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Coastal Erosion on Cape Cod: Some Questions and Answers
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Questions and Answers

  • Has the rate of coastal erosion on cape cod fluctuated over the past few decades, or has erosion always occurred at a constant rate?

    Measurements of shoreline erosion over time are highly variable. Between 1938 and 1974, average rates of erosion along the cliffed section of the ocean side of Cape Cod ranged from a high of about nine feet/year to a low of a few inches/year, and the average rates for North Beach (the barrier off Chatham) ranged from about 19 feet/year to three feet/year. The average erosion for the cliffed section from Eastham to Truro was about three feet/year and the average erosion for North Beach was almost eight feet/year. Short-term average rates of erosion can be very misleading because the locus of erosion will change over time and one part of the lower Cape may be undergoing rapid erosion while other parts may be stable for decades. In addition, in other places, the beach may become wider as sand is added to the beach face, causing the shoreline to move seaward.

    We don't know if erosion has occurred at a constant rate over long periods of time. Over about the last hundred years, the average rate of erosion for the ocean side of the lower Cape is about three feet/year, according to the position of the shoreline shown on maps of different ages. At other times in the past, the rate of erosion may have been more or less.

  • If the rate of erosion is increasing, what factors have contributed to this increase?

    Although we don't really know if the rate of erosion has increased, decreased, or stayed the same, we can guess that it may have increased over time. The most important factor may be the recent rapid sea-level rise indicated by tide gauge records. Over the last 2,000 years, the sea level rose only about 61/2 feet, but tide gauge records show that sea level rose about 1 ft over just the past 100 years. This much faster rate of sea-level rise would be expected to cause greater rates of erosion, and, if this increase in the rate of sea-level rise continues, it is logical to assume that the rates of erosion will increase too.

  • What impact will sea-level rise and marine erosion have on the future of Cape Cod?

    Without a doubt, the Cape and Islands will continue to erode because, with or without sea-level rise, the loose sand of the glacial cape has no resistance to wave attack. Continued sea-level rise will accelerate the erosion and cause the demise of the Cape to occur sooner. However, it will take thousands of years before the Cape is reduced to shoals and low-lying islands and perhaps five or six thousand years be completely drowned by the sea. The Cape region will probably not be deeply submerged because the present sea level is approaching the norm for glacial to interglacial sea-level change (about 300 feet). Nantucket Shoals south and southeast of Nantucket Island is probably a good example of what the Cape Cod region may look like when it is completely eroded away and submerged by the sea.

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  • Which parts of the Cape face the greatest peril from storm wave erosion?

    The shorelines on Cape Cod that are exposed to the full fury of northeast storms will suffer the most from shore erosion. This includes the ocean side of lower Cape Cod and the western part of the north shore of upper Cape Cod in Sandwich that is beyond the lee of the lower Cape. These reaches of the coast are exposed to the large storm waves generated by northeast winds blowing across the great fetch of the Gulf of Maine. The south shore of upper Cape Cod and the shore of Buzzards Bay suffer rapid erosion during hurricanes, especially those that cross the coast to the west of the Cape, as did Hurricane Bob in 1991. However, the frequency of hurricanes is much less than the frequency of northeast storms and, unlike northeast storms that may attack long stretches of the coast for several days at a time, fast-moving hurricanes generally impact a much smaller section of the coast and for only a few hours. Although hurricane erosion can be serious and dramatic, in the long run, the northeast storms do the most damage. Recent northeast storms cut Monomoy Island into two parts and wiped 40 feet high dunes and houses (including the National Seashore's bathhouse and Henry Beston's Outermost House) off Nauset spit in Eastham (1978), caused the breach in North Beach (1987), and broke through at Ballston Beach in Truro, to temporally make the upper part of the lower Cape an island (the Halloween storm of 1991).

    A geologist would not likely use the word "peril" to refer to Cape Cod's exposure to wave erosion during northeast storms or hurricanes. To many geologists, coastal erosion is not a peril, but a process that is vital to the Cape's existence. It is a natural process that allows the Cape to adjust to rising sea level. Erosion is only a peril to property. If we build on the shore, we must accept the fact that sooner or later coastal erosion will take the property away.

  • What is longshore drift and what part does it play in beach retreat?

    Figure 4. Diagrammatic illustration of longshore drift and longshore current generated by oblique approach of waves.
    Figure 4.  Diagrammatic illustration of longshore drift and longshore current generated by oblique approach of waves. Click on figure for larger image. (65KB)

    Longshore drift is the movement of sand grains along the beach by waves. Waves that approach the shore at an angle rush diagonally up the beach. The water then returns directly down the beach under the force of gravity. Sand grains carried by the rush and backwash of the waves are moved along the beach in a sawtooth fashion (Fig. 4).

    Other grains are carried along just seaward of the beach by the longshore current, which is also generated by the oblique approach of the waves.

    Longshore currents and longshore drift are generally considered to be constructive processes. Unlike storm waves, they are not significant in coastal erosion. They are the continuing processes that nourish the beach and carry sand along the shore of a barrier spit to deposit it at the end of the spit so that the spit grows in length.

  • How is beach erosion and the movement of sand affecting the marine life on Cape Cod?

    As far as beach erosion is concerned, marine life on Cape Cod is as happy as a clam. The natural processes of erosion, transportation, and deposition of sand may locally destroy marine life, but, in the long run, these processes provide the habitats where much of our marine life flourishes.

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  • Is the break in North Beach in Chatham part of a cycle of erosion and coastal change that has occurred before and will again in the future?

    Changes in North Beach have been observed over almost 400 years. Historically, breaching of the barrier and the subsequent southward migration of the inlet and the destruction of the barrier south of the breach has occurred at least four times. However, although some scientists consider the breaching and subsequent events to be cyclic with a frequency of about 100 or 150 years, the repetition of these events is better described as being episodic.

  • What causes these cycles to occur?

    The passage of the tides in and out of Chatham Harbor (including Pleasant Bay and Little Pleasant Bay) is controlled by the entrance to Chatham Harbor. As the entrance migrates southward, the tidal flow path becomes longer and longer. This leads to an increasing difference between the stage of the tide in the harbor and in the adjacent open ocean. Eventually, when North Beach has grown southward almost to or past the north end of Monomoy, the tidal differential is large enough to support a new breach in the barrier some distance to the north of the existing entrance. With a tide level difference of several feet on either side of the barrier, a small breach in the barrier, caused by a northeast storm, quickly becomes wider as the water rushes through and the breach becomes an inlet. With a new inlet, inlet migration and destruction of the barrier to the south begins again. The inlet alters the sand budget of the barrier. Sand carried southward along the north part of the barrier by longshore drift and longshore currents is swept into the lagoon by tidal currents. The southern part of the barrier is deprived of nourishment and migrates landward to preserve itself and to eventually weld itself to the upland. The combination of deposition at the south end of the northern part of the barrier and erosion of the north end of the southern part of the barrier causes the inlet to migrate southward.

  • Are there other known cycles of erosion of the Cape Cod Coast?

    Although less well studied, other episodes (cycles ?) of breaching and inlet migration do occur along the ocean shore of lower Cape Cod. The south to north migration of Nauset Inlet in Orleans and Eastham is well documented and has occurred more than once. Breaching has occurred a number of times on Monomoy. These breaches detach all or some of Monomoy from the mainland so that it becomes an island. Elsewhere around most of Cape Cod, the shoreline is better protected from large storm waves (a critical factor in causing a breach) and the barriers are more stable.

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  • Do scientists have an idea as to why erosion occurs in only certain spots on beaches and along sea cliffs or why erosion cuts through barrier islands at particular points?

    Over time, erosion occurs everywhere along the seaward side of barriers and along the cliffed shore. One determining factor as to where storm wave erosion occurs is the configuration of the adjacent seafloor. Shoals and bars offer protection from wave erosion by causing storm waves to break and dissipate their energy before reaching the shore. The configuration of the nearby seafloor is ever changing and changes in the location of shoals and bars may cause the locus of beach or cliff erosion to change position along the shore. Erosion on the seaward side coupled with deposition on the landward side has caused North Beach to migrate westward. In the long run, this rate of migration is the same as the rate of shoreline retreat for the cliffed part of the lower Cape to the north. The smooth arcuate shoreline from Provincetown to the south end of North Beach is evidence of this.

    Breaching does not always occur in the same place. The most likely place for a breach to occur is where the barrier is narrow and where previous storm waves have washed across the barrier and removed the dunes. These conditions were present at the site of the 1987 breach in South Beach. On some barriers, beach dwellers have removed the dune to have a sea view. Once the protection offered by the dunes is gone, the barrier is more easily washed over and more subject to breaching.

  • Although various shore protection techniques are being carried out by towns and individuals, some scientists have said that the best method is to do nothing. Why is this?

    On coasts exposed to large storm waves and high storm surges, even the most massive seawalls will not always provide protection from wave attack. The storm surge may overtop a seawall to allow the storm waves to break against buildings and erode the land behind the seawall. Even if the storm waves do not overtop the wall, waves breaking against it will release the wave energy all at once and cause the beach to erode. Eventually all of the beach may be eroded away and then even fair weather waves will break directly against the seawall preventing the formation of a new beach. Jetties and groins generally do not stop erosion, but interfere with longshore drift and longshore currents to stop the passage of sand along the beach (Leatherman, 1988). All of these structures alter the natural system that created and maintains the shore. Coastal erosion, accretion, and shoreline retreat are vital to Cape Cod. Tampering with these processes may eventually destroy the very Cape Cod that towns and individual property owners are trying to save.

    Geologists think in geologic time, which is made up of thousands and millions of years. They know that over the long run all attempts to control the coast will fail. To many geologists, coastal erosion is not the problem, people and private property are.

  • Can you explain how global warming is causing a rise in sea level and subsequent erosion?

    In the short term (decades and centuries) we don't know if sea level around the world is rising, falling, or unchanging. Sea-level change may be local, the result of the sinking or rising of the land and not evidence of a worldwide change. Global warming, if it is really happening, may cause a rise in sea level if the remaining glaciers melt and the water is returned to the ocean basins, and if the sea water warms and expands. However, we are now probably far into the present interglacial stage (Imbrie and Imbrie, 1979) and sea-level rise resulting from the melting of the middle latitude continental ice sheets is complete. Thus, any remaining sea-level rise, caused by partial melting of the Greenland and Antarctic ice sheets, will likely be small. On the other hand, global warming might cause sea level to fall. Warmer air over Greenland and Antarctica that are now polar deserts (the air is very dry and there is only small amounts of precipitation) might cause more snow to fall. In this case, the glaciers would grow by removing water from the ocean basins and sea level would fall.

    As the sea level rises, the land must adjust or drown. It does this by the erosion of headlands and by the migration of barrier beaches and barrier islands landward. The migration is accomplished by a combination of erosion along the seaward side of the barrier and by the deposition of sand in the lagoon by overwash across the barrier or by flood tidal currents in inlets. Like a tank tread, the barrier rolls over itself and stays in shallow water and thus remains above sea level.

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  • What benefits, if any, come with the coastal erosion on Cape Cod?

    Coastal erosion, sand transport, and deposition are the natural processes that are responsible for the Cape as we know it. Without these processes there would be no beaches, no barrier islands or barrier spits, and no sea cliffs. Without them there would be fewer sheltered harbors, bays, lagoons, and salt marshes. The dunes of Provincetown and Sandy Neck, near Barnstable, would not exist.

  • In the book Cape Cod and the Islands, The Geologic Story, you say "...much can be done now to preserve what is left (the Cape and Islands) and, in many cases, to restore the natural environment and to begin a much-needed process of healing..." What is your feeling about what can be done today to save Cape Cod?

    I made this statement first in a contribution to a book on the natural history of the Cape and Islands (Oldale, 1990, p. 26) and more recently in a book on the geology of the Cape and Islands (Oldale, 1992, p. 168). Both times, I meant that we can restore and preserve what pleases us about Cape Cod and the Islands, by careful consideration of the effect of man's past and future activities on the natural environment. In no way does it mean that we should try to stop the natural processes that have shaped the Cape for many thousands of years and that will continue to shape the Cape in the future. Thus, I would urge that we should do the least possible to prevent these natural processes from operating, especially on the open coast. Certainly, in terms of thousands of years, we can do nothing to prevent Cape Cod from eroding away.

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