Re: Why do ocean waves curl and break, especially at the beach?

It is all a matter of different forces acting on the wave at different
heights.

At the shore, the "bottom" of the wave hits the beach, causing it to
slow down.  The water at the top of the wave, however, is still moving 
forward.  Its momentum causes it to move past the bottom part of the wave,
and as it does so, it literally flows down over the front as though it had
struck a wall.  

Out at sea, waves do not usually curl or break.  The exception is when they
are acted upon by strong winds.  Here, the top of the wave is pushed 
forward by the wind at a speed greater than the bottom part of the wave
is traveling.  Once again, the water near the top of the wave is pushed
ahead of the slower-moving water in front of it.