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The extent to which the shape of a beach or coast is altered depends largely on the action of waves upon it. Waves can be gentle and infrequent or larger, more frequent and more powerful.
The formation of waves and their size and shape is a result of the exchange of energy from wind blowing over the sea. The longer the wind blows for, and the greater the distance it blows over, the larger the waves that result, and the greater their energy.
Other factors include:
- Wind strength.
- Time wind blows for.
- Distance (fetch).
In the UK, the direction of maximum fetch is from the South West (for example, if you stand at Lands End, your nearest land mass is the USA) this is why the Cornish Coastline can experience huge high-energy waves.
These are depositional waves as they lead to sediment build up, and are most common where a large fetch exists. They tend to have a low gradient, a larger swash than backwash, low energy and an elliptical orbit. The wave period is long, with 6-8 waves breaking in a minute.
These act as agents of erosion, because backwash is greater than swash. They are most common where fetch is short, have a mainly circular orbit, a steep gradient, and 'plunge' onto the beach. The wave period is short, with 12-14 waves breaking per minute.
Wave fetch: The distance of open water over which a wave has passed. Maximum fetch is the distance from one coastline to the next landmass, it often coincides with prevailing wind direction (South West in the UK).
Wave crest: Highest point of a wave.
Wave trough: Lowest point of a wave.
Wave height: Distance between trough and crest.
Wave length: Distance between one crest/trough and the next.
Swash: Water movement up a beach.
Backwash: Water movement down a beach.
It is very rare for waves to approach a regular uniform coastline, as most have a variety of bays, beaches and headlands.
Because of these features, the depth of water around a coast varies and as a wave approaches a coast its progress is modified due to friction from the seabed, halting the motion of waves.
As waves approach a coast they are refracted so that their energy is concentrated around headlands but reduced around bays. Waves then tend to approach coastline parallel to it, and their energy decreases as water depth decreases.
The process of refraction is outlined below: