The Cape Naturalist: Vol 1 No 1
June, 1972
The outer beach of Cape Cod from Provincetown to Monomoy Point may be considered to be 30 miles out to sea, and as such, is subject to the relentless attack of the waves. However, the outer beach has shown that it is capable of adjusting to the continuous onslaught of the waves through changes in the shoreline configuration. Two of these coastal processes will be discussed in detail.
The Growing Shield. A glance at a map shows that the outer beach of Cape Cod resembles a curved shield which appears to protect the Cape from the storm waves generated out in the Atlantic. Most of the waves approach Cape Cod from the east-northeast. The cliff and beach of the outer Cape in the Truro vicinity, in the center of the shield, are oriented perpendicular to the dominant wave approach direction, and so receive the brunt of the wave attack. North of Highland Light and south of Marconi Station the east-northeast waves approach the outer Cape shoreline at an oblique angle, and some of the wave energy is transformed into longshore currents which flow parallel to the shore and in the general direction of the wave advance. These currents transport much of the sediment eroded from the Truro cliffs to Race Point and Long Point to the northwest, and to Nauset Spit and Monomoy Island to the south. Indeed all four of these features were formed as a direct result of the processes of wave induced longshore currents discussed above.
Protuberances on the shield If one stands on the outer beach of Cape Cod and gazes up and down the beach, one will notice that the shoreline is not absolutely straight. In fact, portions of the shoreline are seen to project seaward, forming shoreline protuberances of sand approximately one-half mile to one and one-half miles apart. These protuberances occur on most beaches of the Atlantic Coast of the United States, but are especially pronounced on the outer beach of Cape Cod.
A process has been suggested which explains the origin of these beach protuberances. Wave fronts, sometimes called wave crests, impinging on the outer Cape are slowed down as they begin to feel the bottom. This slowing down begins at depths of approximately 250 feet for big waves and 125 feet for smaller waves. The shoals on the sea floor off Cape Cod cause the portions of the incoming wave fronts which pass directly over the shoals to slow down first. This results in bending of the wave fronts and changes the amount of waver energy within the different portions of the wave front. Where the wave orthogonals, i.e., lines drawn everywhere perpendicular to the wave front, converge, there is a concentration of wave energy, and the shoreline opposite this part of the wave front undergoes increased erosion. Where the orthogonals diverge, there is a lesser amount of wave energy within the wave front. These variations in wave energy result in different rates of beach erosion, and cause small segments of the shoreline to change orientation and become perpendicular to the dominant east-northeast wave approach direction.
Knowing the locations of the severe erosion zones becomes critical in any coastal zone planning, whether for the construction of a major engineering structure or the building of a summer home. On Monomoy Island, the authors observed differences in rates of beach erosion of over 50 feet per year between two locations just one mile apart.
How long have these processes been operating? The glacial sediments that make up the backbone of Cape Cod were deposited by the melting ice sheets that covered the Cape, previous to 12,000 years ago. As these huge ice sheets melted, sea level rose until it reached the present level, approximately 6000 years ago. Thus the outer beach of Cape Cod has been under the influence of waves for the last six thousand years. Within historical times ,the 150 foot high cliffs from Truro to Eastham have eroded back at a rate of three feet per year. If the present rate of erosion can be extended into the past, these cliffs must have eroded back approximately three miles. The great bulk of this eroded sediment has been redeposited as sand spits growing outward from the center of the shield north to Long Point and south to Monomoy Point.
Thus, by extending our present observations and studies into the past, we can see that through natural processes land eroded by the waves is transported by wave-formed currents and then shaped by the waves into new landforms. Man must carefully learn nature’s ways before he can hope to interfere without risking unforeseen repercussions.
References:
Bascomb, W., 1964, “Waves and beaches”: Doubleday, Garden City, New York, 267 p.
Chamberlain, B., 1964, “These fragile outposts”: Natural History Press, Garden City, New York, 327 p.
Goldsmith, V., and Colonell, j., 1970, “Effects of nonuniform wave energy in the littoral zone”: Proc. Twelfth Coastal Engineering Conf., A.S.C.E., p. 767-785.
Hay, john, 1963, “The Great Beach”: Doubleday, Garden City, New York, 131 p. Strahler, A. N., 1966, “A geologist’s view of Cape Cod”: Natural History Press, Garden City, New York, 115 p.
Zeigler, j. M., Tuttle, S. D., Giese, G. 5., and Tasha, H. j., 1964, “Residence time of sand composing the beaches and bars of outer Cape Cod”: Proc. Ninth Coastal Engineering Conf., A. S. C. E., p. 403-416.
DR. GOLDSMITH was formally Faculty Research Associate with the Coastal Research Center in the Department of Geology at The University of Massachusetts, and is now Associate Marine Scientist at The Virginia Institute of Marine Science.
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