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USGS Geology in the Parks


Beaches: Sand on the move!

Description of longshore currents and transport

Breaking waves disintegrate into churning sheets of water called swash that carry sand and gravel up onto the beach. Each wave brings new sediment and carries some sediment back into the surf zone. If more sediment is deposited by waves than is carried away, a beach builds up. If more sediment is removed than is deposited, the beach erodes.

A section of the Pacific Coast with waves
Waves from the Pacific Ocean breaking on the shore with bluffs.
A section of the Pacific Coast with waves
Labels illustrating the effect of a longshore current.

Longshore drift

  1. Wind blows toward the shore, creating waves
  2. Waves move sediment towards the shore at an angle (oblique)
  3. Backwash (retreating waves) carry sediment away from beach at 90°, perpendicular to the shore
  4. The effect of these processes is longshore transport (beach drift) of sediment parallel to shore in the general direction of the prevailing winds
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