What are Groynes made of?

When you have visited the seaside, you may have seen what look like garden fences running straight down the beach into the sea, and then coming to a halt. But they do not seem to be fencing anything off, do they? These barriers are called grayness. Let’s know more about what are groynes.

If you look more closely at them you will probably find that they are covered with various types of seaweeds and barnacles (small limpet-like shells but with a hole in the top). More important than this, however, you should be able to see that the sand or shingle has built up higher on one side of the   groyne that the other.

In fact, on the south coast of Britain, it is very likely that the sand will be higher on the western side, and on the east coast it will probably be higher in the north of the groyne. Why should this be happening? Andy, why are the groynes there in the first place?

What are groynes

Groynes prevent sand and shingle from being transported very far by the action of longshore drift. Longshore drift may cause a spit to form at an estuary and salt marshes then develop behind it.

We have already explained how waves are formed by the wind and how these waves break when they reach the shore. Although it is only  the wave shape that  moves in  the open sea,  on the shore the breaking  waves of water tend to have a motion shore the breaking waves of water  tend to have a motion  shoreward’s.

This motion is called the swash. But as you will have seen, after the wash up the beach, the water runs back again this is called the backwash. If  the force of  the swash  is greater than the  force of the  backwash, the  waves  are called  constructive, and the  sand, mud, and shingle that may  be carried with the wave tends  to be moved in  the  direction of the  wave.

When the reverse occurs the waves are called destructive. Generally, constructive waves are formed in quiet weather conditions and destructive waves during storms.

What are groynes

When the amount of sediment that is being carried by a river is greater than the amount that can be removed by longshore drift and other currents, a delta may form. The inset shows how the sediments in a delta build up.

Generally, the waves do not come straight up the beach in Britain. This is because the wind most commonly blows from the west, that is, the prevailing winds are the westerlies. Therefore, on the south coast, for example, the waves travel up the beach and along from west to east.

The sand and shingle are carried diagonally up the beach by the swash and directly down the beach b the backwash. This means that sand and shingle tend to move from west to east. What would happen if this were allowed to carry on unchecked?

Of course, the beaches in the west would be washed away and those in the east built up. To prevent this groynes are erected and the sand builds up on one side. Sometimes, the movement of shingle and sand may build up material into bars and spits. This is called longshore drift, and a good example of this is Chesil Beach near Weymouth in Dorset, England.

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