John Falloon, of Ardgowan School, asks :-

Why are waves always parallel to the beach?

John Campbell, a physicist at the University of Canterbury, responded.

Because the speed of waves in shallow water depends on the depth of water.

In deep ocean water waves build up due to the interaction of wind blowing over the sea. 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 wave speed is proportional to the square root of the wavelength. The longer the wavelength the faster the wave travels. We observe this after the ocean has been calm. The first sign of a distant storm is often the gentle swells with a large distance between wave crests. An oily ocean.

In shallow water, say where the depth is less than ten times the wavelength of the wave, the bottom has increasing influence on the up and down motion of the passing water wave. The motion of the water particles now goes in and out whilst going up and down. A particle travels in an oval fashion. It is then the depth which becomes the important length parameter and the wave speed depends on the depth of the water. The wave speed dis proportional to the square root of the water depth. The shallower the water the slower the wave travels.

The next time you are at the beach measure the wave speed of unbroken waves at different depths of water. This can sometimes be done whilst keeping dry by walking along a pier.

If the wave comes in at an angle from deeper water, the part of the wave closest to the beach slows down, allowing the rest to catch up until the entire wave crest is travelling at the same speed, ie has the same depth of water under it.

Hence for gently sloping sandy beaches the waves are always parallel to the shore. There are moves to sink artificial reefs near beaches such that the depth of water above the reef changes slowly along the beach. This would give a uniform shoulder for board riders to ride.