Breaking wave

When a deep water wave approaches a sloping beach, it changes into a shallow water wave. As the depth of the water continues to decrease, the wave will steepen and eventually topple over or collapse. This collapsing waves is called a breaking wave.

A mathematically rigorous description of a breaking wave has never been found. It's really that complicated. Fortunately, an intuitive description isn't that hard.

Consider the motion of a particle of water in a wave far off shore. If you follow this particle, you'll see that its horizontal position doesn't change. It just moves up and down in a circular motion. This holds for all the particles in a wave -- they move in a oscillating, circular motion.

That circular motion must be supported by a column of water. In a deep water wave, bottom effects are absent and there is no problem with supporting the wave. As the depth of the water decreases, however, the bottom will begin to interfere with the motion of water in the supporting column. Eventually, it will slow down the circular motion of the water particles in the wave. At this point, the motion at to top of the column is unimpeded and hence faster than motion near the bottom of the column. This drag effect on the bottom of the column steepens the wave.

Eventually, as the water gets more and more shallow, the drag on the bottom of the column grows and the wave gets so steep that it can't support its own weight. At this point, the whole wave topples over -- i.e. the wave breaks. The most obvious visual manifestation of the circular motion becomes apparent - the top of the wave curls forward and, in a large wave, can form a tube.

As the wave breaks, the gravitational potential energy is converted into rotational kinetic energy. This can result in potentially violent angular motions in the breaking wave. A kayaker can use this motion to help them roll. Its not that hard and when executed properly, Surf rolling looks really cool.

See Kayak surfing