Longshore drift is the name given to the process by which beach material is transported along the coast by the action of waves. Waves rarely hit the beach at exactly right angles to the coast, and are far more likely to hit the beach at an angle. This is because in many areas the prevailing wind controls the direction of the waves and, obviously, very few long sections of coast are dead straight for miles and miles.
As waves approach a beach the base of the wave hits the sea bed. This is what causes it to topple forward and to 'break' but it also allows the wave to pick up sediment. The size of the sediment particles moved by the wave is determined by what is available on the sea bed, and by the power of the wave. More powerful waves can move heavier sediment particles.
If the wave was to hit the beach at exactly right angles to the line of the coast, the water would go straight up the beach (swash) and straight back down again (backwash). The only movement of the sediment would be up the beach with the swash, and back down the beach with the backwash.
When waves break on to a beach at an angle, material is pushed up the beach at the same angle by the swash, but pulled back down the beach by the backwash at ninety degrees to the coast. The sediment is moved across the beach as well as up it. When the wave runs out of energy the water starts to flow back towards the sea. Gravity pulls it straight down the beach, so the returning water follows a different path to the one it followed on the way up it. Each wave can move the sediment a little further across the beach.
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Groynes are effective at trapping material as it is moved along along the coast by longshore drift. Groynes are typically made from sturdy wooden bars, similar to railway sleepers, and have a life expectancy of around 30 years, although their actual lifespan can vary enormously according to their location and the type of material found on the beach. |
The photograph shows a series of wooden groynes stretching down a sand and shingle beach. You can see that this side of the nearest groyne has plenty of sediment built up against it. The sediment includes large items like pebbles and shingle. On the other side of the groyne the beach level is lower because sediment has been caught on this side and cannot move further along the beach. On many beaches you can find height differences of several meters on different sides of a groyne. Many a beach walker has had a nasty surprise when he leaps over a low groyne and finds a 2 meter drop the other side! |
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This flash animation shows how waves breaking at an angle up a beach can move an object along it in a series of moves. The rubber duck in the animation is moved by the waves as they move up the beach and back to the sea again. On this coast the waves come into the beach from the left of the picture, and slowly move sediment (and rubber ducks) towards the right end of the beach. |
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