Beaches and Coastal Processes

The shoreline is the meeting place for the interaction of land, water, and atmosphere, and rapid changes are the rule rather than the exception.

Beach Composition

The processes that create, erode, and modify beaches are many. Marine processes, such as waves, wave refraction, currents, and tides, work concurrently. This suggests that a beach is a sensitive landform, and indeed it is. Generally a beach is a sedimentary deposit made by waves and related processes. Beaches are often regarded as sandy deposits created by wave action; however, beaches may be composed of broken fragments of lava, sea shells, coral reef fragments, gravel, or even large stones. A beach is composed of whatever sediment is available. Beaches have remarkably resilient characteristics. They are landforms made of loose sediment and are constantly exposed to wave and current action. On occasion, the coastal processes may be very intensive, such as during a hurricane or tropical storm. Yet, in spite of the intensity of wave processes, these rather thin and narrow landforms, although perhaps displaced, restore themselves within a matter of days. Coastal scientists are inclined to believe that the occurrence and maintenance of beaches are related to their flexibility and rapid readjustment to the varying intensity of persistent processes.

Sediment deposited on beaches is often derived from the continents. Rivers are one source of beach sediment, and the coast is another. As rivers erode the land, the sediment they carry ultimately finds its way to the shore. Because the sediment may be transported several tens or hundreds of kilometers, it is refined and broken down even further to finer-sized particles. Once the river reaches the sea, the sediment is distributed by longshore currents along the shoreline as a beach. Beaches also occur where the shoreline is composed of cliffs, such as along the Pacific coast of North America. Here, waves erode the sea cliffs, and the sediment is deposited locally as a beach. Under these conditions, the beach deposit is most often gravelly because the sediment is transported only a short distance and has not had an opportunity to break up into finer-sized sediment such as sand. Some island beaches are made up entirely of shell fragments.

Waves

The most obvious mechanical energy source working on beaches is waves. Waves approaching a beach are generally created by winds at sea. As wind velocities increase, a wave form develops and radiates out from the storm. An oscillatory motion of the water occurs as the wave form moves across the water surface. It is important to note that the water movement within a wave is not the same as the movement of a wave form. In a wave created in deep water that is approaching a beach, the water particles move in a circular orbit, and little forward movement of the water occurs. The water at the surface moves from the top of the orbit (the wave crest) to the base of the orbit (the wave trough) and then back up. Thus the water particles move in an oscillatory wave; this motion continues down into the water. Although the size of the orbits in the water column decreases, motion occurs to a depth referred to as the wave base. At this point, the depth of the wave base is less than the depth of the water. As wave crests and troughs move into shallow water, the water depth decreases to a point where it is equal to the wave base. From this point, the orbital motion is confined because of the shallowness of the water and takes on an elliptical path. The ellipse becomes more confined as the waves enter ever shallower water and eventually becomes a horizontal line. At this point, there is a net forward movement of water in the form of a breaker.

As a wave enters shallow water, many adjustments occur, such as a change in the orbital path of water particles already described and a change in the velocity of the wave form. Since the wave base is “feeling the bottom” in shallower water, friction occurs, slowing the wave down. As seen from an airplane, waves entering shallow water do so at an angle, not parallel to the shoreline. Therefore, one part of the wave enters shallow water and slows down relative to the rest of the wave. Thus a part of the wave crest is feeling bottom sooner than the rest of the wave, causing the breaking action of the wave to appear to peel itself along the beachfront. Because the wave crests and troughs have different velocities, the wave refracts or bends. In so doing, the wave crests and troughs try to parallel the shallow bottom topography that they have encountered.

The wave refraction is seldom completed, and the breaking wave surges obliquely up the slope of the beach and then returns perpendicular to the shoreline. The result is a current that basically moves water in one direction parallel to the beach in a zigzag pattern (a longshore, or littoral, current). It is a slow-moving current that is located between the breaking wave out in the water and the beach. Because longshore-current movement operates in shallow water and along the beach, it is capable of transporting sediment along the shoreline. “Longshore drift” refers to the movement of sediment along beaches. In a sense, the longshore current is like a river moving sand and other material parallel to the shore. Beaches are always in a state of flux. Although they appear to be somewhat permanent, they are constantly being moved in the direction of the longshore current. Along any shoreline, several thousand cubic meters of sediment are constantly in motion as longshore drift. Along the beaches of the eastern United States, about 200,000 cubic meters of sediment is transported annually within a longshore drift system.

A different type of ocean wave is one that is generated on the ocean floor rather than by the wind. Such waves are properly termed tsunamis. Although they popularly have been coined “tidal waves,” they are completely unrelated to tides or the movement of planets. Some type of submarine displacement, such as the creation of a volcano, a landslide on the sea floor, or an earthquake beneath the ocean bottom, displaces a volume of water, which triggers waves. Tsunami waves are low, subdued forms traveling thousands of kilometers over the ocean surface at extremely high velocities, exceeding 800 kilometers per hour under the proper conditions. As a tsunami approaches shallow water or a confined bay, its height increases. There is no method for direct measurement of the heights of tsunami waves. However, in 1946, a lighthouse at Scotch Cap, Alaska, located on a headland 31 meters above the Pacific Ocean, was destroyed by waves caused by a landslide-generated tsunami.

The changing character of a beach is very dynamic because the properties of waves are variable. During storm conditions or when more powerful waves strike a beach during the winter season, for example, the beach commonly is eroded, has a steeper slope, and is composed of a residue of coarser sediment, such as gravel, that is more difficult to remove. During fair-weather or summer conditions, however, beaches are redeposited and built up.

Study of Beaches and Coastal Processes

The study of beaches and coastal processes is not particularly easy because of constant wave motion and changes in the beach shape. Scientists have, however, developed field methods as well as laboratory techniques to study these phenomena. To study wave motion and related current action, several techniques have been devised that include tracers, current meters, aerial photography, and satellite-based monitoring. Two types of tracers are commonly used to determine the direction and velocity of sand movement: radioactive isotopes and fluorescent coatings to produce luminophors. (Luminophors are sediment particles coated with selected organic or inorganic substances that glow under certain light conditions.) In the former type, a radioactive isotope such as gold, chromium, or iridium is placed on the surface of the grains of sediment. Alternatively, grains of glass may be coated with a radioactive element. The radioactive sediment can then be detected relatively easily and quickly using just a Geiger counter. Both techniques can trace the direction and abundance of sediment along the seashore. Bright-colored dyes or current meters can also be used to document the direction of longshore currents. Surveying instruments are used to determine the high and low topography of beaches and adjacent sand bars. By measuring beach topography before and after a storm, for example, scientists can record immediate changes. In this way, they can document the volume and dimensions of beach erosion or deposition that has taken place during a storm. Measurement of wave height and other wave characteristics can be done with varying degrees of sophistication. Holding a graduated pole in the water and visually observing wave crest and troughs is both the simplest and the cheapest method. To achieve more refined measurements, however, scientists place pressure transducers or ultrasonic devices on the shallow sea floor to record pressure differences or fluctuations of the sea surface. These more precise instruments also record other wave data for later study and analysis.

All these methods are detailed field techniques. By comparing aerial photographs, detailed maps, satellite pictures, and in some cases government studies in a coastal sector, changes over long periods of time may be detected. Finally, because wave motion cannot be controlled on a shoreline, wave tank studies are used to derive wave theories. Normally, an elongated glass-lined tank containing water from 0.3 to 1 meter in depth is used. A mechanical device called a wave machine displaces water at one end of the wave tank to create waves and sediment is introduced at the other end. The heights and other characteristics of the waves can be varied, as can the type of sediment, to form the beach. In this controlled way, the various beach and wave relationships can be studied.

Significance

More than 70 percent of the population of the United States lives in a shoreline setting, and thus an understanding of how waves interact with beaches is important. Shoreline property is prized for residential use and hence highly valuable. On many beaches, the great investments made in hotels and condominiums suggest that beaches are the most sought-after environment on Earth, and shoreline frontage is sold by the foot or meter, not by the acre or hectare. Currently, however, such demand and investment are threatened with rising sea levels and continued coastal erosion.

Beaches represent the buffer zone that mitigates wave erosion. Waves generated in the open sea slow down in shallow water, and the beach deposit absorbs the impact of waves. Beaches are therefore constantly changing and are one of the most ephemeral environments on Earth. Thus a sound knowledge of longshore currents and beach development is necessary prior to nearshore or marine construction. Sea walls and groins, for example, interfere with waves and longshore currents and may cause considerable erosion in selected areas, while inducing abnormal sedimentation in others. Sea walls are often constructed perpendicular to a shoreline to slow longshore currents so that a beach can be deposited. Downcurrent, beyond the area of beach deposition, erosion will take place. Similarly, rivers, which are a major source of the sediment that creates beaches, are sometimes dammed, thus depriving beaches of sediment and resulting in their erosion. Unless planners, developers, and builders understand these processes, major damage can result: Failure to understand coastal processes has, on occasion, caused a property owner to sue a neighbor who caused beach erosion to take place.

Principal Terms

beach: an accumulation of loose material, such as sand or gravel, that is deposited by waves and currents at the border of a body or stream of water

longshore current: a slow-moving current between a beach and the breakers, moving parallel to the beach; the current direction is determined by the wave refraction pattern

longshore drift: the movement of sediment parallel to the beach by a longshore current

oscillatory wave: a wind-generated wave in which each water particle describes a circular motion; such waves develop far from shore, where the water is deep

tsunami: a low, rapidly moving wave created by a disturbance on the ocean floor, such as a submarine landslide or earthquake

wave base: the depth to which water particles of an oscillatory wave have an orbital motion; generally the wave base is equal to one-half the distance between successive waves

wave refraction: the process by which the vertical angle of a wave moving into shallow water is changed or bent

Bibliography

Bird, Eric. Beach Management. New York: John Wiley, 1996. Includes chapters on waves and on beaches. Covers fundamental concepts, although most examples are Australian. A good introduction for anyone with a high school-level earth background.

Cameron, Silver Donald. The Living Beach. Toronto: Macmillan Canada, 1998. Looks at the ecosystems and biological systems of North American beaches. Focuses on the relationships between beaches and the environment. Index and bibliography.

Davis, Richard Jr., and Duncan Fitzgerald. Beaches and Coasts. John Wiley & Sons, 2009. Provides an exhaustive overview of the world’s different coasts and the processes that shape them, combining tectonics, hydrography, climate, and geology to explain the morphology of coasts and coastal processes.

Dean, Robert G., and Robert A. Dalrymple. Coastal Processes with Engineering Applications. Cambridge, England: Cambridge University Press, 2002. Covers coastal engineering and oceanography theories and applications intended to mitigate coastal erosion. For advanced students in fields related to coast morphology.

Fall, J. A., et al. Long-Term Consequences of the Exxon Valdez Oil Spill for Coastal Communities of South-central Alaska. Alaska Department of Fish and Game, Division of Subsistence, Anchorage, Alaska. Technical Report 163. 2001. Covers social, economic, and environmental impact of the oil spill.

Haslett, Simon K. Coastal Systems.2d ed. Abingdon: Routledge, 2009. Offers a concise introduction to the global environment of coasts in which ocean, land, and atmosphere all play a role in the physical and ecological evolution of coastlines.

Holthujsen, Leo H. Waves in Oceanic and Coastal Waters. New York: Cambridge University Press. The text is slightly above introductory level. It contains a chapter on observation techniques such as remote-sensing, altimetry, and wave buoys. The physics of water waves and linear wave theory are thoroughly discussed.

Kaufman, Wallace, and Orrin Pilkey. The Beaches Are Moving: The Drowning of America’s Shoreline. Durham, N.C.: Duke University Press, 1983. Deals with the processes working in the coastal zone, such as winds, waves, and tides. Highlights the impact of rising sea levels and the modification and urbanization of the coast. Written in a nontechnical style. A narrative text suitable for all ages.

Komar, Paul D. Beach Process and Sedimentation. Upper Saddle River, N.J.: Prentice Hall, 1998. Extensive treatment of waves, longshore currents, and sand transport on beaches. Equations and mathematical relationships are presented and elaborated upon. College-level material. Appropriate for those interested in the specifics of coastal processes.

Leatherman, Stephen P. Barrier Island Handbook. 3d ed. College Park, Md.: Laboratory for Coastal Reasearch, University of Maryland, 1988. Based on actual field studies along the East Coast of the United States. Includes numerous photographs, diagrams, and tables. Most suitable for coastal managers and government employees; however, very readable and suitable for nonscientists as well as the general scientist. Emphasizes the dynamic nature of beaches, recreation and construction impacts, and nearshore processes.

Lencek, Lena, and Gideon Boskar. The Beach: The History of Paradise on Earth. New York: Viking, 1998. A natural history of the environmental and social importance of coastal areas. Includes illustrations.

Masselink, G., and M. G. Hughes. Introduction to Coastal Processes and Geomorphology. London: Hodder Arnold Publication, 2003. The book is an excellent resource for students of geomorphology. Contains many illustrations and diagrams.

Micallef, Anton, and Allan Williams. Beach Management: Principles and Practice. London: Earthscan, 2009. Uses case studies from the United Kingdom, the United States, New Zealand, the Mediterranean, and Latin America. Provides a comprehensive coverage of beach management principles and practice.

Pethick, John. An Introduction to Coastal Geomorphology.Baltimore, Md.: Edward Arnold, 1984. A thorough survey of coastal processes. Discusses wave energy on the coast and its characteristics, the relationship between currents and the movement of beach material along the shore, and the landforms, such as beaches, mud flats, and estuaries. Most suitable for anyone needing an equation or a technical explanation of selected processes operating in a coastal zone.

Schwartz, Maurice L. Encyclopedia of Coastal Science. Dordrecht: Springer, 2005. Features contributions by well-known international specialists in their respective fields. Covers geomorphology, ecology, engineering, technology, oceanography, and human activities as they relate to coasts.

Thorsen, G. W. “Overview of Earthquake-Induced Water Waves in Washington and Oregon.” Washington Geologic Newsletter 16 (October, 1988). Offers an introduction to the impact of a tsunami on the coasts of Washington and Oregon following an earthquake in March 1964. Discusses the tsunami in nontechnical language. Estimates the damage and economic losses. Presents maps and tidal gauge records. A good review of wave activity, intended for interested laypersons and teachers.

Walker, H. J. “Coastal Morphology.” Soil Science 119 (January, 1975). A nontechnical overview of coastal landforms. Discussion includes beaches, deltas, and lagoons. Includes maps illustrating processes. Useful for a nonscientist interested in the causes and distribution of coastal features from a geographical perspective.