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How Do Tides and Waves Work?


How Do Tides and Waves Work?
Photo © Christophe Libert / stock.xchng.

Waves give rhythm to the ocean. They transport energy over vast distances. Where they make landfall, waves help to sculpt a unique and dynamic mosaic of coastal habitats. They impart a watery pulse upon intertidal zones and trim back coastal sand dunes as they creep towards the sea. Where coasts are rocky, waves and tides can, over time, erode the shoreline leaving dramatic sea cliffs. Thus, understanding ocean waves is an important part of understanding the coastal habitats they influence. In general, there are three types of ocean waves: wind-driven waves, tidal waves, and tsunamis.

  • Wind-Driven Waves—Wind-driven waves are waves that form as wind passes over the surface of the open water. Energy from the wind is transfered into the topmost layers of water via friction and pressure. These forces develop a disturbance that is transported through the sea water. It should be noted that it is the wave that moves, not the water itself (for the most part). For a demonstration of this principle, see What is a Wave?. Additionally, the behavior of waves in water adheres to the same principles that governs the behavior of other waves such as sound waves in air.
  • Tidal Waves—Tidal waves are the largest oceanic waves on our planet. Tidal waves are formed by the gravitational forces of the earth, sun, and moon. The gravitational forces of the sun and (to a greater extent) the moon pull on the oceans causing the oceans to swell on either side of the earth (the side closest to the moon and the side farthest from the moon). As the earth rotates, the tides go 'in' and 'out' (the earth moves but the bulge of water remains in line with the moon, giving the appearance that the tides are moving when it is in fact the earth that is moving).
  • Tsunamis—Tsunamis are large, powerful oceanic waves caused by geological disturbances (earthquakes, landslides, volcanic eruptions) and are normally very large waves.

When Waves Meet

Now that we've defined some types of ocean waves, we'll look at how waves behave when they encounter other waves (this gets tricky so you may want to refer to the sources listed at the end of this article for more information). When ocean waves (or for that matter any waves such as sound waves) meet one another the following principles apply:

  • Superposition—When the waves traveling through the same medium at the same time pass through one another, they do not disturb each other. At any point in space or time, the net displacement that is observed in the medium (in the case of ocean waves, the medium is sea water) is the sum of the individual wave displacements (Source: Russell, 2007).
  • Destructive Interference—Destructive interference occurs when two waves collide and the crest of one wave aligns with the trough of another wave. The result is that the waves cancel each other out.
  • Constructive Interference—Constructive interference occurs when two waves collide and the crest of one wave aligns with the crest of another wave. The result is that the waves add together each other out.

Where Land Meets Sea

When waves meet the shore, they are reflected which means that wave is pushed back or resisted by the shore (or any hard surface) such that the wave motion is sent back in the other direction. Additionally, when waves meet a shore, it is refracted. As the wave approaches the shore it experiences friction as it moves over the sea floor. This frictional force bends (or refracts) the wave differently depending on the characteristics of the sea floor.



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