Waves do much of the work of marine processes. They erode, transport and deposit materials. Waves are created by winds as they blow over the surface of the sea. It is the friction between the wind and water that sets waves in motion. The strength of waves depends on the strength of the wind depends on the strength of the wind. It also depends on the length of time and distance over which the wind has been blowing (the fetch).
As waves near the coast, they enter into shallower water. Friction with the sea bed causes the wave to tip forward so that it eventually breaks. The resulting forward movement of the water, called the swash, runs up the beach until it runs out of energy. The water then runs back down the beach under gravity. This is called the backwash.
The balance between the swash and the backwash of waves created the difference between constructive and destructive waves. In constructive waves, the swash is stronger than the backwash. As a result, material is moved up the beach and much is left there (deposition). In destructive waves, the backwash is stronger. Material is dragged back down the beach (erosion) and moved along the coast by Long Shore Drift (LSD).
Processes of Erosion
It is destructive waves that do much of the erosion along a coast. They cut away at the coastline in a number of different ways:
- Hydraulic action – this results from the force of the waves hitting the cliffs and forcing pockets of air into cracks and crevices.
- Abrasion – this is caused by waves picking up stones and hurling them at cliffs and thus wearing the cliff away.
- Corrosion – the dissolving of rocks by the sea water.
Attrition is a process whereby the material carried by the waves becomes rounded and smaller over time as it collides with other material. It does not erode the coast as such but does form small pebbles and sand.
Long Shore Drift (LSD)
Once rocks are detached from the cliff, waves can move them along the coastline for quite long distances, this process is known as longshore drift. Generally speaking, the smaller the material, the further it is likely to be moved by waves as it is lighter. Eventually, the waves are unable to move so much materials and the materials will be deposited to create new land forms.
There are three main processes at work on the landward side of the coastline:
- Weathering – the breakdown of rocks which is caused by freeze-thaw and the growth of salt crystals, by acid rain and by the growth of vegetation roots
- Erosion – the weathering away of rocks by wind and rain
- Mass Movement – the removal of cliff-face material under the influence of gravity in the form of rock falls, slumping and landslides.