The power of waves is one of the most important forces that changes the shape of the coast. Waves are created by wind blowing over the surface of the sea.
The size of a wave depends on:
When a wave breaks, water is washed up the beach. This is called the swash. Then the water runs back down the beach, which is called the backwash. With a constructive wave, the swash is stronger than the backwash. With a destructive wave, the backwash is stronger than the swash.
The table below outlines the key differences between the two types of wave.
|Beach shape caused by this type of wave||Wide and flat||Steep and narrow|
|Frequency||Low (6-8 per minute)||High (10-14 per minute)|
If the swash is stronger than the backwash (constructive wave), some of the sediment carried in the wave will be left behind to build up the beach. This means that the beach increases in size.
If the swash is weaker than the backwash (destructive wave), very little sediment is carried up the beach. With a strong backwash, material will be removed and the beach will decrease in size.