As lockdown eases and summer draws holiday makers to the beach, many people will enjoy the sea, whether paddling in the waves or being more adventurous and taking to surf and body boards. We take these constant but etherial phenomena for granted, but have you ever thought what causes waves and why conditions can be so different from day to day, or even hour to hour?
Ocean waves are generated by the wind. When the wind blows across the water surface, the friction between the wind and the surface ruffles the surface of the sea. This causes little, ‘capillary’ waves to be created. If the wind continues to blow on these little undulations, they get bigger.
There are three factors that are needed for big waves to be created: You need a strong wind, blowing for a long time, over a long distance. The requirement for a long fetch (the distance upon which the wind blows) and no obstacles is why the biggest waves are generated in the open ocean.
Waves can travel long distances and so may end up thousands of miles from the original storm that formed them. Scientific studies have tracked waves that were started off the coast of New Zealand all the way to the coast of California.
There are two fundamentally different types of wave in the sea. There are wind-driven waves, which are the ones generated, grown and pushed along by the wind. Once they have left the storm area, they organise themselves into swell waves. So wave is the general term and swell is a specific term, meaning a wave generated by distant winds.
The swell waves propagate away from the generating area and travel until they reach shallower water. So even if it’s a clam day, your local beach might still have large swell waves that have come from a distant storm.
Waves travel about 20 to 30 miles an hour in the open ocean. So big waves can take 2 to 3 days to get to Cornwall, by which time conditions at the coast might be calm and sunny. This is one of the most misunderstood things about waves – you can still have big waves even when local winds are calm!
Types of breaker
When waves come into shallower water, they break. The wave reaches a point when the crest is travelling faster than the trough (due to friction against the beach), so the wave becomes unstable and breaks. There are two types of breaker: a spilling breaker and a plunging breaker.
A spilling breaker is a gentle breaking wave and occurs on flat, gently sloping beaches such as Polzeath. Spilling breakers topple over gently and spill shorewards, often accompanied by an on-shore wind. These waves are popular with learner surfers.
The converse is the plunging breaker, which breaks in a more violent way, when the transition in seabed from steep to shallow is quicker.
The plunging breaker curls over at the top and forms a tube that good surfers can ride in. The ideal combination is a steep slope for the wave to break on together with an off-shore wind that helps hold the wave up, delaying it break. On UK beaches, this often occurs at low tide because the low water mark beaches tend to drop off an make a steeper slope. You can see this on the popular Fistral beach.
Various factors on individual beaches will affect waves, including the shape of the beach, the sea bottom and other obstructions such as islands or curved headlands. In the summer, smaller waves bring in sand, creating sandbanks that waves break around. This is good for surfing, but the rip currents that develop between the sandbanks are hazardous for bathers and learner surfers.
A ‘rogue’ wave occurs when waves that are travelling in different directions come together and combine their heights into one larger wave. This sudden increase in size can be hazardous for water users out at sea.
Closer to shore, waves typically arrive in sets (a group of a few large waves, one after another) every few minutes, with a few smaller wave in between. But a particularly large set is often responsible for washing anglers and coastal walkers from rocks into the sea.
In the past, forecasting waves required analysis of weather charts, tide, wind and local conditions. Now there are forecasting apps just a click away. These include magicseaweed.com and windy.com.