As a wave approaches the shore and enters shallower water, the bottom of the wave slows down due to friction with the seabed. The top of the wave, however, maintains its speed. This causes the wave to steepen until it becomes unstable and collapses, resulting in the wave breaking. Generally, a wave will break when the water depth is approximately 1.3 times the wave height.
Here's a more detailed breakdown:
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Shallowing Water: Waves begin their journey in deep water, unaffected by the seabed. As they move closer to the shore and into shallower water, the seafloor begins to influence their behavior.
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Friction: The bottom of the wave experiences friction as it drags against the seabed. This friction slows down the lower portion of the wave.
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Wave Steepening: While the bottom of the wave slows, the top continues to move at its original speed. This differential in speed causes the wave to become increasingly steep and its wavelength to shorten.
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Instability: The wave continues to steepen until it reaches a point of instability. This point is generally reached when the wave height is a significant portion of the water depth.
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Breaking: The unstable wave collapses, causing the top of the wave to spill forward. This is what we observe as a breaking wave.
Different types of breaking waves are determined by the slope of the seabed:
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Spilling breakers: Occur on gently sloping seabeds. The wave breaks gradually, with foam spilling down the front of the wave.
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Plunging breakers: Occur on moderately steep seabeds. The wave curls over, trapping air underneath, before crashing down with considerable force. This type of wave is popular among surfers.
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Surging breakers: Occur on very steep seabeds. The wave doesn't break in the traditional sense but surges up the beach.
