How To: Understand Waves for Swimmers – Part 2

Part One.

Waves break when the bottom of the wave touches the sea floor or an obstruction, such as a reef , beach or sandbar and is then slowed down. The top of the wave is still moving

faster and the wave breaks, with the top crashing forward at greater speed.

Because waves moves at different speeds, surfers particularly talk of “sets”. “Sets” or “Set waves” are infrequent and much bigger, faster and more powerful waves that arrive at different times during any particular period of waves.
Set waves can arrive singly, or in groups ( two or three set waves following each other). There is no fixed pattern. Stories of “the seventh” or “the ninth wave” being the biggest are just stories. It can be every 3rd, 4th, 5th wave, etc. I’ve experienced times between set waves, (not between the other ordinary waves) up to 15 minutes.

Because of their size and speed, set waves can move faster, have more power and break further out.
When people on the shore are regularly described as having been swept out to sea and often drowned by “freak waves”, what is usually meant is a set wave, which was bigger and more powerful than the preceding waves, able to sweep up over previously uncovered land, and pull people back out to sea when they ebb.

There is a surfer’s saying; “watch the waves for twice as long as they are high”. In other words if the front of the wave looks about 4 feet high, watch the water for 8 minutes. But unless you are experienced you will almost definitely under-estimate the size of the wave front.

If you look at a weather forecast and it says wave height of 3 metres, that means the height difference between the trough and the peak, in open water, is 3 metres. It does NOT mean the waves will be 3 metres high.

The water from waves breaking on a beach must go back out to sea. This can happen two ways: An Undertow: or a Channel.


Undertows occur mainly on steep beaches with destructive waves and the water goes down underneath the incoming waves. Undertows are very dangerous but luckily not too common.
A Channel occurs when there is one place where the water can flow back out to sea through the incoming waves. It can usually be seen with a bit of practice because the waves break less at this point. Channels are usually narrow, often only about 10 metres wide or less. This is the point surfers and experienced sea swimmers use to get further out.

However, if the wind is onshore and the beach is fairly flat running out a good distance, breaking waves can occur out for a long distance. In Tramore Bay, when the swell is 2 or 3 metres (significant but not huge), breaking waves can stretch the whole way out to the mouth of the bay, about 3 miles out, because the bottom is flat and the depth drops only very slowly.

Beyond the breaking waves is called “out back”, again in surfing terms.

If you can see however that the breaking waves only stretch out a few hundred metres, and know you are capable of this, then the best way out is find the Channel, follow it out, being aware that you may being also pushed to either side of the Channel. Waves will still break in front of you and the easiest wave through them is to dive under just before the wave reaches you. The wave will pass quickly over you and allow you to pop up behind it in calmer water. Always be careful though, and immediately check where the next wave is. If the wave period is only three or four seconds the next will be upon you almost immediately. Honestly, you probably shouldn’t be swimming in these conditions.

Finally, before getting in in wavy conditions make sure that you can get out, know where to get out and how to recognise the spot, from the sea, in rough conditions, and that you know how to get out.

At my local swimming spot, because it is a ladder entry, it’s easy to get in, but if there is more than small swell it is difficult or impossible to get out at the same place.

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One thought on “How To: Understand Waves for Swimmers – Part 2

  1. Pingback: How To: Understand Waves for Swimmers – Part 1 | LoneSwimmer

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