When Waves arrive at the beach at an angle to the shore line, sand will be pushed up the beach in that direction however in the backwash the sand will usually moved directly down the beach, perpendicular to the shore line. So the sand follows a zigzag pattern. The result is a net movement of sand along the beach. An enormous amount of sand can moved in this manner.
By looking at new sand bars you can determine the longshore drift. If the sand bar is attached to the beach at its northern end the longshore drift is to the south. Conversely if the sandbar is attached to the beach at its southern end the longshore drift is to the north.
Longshore drift can fluctuate from day to day, and may vary locally as the beach tries to readjust its shape to be in better equilibrium with the waves and currents.
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