Waves: Characteristics, Coastal Impact, and Wave Energy

Waves are the forward movements of ocean water and energy over the sea surface. Most waves are caused by the oscillation (back-and-forth movement) of water molecules generated by the frictional drag of wind. Some very large waves, called tsunamis, are generated by undersea earthquakes and other sudden, jarring movements of the sea floor. Waves differ from tides and currents. Tides are the regular up-and-down movements of the ocean surface caused by the gravitational pulls of the moon and sun. Ocean currents are continuous surface and deep-water flows of water driven by several forces, including the Coriolis effect, wind, and temperature and density gradients.

CHARACTERISTICS AND ENERGY. Waves can be described by certain physical characteristics. The crest of a wave is the wave’s highest point, or peak. The trough of a wave is its lowest point. The wavelength is the horizontal length of the wave as measured between either two consecutive crests or two consecutive troughs. Wave height is the vertical distance between the top of a crest and the bottom of the adjacent trough. A wave train is a group of waves that travel together. The size and strength of waves are generally dependent on the speed of the wind and the friction caused by the wind on the sea surface. The powerful winds of storms can generate wave trains of great energy known as storm surges. “Rogue waves,” also called extreme storm waves, are waves that are unusually high compared with surrounding waves; they are generated unexpectedly and unpredictably and may be more than 100 feet (30 meters) high. Swells are the regular undulations of smooth, rounded waves that are left in the open ocean after high levels of energy, such as strong winds, have left the region. Some swells are only small ripples, whereas others are made up of large, flat-crested waves.

The velocity, or speed, of a wave is dependent on its wavelength and the depth of the water. The longer the wavelength, the faster the wave. Waves tend to slow down—and become shorter and higher—as the seawater becomes shallower closer to the coast. Faster wave speeds generally mean more wave energy. Wave energy tends to be greater in certain coastal regions than in others. Some of the most energetic and powerful waves can be found along the northwestern coast of the United States, western and northern Canada, western Scotland, and southern Africa.

The movement of a wave is more a movement of energy than of water. The water serves primarily as a flexible medium through which the energy moves. The energy in the water is generated by the friction of the wind over the open ocean. This energy is transferred between individual water molecules, causing the receiving molecules to shift forward a short distance and to form a vertical circular pattern. As the energy moves steadily toward the shore, more and more waves develop in the wave train, and the waves bunch up closer together, slowing in speed. The waves also increase in height as they approach the shore. Ultimately, the crests crash onto the coast in a so-called breaker, dissipating the energy onto the shore.

Wave height can be increased or decreased when two wave trains moving in different directions meet in an interaction known as interference. If the crests and troughs of one wave align with those of another wave, wave height is increased. If the crests align with the troughs, however, wave height is decreased.

COASTAL IMPACTS. The constant year-after-year pounding force of waves serves to shape coastlines around the world. Waves tend to erode and straighten most parts of a coastline. Land that is particularly resistant to erosion forms into headlands that jut out into the sea. Cliffs and caves are other features formed by wave erosion. Waves that are refracted, or change direction, after they make contact with headlands or other irregular parts of the coast may develop into longshore currents, which flow parallel to the coast. The sediment carried by these currents, called longshore drift, is deposited along the coast to form the sand and gravel of beaches, as well as the depositional features of bay barriers and lagoons.

Sudden substantial movements of the sea floor—such as earthquakes, landslides, and volcanic eruptions—generate a series of large, long, fast-moving waves in tsunamis as large amounts of water become displaced. Tsunamis can move as rapidly as 550 miles (885 kilometers) per hour, compared with normal wave speeds of 5 to 60 miles (8 to 97 kilometers) per hour. Each wave in a tsunami may be as long as 186 miles (300 kilometers) in the open ocean and as high as 100 feet (30 meters) as it reaches shore. When these powerful waves crash upon the shore, they can cause enormous destruction of natural environments and human-built structures, as well as huge numbers of deaths. Tsunamis are sometimes incorrectly called tidal waves.

ELECTRICITY GENERATION. Electricity can be generated using wave-power devices, which extract energy from the surface motion of waves or from subsurface pressure fluctuations associated with waves. Some of these devices are designed to operate near the shore, whereas others are positioned far offshore. Most wave-power technology remained in the experimental stage as of 2016. Concerns about environmental damage from wave-power systems and questions about the economic viability of these systems have limited commercial applications of wave power.

FURTHER READING: Casey, Susan. 2011. The Wave: In Pursuit of the Rogues, Freaks, and Giants of the Ocean. New York: Anchor Books.

National Weather Service, National Oceanic and Atmospheric Administration. “JetStream Max: Anatomy of a Wave.” http://www.srh.noaa.gov/jetstream/ocean/wave_max.html. Accessed December 8, 2016.

National Weather Service, National Oceanic and Atmospheric Administration. “Wind, Swell and Rogue Waves.” http://www.srh.noaa.gov/jetstream/ocean/waves.html. Accessed December 8, 2016.

SMS Tsunami Warning. “Tsunamis: Main Features.” http://www.sms-tsunami-warning.com/pages/ tsunami-features#.WEnULHeZMlV Accessed December 8, 2016.

Zirker, J. B. 2013. The Science of Ocean Waves: Ripples, Tsunamis, and Stormy Seas. Baltimore, MD: Johns Hopkins University Press.