Waves are a primary agent in processes which shape the coast.
Wave characteristics include:
- Wave height: vertical distance between crest and trough
- Wave length/amplitude: distance between 2 successive crests
- Wave frequency/period: time for one wave to travel the distance of one wavelength, or time between one crest and the following crest passing a fixed point
- Wave velocity: speed of movement of crest in a given time period
- Wave steepness: ratio of wave height to wave length – at a ratio of 1:7 (1/7), the wave will break so steepness does not exceed this point
Formation of waves
In the sea, waves tend to move with a circular orbit near the surface. As the wave moves in towards shallow water, more friction occurs with the seabed, so the base of the wave moves down. The crest rises as it moves forward, decreasing the velocity and wavelength. The orbit becomes more elliptical as a result of this pattern. The wave keeps steepening until it reaches ratio of 1:7, at which point it breaks; the upper part plunges. Some water rushes up the beach, and this is called the swash. Water which then runs back down the beach is the backwash.
Waves formed by distant storms and travelling long distances are swell waves (similar to constructive):
- Low height in relation to length
- Gentle steepness
- Long length
- Long period
Waves from local winds and having travelled short distances are sea waves (similar to destructive):
- High height
- Steep
- Short length
- Short period
Constructive waves:
- Low wave height
- Low frequency of 6-8/minute
- Wave front steepens slowly, leading to a gentle spill
- Material is slowly moved up the beach leading to ridge formations – berms
- Weak backwash – insufficient force to erode beach
- Long wavelength of up to 100m
- Beach has a more gentle profile
Destructive waves:
- High wave height with steep form
- High frequency of 10-14/minute
- Powerful swash inhibiting swash of next wave
- Rapidly steepens in approaching the beach, vigorous plunge when they break
- Weak swash so little material is moved up the beach
- Erosive power so steeper beach profiles formed
- Forms storm beaches
The cycle between constructive and destructive waves is an example of negative feedback. Constructive waves build up the beach due to strong swash. This then encourages waves to become more destructive – they move material back towards the sea, reducing beach angle and therefore encouraging more constructive waves. This maintains the state of dynamic equilibrium. However, it often does not happen exactly like this because of other factors.