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Chapter Overview

The previous five chapters have dealt with the sculpturing of the land by mass wasting, streams, ground water, glaciers, and wind. Water waves are another agent of erosion, transportation, and deposition of sediment. Along the shores of oceans and lakes, waves break against the land, building it up in some places and tearing down in others.

The energy of the waves comes from the wind. This energy is used to a large extent in eroding and transporting sediment along the shoreline. Understanding how waves travel and move sediment can help you see how easily the balance of supply, transportation, and deposition of beach sediment can be disturbed. Such disturbances can be natural or human-made, and the changes that result often destroy beachfront homes and block harbors with sand.

Beaches have been called "rivers of sand" because breaking waves, as they sort and transport sediment, tend to move sand parallel to the shoreline. In this chapter we look at how beaches are formed and also examine the influence of wave action on such coastal features as sea cliffs, barrier islands, and terraces.

Learning Objectives

1. Wind energy is transferred to water surfaces as waves. Wave height (distance from crest to trough) reflects wind speed, duration and distance. Wavelength is the distance between crests. Waves passing a point move water particles in a circular orbit. At the surface, diameter of the orbit equals wave height. At depth, effects of wave passage are lost below depths equal to half the wavelength.

2. Wave refraction is the change in direction along a wave crest as it comes in contact with the bottom while approaching shore. Wave refraction produces longshore currents that are parallel to the shoreline and transport considerable sediment parallel to the shoreline in the surf zone. Rip currents are perpendicular to shore and carry fine sediment offshore.

3. Beaches are strips of sand or gravel that extend from marine terraces offshore to cliffs or permanent vegetation zones onshore. The beach face is the steepest part and is exposed to wave action. The berm extends landward from the beach face.

4. Summer beaches have wide berms, while winter beaches have narrow berms and sandbars offshore. Most sediment on beaches was brought to the coast by rivers and streams. Damming free flowing rivers reduces sediment supply to beaches and promotes beach erosion.

5. Longshore drift is the movement of sediment parallel to shoreline by either swash and backwash along the beach face, or by longshore currents. Spits are fingerlike ridges of sediment deposited into open water. Baymouth bars extend from headland to headland cutting off bays from the ocean. Tombolos connect offshore islands to the mainland. All three depositional features reflect longshore drift. Jetties and groins are structures made by humans to interrupt sand being transported by longshore drift.

6. Coastal areas are classified as erosional, depositional, drowned, uplifted, or shaped by organisms. Erosional coasts are subject to coastal straightening (erosion on headlands and deposition in bays), and exhibit sea cliffs, wave-cut platforms, stacks and arches.

7. Depositional coasts have barrier islands, deltas, tidal deltas, and may preserve glacial deposits such as moraines. Fiords and estuaries are typical of drowned (submergent) coasts. Uplifted coasts exhibit uplifted marine terraces. Reefs and mangroves may shape coasts.

Related Readings

Bascom, W. 1980. Waves and Beaches. Rev. ed. New York: Doubleday Anchor Books.

Bird, E. C. F. 1985. Coastline Changes: A Global Review. New York: John Wiley and Sons.

Bird, E. C. F., and M. L. Schwartz. 1985. The World's Coastlines. New York: Van Nostrand Reinhold.

Davis, R. A., Jr., and R. L. Ethington. 1976. Beach and Nearshore Sedimentation. Tulsa: Society of Economic Paleontologists and Mineralogists, Special Publication 24.

Dolan, R., B. Hayden, and H. Lins. 1980. Barrier Islands. American Scientist 68: 16-25.

Dolan, R., and H. Lins. 1987. Beaches and Barrier Islands. Scientific American 257(7): 68-77.

Easterbrook, D. J. 1993. Surface Processes and Landforms. New York: Macmillan.

Hardisty, J. 1990. Beaches, Form and Process. New York: Harper Collins Academic.

Inman, D. L., and B. M. Brush. 1973. The Coastal Challenge. Science 180 (4094): 20-32.

Kaufman, W., and O. H. Pilkey, Jr. 1983. The Beaches are Moving: The Drowning of America's Shoreline. Durham, NC: Duke University Press.

Komar, P. D. 1976. Beach Processes and Sedimentation. Englewood Cliffs, NJ: Prentice-Hall.

Mack, W. N., and E. A. Leitikow. 1996. Sands of the World. Scientific American. August: 62-67.

Nuhfer, E. B., R. J. Proctor, and P. H. Moser. 1993. The Citizens' Guide to Geologic Hazards. Arvada, CO: American Institute of Professional Geologists.

Shepard, F. P. 1973. Submarine Geology. 3d ed. New York: Harper and Row.

Shepard, F. P., and H. R. Wanless. 1971. Our Changing Coastlines. New York: McGraw-Hill.

Snead, R. E. 1982. Coastal Landforms and Surface Features: A Photographic Atlas and Glossary. Stroudsburg, PA: Hutchinson Ross.

Williams, S. J., K. Dodd, and K. K. Gohn. 1990. Coasts in Crisis. U.S. Geological Survey Circular 1075.

Answers to EOC Questions

Following are answers to the End of Chapter Questions for Chapter 14:

11.B, 12.B, 13.C, 14.A, 15.D, 16.A, 17.E, 18.E, 19.A, 20.D, 21.C

Boxed Readings

This chapter contains the following boxed readings:

Environmental Geology
Box 14.1: The Effects of Rising Sea Level





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