John Falloon, of Ardgowan School, asks :-

Why do waves curl over?

John Campbell, an old surf guard and a physicist at the University of Canterbury, responded.

Because the top of the wave is travelling faster than the bottom. It all has to do with the speed of waves on the surface of water.

In the deep ocean water a particle of water just goes up and down as the wave passes by. In this case the important distance parameter is the distance between wave crests, which we call the wavelength for short. The speed at which waves in the deep ocean travel depends on this wavelength, the longer the wavelength the faster the wave travels.

In shallow water, say where the depth is less than 10 times the wavelength of the wave, the bottom has increasing influence on the up and down motion of the passing water wave. Now the depth becomes the important length parameter and the wave speed depends on the depth of the water. The shallower the water the slower the wave speed. So it is the bottom of the wave which is trying to slow down whereas the top is trying to maintain its earlier speed. Finally the top falls off the bottom of the wave and it breaks.

It is a little more complicated than that. For waves in shallow water a particle of water describes more of a circular motion. You can see this in the sand disturbance under the wave at the beach. As the top of the wave passes overhead the sand disturbance shows the water at the bottom going away from the beach. The shallower the water the more circular the motion, causing the top of the wave to be thrown forward off the bottom of the wave with even more vigor.

When at the beach you can tell where any sandbanks are (the waves break first over them because it is shallower) and where any channels and potholes are (the waves may not break over them as it is deeper).