How To: Understand Waves for Swimmers – Part 1

If you want to swim Open Water more safely at the coast it’s a good idea to develop some understanding of various aspects of your environment. This is  a thumbnail sketch of breaking coastal waves for swimmers.

1: Waves are mostly caused by wind, either close or far-away.

2: There are many days without waves.

3: But new wave-fronts can arrive seemingly suddenly, regardless of local weather conditions.

4: It’s vitally important to have a general understanding of different waves.

When wind blows across water, it causes turbulence. The longer timer period and greater distance the wind blows over, causes greater turbulence, which leads to waves. The distance that the wind blows over is called the Fetch. An easterly wind blowing across Tramore Bay is only blowing over a fetch of five kilometers, yet can still cause very choppy water at the Guillamene, but this is a very short fetch. A long fetch may be hundreds of kilometers long in big storms in the Caribbean or Arctic, or, in the case of the Pacific, up around the Aleutian islands, leading to huge waves thousands of kilometers away in Hawaii.

Waves caused by close winds in the immediate area will be messy and disorganised, and will be close together. (A short period of much less than 10 seconds between them). this makes them more difficult to get past.

Waves caused by winds far away will have longer gaps between them and will be more potentially dangerous.

In Ireland that period can regularly be 13 to 15 seconds (and could in rare circumstances get up to 20/21 secs). These are called groundswell waves or just groundswell. Without a good understanding, groundswell looks more regular, but can be much more dangerous and unpredictable. Groundswell will generally be bigger and more powerful. As one wave catches up with a sightly different slower speed wave, their height will combine to result in a bigger, faster, more powerful wave.

A five meter wave catching up with a four metre wave will result in a nine metre wave. Groundswell gets more powerful the further it travels (the “fetch”) and the greater distance the wind travels to cause it. Waves hitting Ireland in the spring or autumn from a storm in the Caribbean will have a “fetch” of two and half thousand miles and can be very large and powerful. (I’ve seen an 11 metres swell on the West Coast back in 2002/3. It looked like mountains moving out at sea.) Note: –

If the wind is blowing to land it’s called “on-shore”. This will make the waves messy regardless of their origin. They will “close-out”, that is they will break all along the wave at the same time, and have lots of foam and whitewater in from of the wave.

Note: –

Wind blowing from land to sea is “off-shore”, the conditions sought by surfers. The south of Ireland is unfortunately prevailing on-shore southerly winds, while the north of Ireland is prevailing off-shore. Off-shore winds will cause any wave to “clean-up”. The waves will break gradually from left to right or from right to left, depending on the sea-floor under them. The face of the wave will be unbroken green or grey (for Ireland) water until it breaks. If the off-shore in on a groundswell, these will combine to give the biggest and most powerful waves.

Probably my own favourite wave location ever

Part two.


3 thoughts on “How To: Understand Waves for Swimmers – Part 1

  1. Pingback: How To: Understand the Different Types of Open Water Swimming Locations Features and Hazards – I – The Ocean | LoneSwimmer

  2. Pingback: 11 Things To Hate About Open Water Swimming (And What To Do About Them) | LoneSwimmer

  3. Pingback: How To: Understand Waves for Swimmers, Part 2 | LoneSwimmer

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