally "cleaning out" the ponds. Some fishermen hoped to reestablish a commercial pond fishery. In other cases, the main reason for opening ponds was to lower water levels in ponds and their connected swamps to mitigate flooding of nearby homes and cottages by seasonal high water tables. Some long-term Nantucket residents also felt that pond opening formed part of their heritage and was one more tradition being usurped by meddling "off-island experts," particularly those in government or environmental management positions.
Those opposed to pond openings argued that the practice resulted in a significant reduction of Nantucket's limited fresh water supply, promoted salt contamination of well-fields, and resulted in sand loss from breached barrier beaches to deep water. Many doubted that lowering pond levels was effective in deny ing stagnant water to breeding mosquitoes. Some opponents suggested that clear management objectives for opening be derived, stated and approved, or the practice be discontinued.
In 1981, the Nantucket Conservation Commission, acting under provisions of the Massachusetts Wetlands Protection Act, required that a detailed and extensive environmental impact statement be prepared and approved in advance of any future pond opening. The Commission rejected the arguments of the opening proponents and was upheld on appeal to the Massachusetts Department of Environmental Quality Engineering. No sanctioned pond opening has occurred since 1981. Hummock Pond was illegally breached by unknown individuals on January 14-15,1987, while several attempts have been made to open Sesachacha — one such effort requiring intervention by the police. Such incidents exemplify the emotional content of this issue.
A New Reason for Pond Opening
In the last few years those in favor of opening the ponds have cited an additional reason in favor of the practice. They argue that sand carried out to sea by water rushing through the breached barrier beach contributes to protective offshore shoals, or directly nourishes beaches. This, they claim, decreases wave impact energy and/or provides more sand to beaches, thus mitigating erosion along Nantucket's shores. In specific, opening proponents cite a severe erosion cycle, beginning about 1981 (when Sesachacha Pond was last opened), that has endangered houses and the lighthouse atop the Sankaty bluff, about 1.3 miles south of the pond (figure 1). They suggest that opening Sesachacha Pond again would help control or reduce erosion on this bluff, saving millions of dollars of private property and the historic Sankaty Head Lighthouse.
This contention has added new fuel to an already overheated controversy. Our purpose in this study is to evaluate the possible relationship between the sand that would be contributed to the coastal system by opening Sesachacha Pond and the catastrophic erosion of the Sankaty bluff.
Opening Sesachacha Pond
Sesachacha Pond, on the eastern side of Nantucket, is separated from the Atlantic Ocean by a narrow barrier beach. It is a large pond of about 280 acres. A fine Indian site just north of the pond contains an extensive oyster-shell midden, suggesting that the pond was either breached naturally on a regular basis or that Indians opened it to promote oyster growth and perhaps to encourage fish runs. Since English settlement the pond has probably been opened regularly. Sesachacha has become the test case in the controversy between the proponents and opponents of the opening for all coastal ponds on Nantucket.
When the water table is high, we have observed the pond level at as much as six feet above mean low water on the ocean side of the barrier beach. This represents a water volume of 2,710,400 cubic yards. In a successful pond opening, this large volume of water pours out through an initially narrow, machine-dug ditch with increasing force and velocity. The beach sand is easily carved away; and when the pond drops to the ocean's low-tide level, a channel some two hundred feet wide remains. Longshore transport of sand closes the opening, usually within two to ten days, depending on weather conditions, wave intensity and wave direction. However, on two occasions, the pond remained open for about four months. When Sesachacha was opened regularly, our measurements indicated salinities ranging from 10.2 to 26.3 parts per thousand (0/00) (1.0 to 2.6% - normal ocean salinity is about 32 0/00 or 3.2%). On January 24, 1989, after nearly eight years unopened, the salinity was 3.1 0/00 (0.3%).
Erosion at the Sankaty Bluff
The Sankaty bluff runs from Sesachacha Pond south to the town of Siasconset for about two miles and varies in height from a few feet to about one hundred feet near Sankaty Lighthouse. At present, the northern one mile is actively eroding. The bluff is composed of marine sediments comprising unconsolidated sand, gravel, and clay derived from Pleistocene glacial tills deposited from 21,000 to 16,000 years before present (Oldale, 1981; Oldale, Eskenasy and Lian, 1985; Oldale, 1987). Some 16,000 years ago, continental glaciers began to melt and sea level to rise. Waves, driven by oceanic storms and tidal currents, aided by rising sea level, began to attack Sankaty bluff. This process has continued, often dramatically, at Sankaty in specific and on Nantucket in general to the present time (Flint, 1971; Tiffney and Benchley, 1987). Today, only a small part of the once-extensive Sankaty bluff remains.
Photographs and our observations suggest that erosion of the bluff does not proceed as a continuous or predictable process, but as a series of catastrophic episodes. Photographs show that the bluff was actively eroding between 1890 and 1900, but was again vegetated and comparatively stable in the 1930's (J. C. Andrews observation). These cycles are not evident from long-term erosion rates, perhaps due to lack of accuracy in charts used to generate the rates, or lack of sensitivity in the sampling technique. Erosion records from historical charts show slight accretion (1.5 to 2.5 feet) for the period 1846 to 1955, while information from aerial photographs shows no change from 1938 to 1970 (Gutman et al., 1979).
A two-hundred-foot-wide slump on the bank face took place in 1974, and another more general collapse occurred in 1981. These events heralded a new major cycle of bank erosion and retreat. Most of the bank, from south of Sesachacha Pond to south of the Sankaty Lighthouse, was severely undercut by 1982, and it was clear that the vegetated face was due to collapse. By 1983, substantial parts of the vegetated bank face had fallen to the beach and been washed away. Our measurements showan annual loss of 6.8 feet for the period 1981 to 1988 for a total loss of about 48 feet. Further observations during the winter storm season of 1988-89 suggest that this loss may increase dramatically, and extend further to the south, in the near future (Figure 1).
The extensive complex of Nantucket Shoals exists just to the east of Nantucket Island, and a small part of this shoal system fronts the Sankaty bluff. The shallow water of near-shore shoals causes waves to break before their full force strikes the beach, thus decreasing their erosional power. We suggest that sometime during 1981, the near-shore parts of the Nantucket Shoals fronting the Sankaty bluff began moving, or dissipating, resulting in deeper water close to shore. This, in turn, would allow higher energy waves to break directly on the shore, resulting in increased erosion. Periodic movement of the shoals on, off or along shore and/or wavelike surges of sand along shore, may account for the episodic erosion of the Sankaty bluff observed by ourselves and others. Given the wave energies inherent in North Atlantic winter storms, no beach protective structures can stand up against these wave forces or control this erosion.
Some twelve private dwellings, many built between 1900 and 1910, but some as recently as the 1960's (average 1989 value about $1,000,000 each), are located in the area of maximum erosion just south and north of the historic Sankaty Head Lighthouse. Most owners have now moved their endangered buildings to the backs of their lots. However, lots are shallow, the bluff slopes rapidly to a valley on its landward side, and the maximum possible move that can be accomplished by most owners is only about one hundred feet. One individual moved his cottage entirely off the bluff. No attempt has yet been made to move the lighthouse, now two hundred feet from the bank. Given the severe nature of the present erosion and bank retreat, it appears probable that, despite their recent moves, most property owners must soon face moving their buildings entirely off the bank or abandoning them to fall over the cliff.
Given the situation, it is not surprising that property owners all along the Sankaty bluff are intensely interested in the suggestion that sand derived from opening Sesachacha Pond may mitigate erosion of the Sankaty bluff.
Sediment Volumes Along Nantucket's Eastern Shore
On January 18,1989, we surveyed a profile of the Sesachacha pond barrier beach at the point of the 1981 outlet channel (Figures 2 and 3). At this point, the barrier was 325 feet wide from ocean to pond. Our measurements suggested a cross-sectional area of about 1,800 square feet for the barrier (Figure 3). We have observed a width of about two hundred feet for a channel resulting from opening the pond. Hence, we estimated that a full and successful pond opening would result in about 13,000 cubic yards of sand being washed from the barrier into the ocean.
For comparison, using a NOAA chart, we estimated the sediment volume of the north end of Bass Rip Shoal, a very small component of the Nantucket Shoals off Sesachacha Pond. We estimate that about 11,266,400 cubic yards of material are contained in the shoal segment, or about 867 times the quantity of sediment to be expected from opening Sesachacha Pond. Phrased differently, it would require 867 annual pond openings to create a sandbar equivalent to the almost insignificant north end of Bass Rip Shoal.
Finally, we estimated the annual volume of sediment eroded from the Sankaty bluff. We considered an area of the bluff of between about fifty and one hundred feet high, along the present region of maximum erosion, and used our measured annual erosion rate of about seven feet. The total annual loss is about 94,000 cubic yards or about seven times the amount of sediment that would probably be derived from opening the Sesachacha Pond. The material eroded from the bluff, falling directly to the beach surface, may be considered direct beach nourishment. However, the beach under the actively eroding segment of the Sankaty bluff remains narrow and flat (Figure 1). Accretional or stable beaches show a shelf-like area of deposition called a berm developing during the non-storm (usually summer) season. Our observations indicate that the beach below the Sankaty bluff has shown no significant summer berm formation over the last decade.
Littoral Transport Direction
Net longshore current movement and littoral transport of sand are primarily driven by tidal currents and run from south to north along Nantucket's eastern shore (Gutman, et al., 1979). Evidence for northerly flow and movement is provided by the existence of the six-mile-long tombolo and sand spit complex of Great Point, formed of wave and current deposited sediments, and found at the northern end of Nan-tucket Island. Hence, the net movement of sediments eroded from the Sankaty bluff is to the north toward Sesachacha Pond, and any sediment derived from opening the pond will also have a net movement to the north away from the Sankaty bluff. Indeed, if any area has received sand nourishment from breaching Sesachacha's barrier beach in the past, it is probably Quidnet, immediately to the north of Sesachacha.
After comparing the volumes of sediments estimated above and determining that net longshore current and littoral transport is to the north, away from the eroding Sankaty bluff, we conclude that opening Sesachacha Pond would have very little impact on the sediment budget and sand dynamics of Nantucket's eastern shore. Even if, through some miraculous process, the entire 13,000 cubic yards to be expected from an opening could be transported against longshore drift directly to the beach at Sankaty, it would constitute only 14% of the sediment annually falling from the wave-undercut bank on the beach face — and apparently doing very little to moderate the present cycle of severe erosion. For these reasons, we disagree with the suggestion that renewed opening of Sesachacha Pond would help mitigate erosion along the Sankaty bluff.
We sympathize with Nantucket property owners trapped on small lots immediately behind a massive and rapidly eroding sand cliff. However, the fact remains that such cliffs on sea-coasts, riverbanks, or lakeshores are short-term, inappropriate, and indeed dangerous building sites throughout the world. People should be encouraged to avoid developing such areas through educational programs and, as a last resort, through intervention by regulatory agencies.
The authors do not have strong feelings one way or another about opening Sesachacha, or other Nantucket coastal ponds, to the sea. However, they do suggest that the decision to open ponds, or leave them closed, be carefully and rationally thought out, based on observable facts derived from research into the physical and biological characteristics of the specific pond, and be in compliance with due process of law.
Our sincere thanks are due to Gayle and Robert Greenhill for permitting us to survey the Sesachacha barrier beach on their property, to Mr. D. F. Beattie for assistance with surveying and discussion, and to Dr. S. F. Beegel for constructive criticism of the manuscript. We further thank the American Society of Civil Engineers for permission to republish this paper.