Craig C. Freudenrich
One of the first things you notice when you go to the beach is the waves. They come ashore, crest and crash on the beach and surfers make the most of them. For all the water power waves seem to produce, they start with wind energy. Winds blowing across the surface of the ocean push water through the friction forces and build up energy. In fact, wind speeds, the distance over which the winds blow (fetch), and the time that the winds are in contact with the water all determine the height and speed of the wave. Waves can travel thousands of miles from their sources of origin. But it's the wave energy that moves, not the water.
You can see this more clearly by watching the motion of an object (such as a floating seagull) in the deep ocean as a wave passes; the seagull traces out a circular path. It moves upward, forward, downward and backward to return to its original position. The wave has passed, but the object remains. The size of the circular path gets smaller as you go deeper into the water. At a depth that is greater than one-half of a wavelength (one wavelength is the distance from the crest of one wave to the crest of the next wave), there is no motion in the water at all.
As a wave approaches shore, friction between the bottom of the wave and the seafloor affects the wave's motion. The lower portion of the wave slows, while the upper portion continues to travel at the wave's speed in deep water. The wave rises with the slope of the sea bottom. Because the wave's top moves faster than its bottom, it begins to curl, break and collapse. The type of breaker depends upon the steepness of the beach's slope:
- Surging breakers. Steep slopes do not cause waves to break, but rather roll onto the beach. Surging breakers can be quite destructive because waves have their full energy.
- Plunging breakers. Moderate slopes cause waves to curl into tunnels as they break. These are the waves that surfers love.
- Spilling breakers. Shallow slopes cause waves to break far from shore. The surf rolls over the front of a spilling breaker as it moves far up the beach.
Although wind causes waves, occasionally an offshore earthquake, landslide or volcanic eruption creates a wave called a tsunami. The events displace water and cause waves with long wavelengths. Tsunamis have more energy than typical wind-caused waves and cause more destruction.Ocean City, MD -- an urbanized barrier island (Photo courtesy USGS)
Have you ever blown across the top of a cup of hot coffee or cocoa to cool it? The air you blow out moves the surface of the liquid and forms a ridge that travels to the far side of the cup. Waves are created in a similar way. Wind moving along the surface of the ocean pulls water on the surface with it. High wind speeds over vast distances create an enormous buildup of energy. As waves approach the shore they lose stability, causing the top to break and fall over.
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