[thedoc (OP)]

Gail and Alan Alderson  asked the Naked Scientists:
   As  I understand it the waves on the sea are caused by the wind so they can be at any angle. So how is it that regardless of the wind direction waves always arrive at the beach parallel to the beach?

Freddie Alderson

Gran Canaria


   
What do you think?

[alancalverd]

The swell at sea does indeed depend on the wind direction, but the waves that arrive on shore (except in the strongest gales)  result from diffraction of the swell around headlands, with the diffracted wave collapsed by the slope of the beach. 

[Tim the Plumber]

I don't know how many beaches you have been to but they are not always parrallel to the beach.

You presumably have only been observing them in a cove or other inlet or where the wind is driving them straight at the beach.

Try a long beach where the wind is blowing across the beach sideways.

[Colin2B]

As Alan says, this is due to diffraction around headlands.
On long beaches refraction is another effect because waves entering shallow water slow down, so if the wave is coming in at an angle the shore side slows and the ocean side of the wave then catches up causing the whole wave to bend towards the shore. Whether the wave ends up parallel depends on its initial angle and the slope of the sea bed. Gently sloping seabed stretching a long way out gives plenty of time for the waves to curve in.
Headlands, islands and underwater hills/valleys will also cause waves to curve.