Tag Archives: waves

How waves can interfere with swimmers and cut down on their speeds

This phrase is a consistent Google autocorrect search term that bring people to the site so I thought I would use it directly.

Surf at Praia Grande. Porto Covo, Portugal

I’ve previously written a couple of posts on understanding waves, theory and some practical.

Writing recently about the 2010 eight hour pool-training swim followed by a sea swim, I was reminded of the problems waves present for many swimmers.

As we’ve seen in the previous articles waves occur where an open ocean swell meets where water gets shallow, on beaches, reefs, and rocks. Waves are somewhat unpredictable even in good conditions and care must be taken of them. So entering the water in the presence of waves requires some degree of caution, dependent on wave size. Trying to exit on rocks or reefs, in even small waves, is fraught with danger.

So why do waves present such difficulty? It’s simply because water is dense, denser than a human, and heavy and anything heavy has a lot of inertia. Difficult to start, divert or stop.

Everyone has probably stood on a beach in waist high waves and felt how easily the waves can push one around.  One cubic metre ( 1 metre x 1 metre x 1 metre, a fraction of a whole chest high wave) of water weighs one thousand kilograms. Did you ever try pushing against even a small car weighing the same? You are not as powerful as water, a six foot tall man is weaker than a five foot tall wave.

Children learn to jump as the waves approaches to go over the top, or to jump into the wave and let it take them, or to stand with one foot and chest forward to try to hold their position. These are all approaches to the mass of the wave and all and more can be used by swimmers.

Rob Dumouchel shared the video below with me, which perfectly illustrates the problems faced by swimmers unfamiliar with waves.

I hope you noticed the guy on the left at the start, who disappeared pretty quickly. He knew what to do. Instead of standing around like a scared duckling, trying to progress by hopping forward and getting pushed backward, he went under the waves.

Power within a wave is concentrated when it is breaking in the crashing top of the wave. Waves breaking into shallow water, even without being large, will travel fast and slow movement with a lot of lower density white water being pushed ahead.

The water in front of a wave is sucked up into the wave face, while the wave is moving forward so you may get a quick sensation of speed just before the wave hits. You can use this speed to your advantage to get under the wave. Just duck down and forward under the wave and then up and you will pop out well behind the wave lip and past most of the drag of the breaking water.

Remember that water being dumped on beaches by waves needs to escape back outward, so most beaches will have “channels” (some steep beaches will  instead have dangerous undertow).

The trough in front of a wave is lower than the average height, whereas the water behind a wave lip is higher. So if you plunge into a wave face and exit behind, you will be higher up, but if you come up just behind the lip of a crashing wave, you have to be careful not to get dragged back over the edge, “going over the falls”, though is generally not a problem unless you are very close.

In this image of Annestown beach, though the waves are only waist-high, one can see that the shingle isn’t all the same height, some is banked. The areas between the banks are more likely to be deeper, and more likely to be channels as this trough extends outward. The difference will usually look somewhat subtle, but is pretty consistent. If you notice in the image, where the arrow starts, the sand extends further into the shingle as this is a lower trough and this recurs along the beach, so there is actually more than one channel, more visible the more water is trying to escape. However Channels tend to exist closer to the beach and as you escape beyond the initial whitewater, the effect will dissipate.

Wave water escape channel at Annestown beach
  • Don’t panic. As I have said before, there is no situation made better by panic and most will be made worse, especially at sea.
  • Don’t try to get away from waves. You won’t win. Face them and work with what they are doing.
  • Look for channels, the narrow and usually deeper areas where waves aren’t breaking, where the incoming water has to escape back out to sea. That’s your easiest way out. But once in a Channel, don’t try to swim back in against it.
  • In water where you can walk, angle your body sideways to oncoming whitewater, and brace yourself as you move outwards, moving out in the intervals between the wave fronts.
  • Once you reach chest deep water, if you are over sand, it becomes harder to progress by walking even with no waves, so get swimming.
  • The best approach when going out from a beach is to dive under the oncoming waves.
  • Don’t take a huge intake of air, it’ll be harder to submerge. Instead hold the air into your lungs instead of trying to hold a mouthful. Popping under and behind a big wave is a pretty quick task.
  • Don’t try the same thing with waves breaking over rocks. Because idiocy.
  • Swimming against a rip current is a poor decision. Change your angle by 45 to 90° and you will quickly move out of it.
  • As you progress out pass the breaking waves, triangulate your position so you know where you started, might need to finish. Line up two objects, one of front of the other, a house and tree or similar, and you will be able to tell your position along a beach. otherwise you can be 100 metres to either side and it will still look like the same place.
So the simple answer to the initial question, which may be the subject of someone’s homework, (it wouldn’t be the first time, people sometimes include question numbers), is that waves interfere with swimmers by stopping them getting out deeper, by pushing them back into shore, by knocking them over, by pulling their legs from beneath them and by breaking over them. All these problems can be reduced or eliminated with experience and practice.

Related Articles:

Waves for swimmers, Part 1 (loneswimmer.com)

Waves for swimmers, Part 2 (loneswimmer.com)

Exploring freak waves (loneswimmer.com)

Grid waves (loneswimmer.com)

Tides for swimmers, theory (loneswimmer.com)

Tides for swimmers, local effects (loneswimmer.com)

I want to do this

You might recall how I talked about the intense underwater smell associated with plankton blooms in the springtime. And I’ve swam at night and once or twice even surfed at night and love the experience particularly of night swimming, as I’ve previously said. (The real difficulty with night surfing is to see the approaching waves).

The following video is of a red tide in San Diego last week, a plankton bloom, which caused a more intense bio-luminescence in the water than any I’ve ever seen, and is a stunning video of night surfing luminescent waves.

HOW TO: Understanding 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.

HOW TO: Understanding 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.