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The Seashore

Sand Dunes

Constructive Waves

In waves, individual water molecules move in circles . There is little forward motion of the water, it is the wave that moves forward. If you lose your football in the sea (assuming it's not blown somewhere by the wind) the ball will stay roughly in the same place moving up and down as the waves pass. As waves approach shallow water (about two thirds the wave height) the circular movement of the water molecules is impeded by friction from the bottom. The water at the top of the circle therefore is not supported by more water (as it would be if the wave was moving forwards) and it moves forwards into thin air. A crest forms which builds higher and higher until it topples over and breaks on the shore. All the circular motion is now converted to forward motion and the water rushes up the beach. This is called the SWASH of the wave. The water rushes up the beach and then with much less energy drains back down the slope. This is called the BACKWASH of the wave.

If there are not many waves per unit time each wave will complete its cycle of swash and backwash without interference from the next wave. Material pushed up the beach by the swash will be deposited up shore, the backwash will drain away. The next wave will break and its swash will deposit material unimpeded by the backwash of the preceding wave. Hence there will be a net build up of material. The difference between constructive and destructive waves is their frequency.


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