Coastal Processes—Waves

Waves are the dominant force driving the nature of a beach. The energy carried through waves moves beach sediment and transforms beach shape. The more energy, the greater the extent of change. The amount of energy carried through a wave can be determined by the wave structure. Waves are characterized by their wavelength (distance between peaks), height (elevation change between the crest and the trough), velocity (rate of forward motion of the wave peak), and period (the interval of time between successive wave peaks passing the same point). These properties, and the relationships between them, vary greatly depending on the nature of the mechanism generating the wave, the intensity of this generating mechanism, and the environment in which the wave exists.

Wind-generation of waves involves a transfer of energy from moving air to a water surface (Summerfield 1991). The amount of energy exchanged depends mainly on velocity, duration, and fetch (the distance over which the wind blows, which has an important influence on wave height and period) of the wind. The highest waves are produced by strong winds blowing in the same direction over a long distance; such waves can reach heights of 50 feet (15 m). Waves also are generated by low atmospheric pressure (storm surges) and displacement of the ocean floor, in particular by earthquakes (tsunami).

In the deep ocean, little forward motion of water in waves occurs because the wave form moves rather than the water. As waves move toward shallower water, however, their mode of movement changes dramatically. Once the water depth decreases to less than half that of the wave length, the seafloor starts to interfere with the oscillatory motion of waves. The orbit of individual water particles becomes less circular and more elliptical. Forward movement of water now becomes important as the oscillatory (deep-ocean) waves are transformed into translatory waves. This forward motion of water and energy takes along with it newly suspended sediment. As the water depth becomes progressively more shallow, wave length and velocity decrease, wave height increases and consequently the wave steepens. Eventually the wave is over-steepened to the stage where it breaks as its crest crashes forward creating surf. The zone of active breaking waves is known as the surf zone. Once the wave form has been destroyed, the remaining water moves up the shore as swash and returns under the force of gravity as undertow. The energy carried through the wave during this process is dissipated in three ways: formation of subsequent waves or currents, the breaking of the wave itself, and movement of sediment. (Pilkey et.al. 2017).

The wave type, as it approaches the coastline, is dependent on the original wave size and the slope of the seabed. Spilling breakers form when there is a gentle, low angle, slope leading into the beach. The crest of the wave typically “crumbles” forwards in a soft manner. The larger waves crash further out and will often reform as lower energy waves multiple times before reaching the shoreline. Plunging breakers are found where there is a sandbar, reef, or rock structure that abruptly changes the slope of the seabed. The crest of the wave moves faster than the base casing the peak to rise up and curl over the top. These are the waves that form tubes of ocean water often sought out by surfers. Surging breakers are found at the steepest slope shorelines where the water depth quickly changes at the coast. The wave makes landfall with a smooth, sliding movement and no wave break. Strong backwash currents are associated with this type of wave (Pilkey et.al. 2017). Each wave type has a different effect on shoreline weathering and coastal erosion.

Related Links

• Cape Hatteras National Seashore, North Carolina—[Geodiversity Atlas] [Park Home]
• Indiana Dunes National Lakeshore, Indiana—[Geodiversity Atlas] [Park Home]
• Santa Monica Mountains National Recreation Area, California—[Geodiversity Atlas] [Park Home]

• NPS—Coastal Geology
• NPS—Beaches and Coastal Landforms
• NPS—Geology
• NPS—Oceans