Most waves are formed by wind blowing across the surface of the sea or other large body of water. The friction between the wind and the surface of the water causes the water to move in a circular motion. Although the wave moves forward, the water does not move with it. Each particle of water simply moves around in a circular motion. As the wave moves into shallow water, it breaks. When the depth of the sea is less than the height of the wave, friction with the bottom increases and the wave slows down. The wavelength becomes shorter and the front of the wave becomes steeper. The top of the wave then tumbles forward. When this happens, the wave is said to break. This is the only time water is thrown forward by the wave.
When the wave breaks, the water which is thrown onto the beach is called the swash. The water which drains back into the sea is called the backwash. Waves which have a swash which is stronger than their backwash are called constructive waves. They deposit material onto the shore. Waves which have a backwash which is stronger than their swash are called destructive waves. They erode the coastline. Watch the video below.
Waves are powerful agents of erosion. Waves erode the coast in ways which are similar to the ways which rivers erode their bed and banks. Waves erode in the following ways:
- Hydraulic Action: Waves can hit the coastline with a great deal of force. The force with which waves hit the coastline is often enough to cause erosion.
- Abrasion/ corrasion: The material carried by the wave is thrown against the coast, thereby eroding it.
- Attrition: Small stones and other material carried by the wave are thrown against each other. They are further broken down and worn into a round shape.
- Solution: Sea water dissolves soluble minerals in the rocks on the coastline.