Longshore Drift

When a wave comes in, breaks and flows up the beach towards you, the movement is known as “swash”.  The water then drains back down the beach, which is called “backwash”.  The backwash always drains down the slope of the beach, but the swash can come from any direction, depending on the direction of the wind that formed the waves.

If the swash comes in at an angle and the backwash goes straight back, beach material gets gradually shifted along the beach.

To see the direction of longshore drift you need something that you can put into the water and find again easily.

One way to do this is to use an orange.  It will float so you will be able to keep it in sight.  Follow the backwash out and put the orange down as the next wave comes in.  Keep an eye on its movement for 10 minutes.  (Make sure you don’t lose the position that you started from.)  After 10 minutes, if it is safe to rescue your orange, do so.  You can then repeat, starting at a different place on the beach, and see if you get the same result.

  • Is longshore drift happening at your beach?
  • Is it moving in the same direction at each end of the beach?

If your beach is in a bay then the material could be coming into the bay from both directions, as in this picture.

If you are standing on the beach in the foreground, facing the sea, the swash is coming from your right.  If you are on the other side of the bay, facing the sea, the swash is coming from your left.  Material is being moved towards the centre of the bay from both ends, building a nice big beach.