Waves are formed when the wind blows across the surface of the sea. The friction between the wind and the water pushes the water up creating waves.
The height and power of a wave depends on two factors:
- The distance it has had to travel across open water to reach the coastline
- The wind speed
The distance over which the wind has blown is called the fetch.
The greater the fetch, the larger the wave as it has more time to gather energy.
Example: Waves from Brazil travel 10,000km before they hit the south west coast of England making them very powerful.
Constructive waves have three main features:
- They are low in proportion to their length normally less than one metre high
- They have a strong swash which carries material up the beach and have a weak backwash which doesn’t take the material away
- They break gently, with only six and nine waves per minute.
Constructive waves allow material to be deposited along the coastline as material is slowly but constantly moved up the beach. This leads to the formation of features such as spits, tombolos and bars.
Destructive waves have three main features:
- They are high in proportion to their length
- The backwash is much stronger than the swash so that rocks, pebbles and sand are carried back to the sea
- They are frequent waves, breaking at an average rate of between eleven and fifteen per minute
Destructive waves erode the coastline and help to form features such as wave cut platforms, headlands, bays, arches, stacks, caves and stumps.
The video below explains contructive and destructive waves in more detail.