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Why do waves on the sea shore move towards the shore even when the tide is going out?

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up vote 5 down vote accepted

Waves are generated by wind. But aside of small local winds, most of the waves you see at the shore are generated by stronger winds far out on the sea from where they start to propagate into all kinds of directions.

Imagine a point in the middle of the sea where a waves with different directions are created. They will propagate away from this point until they hit a shore, therefore they can only move towards the shore not away from it.

This is also why waves always brake towards the shore and not away from it. Waves in deep water are more or less circular waves which will go unhindered until the ground is too shallow for the circular motion to go on. See here:

enter image description here

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This is a very helpful answer. But I don't understand why a wind thats blowing out to sea cannot propogate waves in that direction. – storabjorn Aug 22 '11 at 14:01
consider a local area out on the sea where there is wind. If this is sufficiently local you can view it actually as something like a point-like pertubation, similar to what happens when you throw a rock into a pond. The resulting wave will propagate outwards from this point. – luksen Aug 22 '11 at 14:08
The key point is that water waves as a whole does not signal the motion of the water as a bulk! Waves are a way for energy to be propagated through a medium without the medium to move as a whole. Wind doesn't create waves by pushing little crests along as if they were tiny sailboats. – Willie Wong Aug 22 '11 at 16:34
The transfer of energy from the wind to the water is mostly "vertical", not horizontal. The motion of the wind creates regions of different air pressure on the surface of the ocean, which sets off undulations on the surface, which then propagates as waves in the manner luksen's post indicates. – Willie Wong Aug 22 '11 at 16:43
Waves do transmit energy downwind. So waves going away from shore that are generated by wind, don't have any fetch to generate waves heights. So the amplitude of these waves should start at zero at the shoreline, and increase as you get further from shore. Also nore for incoming waves, the are diffracted as they reach shallower water -they wave velocity if lower as the water gets shallower, so this means obliquely incoming waves appear to be mostly coming in perpendicular to the shore. – Omega Centauri Aug 22 '11 at 17:58

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