Why Do Waves Travel Towards A Shore No Matter Which Way The Wind Blows?

Answer 1:

Out at sea, waves tend to travel in the same direction as the wind is blowing. But the questioner is right — inshore they often move towards the shoreline. This is caused by the effects of both diffraction and refraction.

Diffraction causes waves to spread out after they pass through a gap — such as a harbour mouth or entrance to an enclosed bay, as shown in the diagram causing the waves to head towards all sides of the harbour. Refraction occurs when the waves travel into changed conditions, such as the shallow waver that is found near a shoreline.

Have you ever noticed that on a curved beach, the waves hit the beach in a parallel fashion all the way along?

This is because as the waves enter the shallower water, they change direction (or refract), slow down and realign as shown in the diagram.

This means that you get a single long wave breaking along the beach, followed by other single waves, rather than a number of individual waves that are breaking randomly. And, to answer the second part of the question, no, there is no place in the centre of the ocean where the water is perpetually still.

Answer 2: 

Shorelines are not all concentric, so if there were some still-water source of waves, it would not be in the centre of the ocean. Anyway, still water is not the origin of waves.

Waves travel in all directions relative to shorelines but only those that are travelling towards a shoreline will reach it.

Those that are travelling away from or parallel to a shoreline either strike another, distant shoreline or peter out as their amplitude shrinks and they eventually become lost in the random surface disturbance.

Because most waves are caused by a steady wind blowing on an open sea, they tend to form in a regular and parallel pattern — in fact it is possible to steer a vessel using just the bearing of the open ocean swell. However, on entering shallower water swells are refracted, because a wave slows as it encounters the shelving sea floor beneath it.

This is what ultimately causes waves to line up with the shore and break as the power part of the waves slows in a line parallel to the shoreline and the faster top tumbles forward.

Published in The Hindu on Feb 14, 2002.