If you have ever thrown a stick out over a lake or past the surf zone on the ocean, you will probably have noticed that it doesn't move much over time, despite the waves passing under it. This is because, while the wave form moves forward over time, the water particles themselves trace out a circular path which leaves them more or less where they started. Figure 17.3 shows these circular paths. Wave speeds depend on the depth of the water and the wavelength. Deeper water and longer wavelengths give faster speeds, so long swells travel faster than do swells with shorter wavelengths.
Waves have a finite vertical extent.
It is also important for explaining why waves break and become surf. When the water depth is about half the wavelength, the wave senses the bottom and begins to steepen. The wave gets progressively more unstable until finally, when the water depth is about 1.3 times the wave height, the wave breaks. Figure 17.4 shows this process, assuming the sea bottom just off the beach is relatively smooth. If the sea bottom is irregular, you may have several places where waves break.