Longshore drift

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Diagram demonstrating longshore drift
3=longshore current direction
4=incoming waves

Longshore drift is the process of the ocean currents forcing sand and other material down a beach.

The material is first pulled into the currents by the backwash (the water going out to sea) and is then pushed back up the beach by the swash (the water going onto the beach). Longshore drift always moves in the direction of the main wind. If longshore drift continues for a long time, beaches can be changed quite a bit. The changes depend on the details, and vary from place to place.

  • The waves carry the rock material up the beach at an angle. (waves move at an angle – wind)
  • The backwash carries the material directly down (gravity) into the sea or it stays there.
  • This process goes again.

There are many inexpensive and effective ways of controlling longshore drift. The most common method is groynes (wooden walls that the sand builds up against). Some people dislike the groynes because they are not very nice to look at.

Longshore drift is the net (average) movement of sand grains across a beach in a zig-zag motion.