Drifting Pebbles

Last week we learnt about longshore drift, and a simple method for finding out its direction on the day of your fieldtrip.

Longshore drift constantly shuffles beach material.

This is Chesil Beach in Dorset.

The beach material is clearly sorted, from small size at the west end…

…to large size at the east end.

(Zoom in on the photos to compare them – if I crop them any more you will lose the sense of scale.)

Longshore drift is usually from west to east on this part of our coast, as the most powerful waves tend to come from the south-west, causing all the beach material to move eastwards.  Less frequently the swash comes from the other direction, and longshore drift goes east to west, but waves from this direction are usually less powerful – only the smaller pebbles get shifted back to the west again.

Here’s another method for looking at longshore drift but this one needs a bit of preparation.  You will need to collect some pebbles, preferably a variety of different sizes, and paint them in a bright colour.  (Don’t use water soluble paint!)

At the beach, put your painted pebbles into the water, all at the same place.  Best to do this as the tide is coming in.  Otherwise they may not move very much before being left high and dry.  Leave them as long as you like, the longer the better.  Just make sure that you know where they started from.  Hopefully you will be able to find some of them at the end of your experiment.

  • Did they move along the beach?
  • Did they all move in the same direction along the beach?
  • Which moved further, bigger or smaller?