How Waves Form
Waves are generated by wind blowing over the sea surface. The wind disturbs the water, creating ripples that grow into waves as energy is transferred.
Factors affecting wave size:
| Factor | Effect |
|---|---|
| Wind speed | Faster wind → larger waves |
| Wind duration | Longer wind → more energy transferred |
| Fetch | Greater fetch → larger waves |
Fetch is the length of open water over which wind has blown. The Atlantic coast of Ireland has a huge fetch (open ocean to the west), so waves are large. The North Sea has a smaller fetch, so waves are generally smaller.
Parts of a Wave
- Crest: top of the wave
- Trough: bottom of the wave
- Wavelength: distance between two crests
- Wave height: vertical distance from trough to crest
Water particles in a wave move in circles — they do not travel forward with the wave. Only the energy travels forward; the water stays roughly in place until the wave breaks.
Breaking Waves
As a wave approaches shore and the water gets shallower, the base of the wave slows down (friction with the seabed) while the top keeps moving. The wave steepens and eventually breaks, water rushes up the beach (swash) and then returns down the beach (backwash).
Constructive and Destructive Waves
| Feature | Constructive | Destructive |
|---|---|---|
| Wave height | Low (< 1 m) | High (> 1 m) |
| Wavelength | Long | Short |
| Frequency | ~6–8 per min | ~10–14 per min |
| Swash vs backwash | Strong swash | Strong backwash |
| Effect on coast | Deposits sand → builds beach | Erodes coast |
| Common location | Gentle, low-energy coastlines | Exposed, high-energy coasts |
Constructive waves spill up the beach gradually and deposit more material than they remove — beaches are gradually built up. Destructive waves plunge steeply, have a powerful backwash, and remove more material than is deposited — beaches and cliffs are eroded.
Tsunamis are not caused by wind. They form when an underwater earthquake, volcanic eruption, or landslide displaces a large volume of water. In open ocean a tsunami may be only 1 m high but travels at 800 km/h. It slows and grows (up to 30 m+) as it approaches shallow water.
Real world: Surfing waves in Cornwall and Ireland
The southwest coasts of Cornwall (England) and Ireland receive some of Europe's largest waves because of their enormous Atlantic fetch — wind blows thousands of kilometres of open ocean before hitting land. Newquay in Cornwall hosts international surfing competitions with waves regularly over 3–4 metres. Wave energy off the West of Ireland can be so powerful it has moved boulders weighing several tonnes up cliffs — a process called "cliff top boulder emplacement" that scientists are still studying.
Real world: The 2004 Indian Ocean tsunami
On 26 December 2004, a magnitude 9.1 earthquake off Sumatra displaced the seabed and generated a tsunami. In open ocean, the waves were less than a metre high — ships sailed over them without noticing. But as the waves reached shallow coastal waters around the Indian Ocean, they slowed and grew to heights of 10–30 metres, killing 230,000 people across 14 countries. This was the deadliest tsunami in recorded history and led to the creation of the Indian Ocean Tsunami Warning System.
Real world: Wave energy as a power source
Engineers are developing machines that extract electrical energy directly from ocean waves. The Wave Energy Converter (WEC) devices deployed off the coast of Portugal and Scotland convert the up-and-down motion of waves into electricity. Wave energy is renewable and predictable (unlike solar and wind). Portugal's Agucadoura wave farm was the world's first commercial wave energy farm when it opened in 2008 — a direct application of wave physics studied in geography.
Where do you see this in real life? The next time you watch a surf competition or see a beach holiday photo, the wave height and power you see reflects the fetch, wind speed, and wave type described here. Big winter swells come from Atlantic storms far out at sea.