The importance of waves
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:
- length of time the wind has been blowing
- strength of the wind
- 'fetch' - how far the wind has travelled
Wave types
Waves can be constructive waveWaves which help build up material on the beach. They tend to have a low frequency and strong swash. or destructive waveWaves which remove material from the beach. The swash is weaker than the backwash..
When a wave breaks, water is washed up the beach. This is called the swashThe water flowing towards a beach when a wave breaks. . Then the water runs back down the beach, which is called the backwashThe movement of water down the beach.. 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.
| Characteristic | Constructive | Destructive |
| Energy | Low | High |
| Swash | Strong | Weak |
| Backwash | Weak | Strong |
| Wave height | Low | High |
| 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) |
| Characteristic | Energy |
|---|---|
| Constructive | Low |
| Destructive | High |
| Characteristic | Swash |
|---|---|
| Constructive | Strong |
| Destructive | Weak |
| Characteristic | Backwash |
|---|---|
| Constructive | Weak |
| Destructive | Strong |
| Characteristic | Wave height |
|---|---|
| Constructive | Low |
| Destructive | High |
| Characteristic | Beach shape caused by this type of wave |
|---|---|
| Constructive | Wide and flat |
| Destructive | Steep and narrow |
| Characteristic | Frequency |
|---|---|
| Constructive | Low (6-8 per minute) |
| Destructive | 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.