The Invisible Friction
A wave in deep water is not moving water. It is moving energy. The water molecules move in perfect circles. They go up, forward, down, and back. They return to where they started.
The Shoaling Effect
When a wave hits a sandbar, the water gets shallow. The bottom of the circular orbit hits the sand. Friction grabs the water particles at the bottom.
This friction slows the bottom of the wave. However, the top of the wave still carries the momentum from the deep ocean. It keeps moving fast.
The Ellipse
The perfect circle gets squashed. It becomes a flat oval (ellipse). The wave height increases to compensate for the loss of speed. This is called "Shoaling." Eventually, the top outruns the bottom and the wave trips over itself. This is the break.
Orbital Motion Transformation
Deep Water
Perfect circles
No friction
Shallow Water
Flattened ellipse
Bottom slows
Breaking
Top outruns bottom
Wave crashes
The Breaking Ratio
A wave typically breaks when the wave height becomes about 78% of the water depth. This is why you can predict where waves will break by knowing the depth of the sandbar.
Summary
Sandbars act as brakes for the bottom of the wave. The top has no brakes, so it crashes.
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