How Do Waves Affect the Seashore?

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Ocean waves breaking on a beach in Hawaii.
Ocean waves breaking on a beach in Hawaii. (Image: Medioimages/Photodisc/Photodisc/Getty Images)

Any disturbance to the ocean surface causes waves to form. Above the ocean, these can be wind, changes in atmospheric pressure, and the gravitational pull of the sun and moon on the rotating earth. Below the sea surface, disturbances may be submarine landslides, volcanic eruptions and earthquakes that cause a vertical displacement of the sea floor. The waves are a response to the disturbance and an attempt to restore calm as they move from one location to another. Wave height is the vertical distance between the wave crest and wave trough. Wavelength is the horizontal distance between two crests. The maximum height that a wave can attain in the open ocean is one-seventh of its wavelength. White caps form at the crest when the wave height to wave length ratio is higher.

Wave Movement

The movement of waves on the sea surface is the transfer of energy through the motion of water molecules. The energy from a blowing wind or other disturbance changes the sea surface first to ripples and then to waves that absorb the energy. Water molecules move up and down in a vertical orbit and transfer the energy forward. The mass of water does not move forward with the energy. The ocean floor does not affect the wave motion until the water depths are less than half of the prevailing wavelengths. As water depths decrease, wavelengths decrease and wave heights increase. Eventually, the wave curls and breaks on the shore. The area of wave break is the swash zone. Swash consists of two phases: the onshore flow, or uprush, and offshore flow, or backwash.

Longshore Drift

Longshore drift is the transport of sediment along a shoreline and is the most important process that forms its shape. Waves mostly approach a shoreline at an angle because winds control prevailing wave direction. Waves approach a shore at right angles only if the coast is dead straight over hundreds of miles. On approach, the energy of the breaking wave erodes sediment from the beach and transports it forward in its swash zone. Sediment moves forward at the wave’s angle of approach, but the wave’s backwash drags some sediment back to the sea in a direction at right angles to the coast. Over time, waves erode one part of the coast and deposit sediment in a more distant part of the coast, creating new land such as sand spits.

Tides

Gravitational forces of the sun and moon, combined with the rotation of the earth, cause sea levels to rise and fall and create tidal waves. Tidal wavelengths are the longest of all ocean waves and can reach lengths of 12,500 miles. Sea locations experience two high tides and two low tides daily. The rise in water levels varies over a lunar month depending of the relative position of the sun, moon and earth in their orbits. Water levels reach maximum and minimum heights – spring tides and neap tides - twice monthly. Tidal levels affect the heights of breaking waves along a shoreline and their power to erode it. High tides and spring tides can destroy cliffs and sand spits. Tidal energy also channels water upstream of an estuary and create a destructive tidal bore.

Tsunamis

Tsunamis form by the vertical displacement of the sea floor during an earthquake, volcanic eruption or following undersea landslides. Released energy travels through the body of water and emerges at its surface as a wave. Each emission of energy from the earth movement creates another wave at the surface, forming a wave train. The initial wave may be just 3 feet in height. As the waves span over the entire ocean surface, they gather momentum and can grow to a height of 150 feet, a wavelength of 200 miles, and speeds of 600 mph. Tsunami waves lose speed as they approach shallow water on the shore, but transfer this energy into higher waves with a stronger destructive force. The waves can travel several miles inland after striking the shore, destroying all objects in their path.

Damage on the Japanese coast after the Mar 11, 2011 earthquake and tsunami.
Damage on the Japanese coast after the Mar 11, 2011 earthquake and tsunami. (Image: Kiyoshi Ota/Getty Images News/Getty Images)

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