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)

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4 thoughts on “How waves can interfere with swimmers and cut down on their speeds”

  1. Nice write-up. As a surfer you learn these tips very quickly, especially bringing a big chunk of fiberglass out with you. Great vid too, I find diving under and staying horizontal works pretty consistently. Confidence and experience is what it’s all about.

    1. True,as a surfer of many years before I realised by swimming the sea almost never stopped me doing what I wanted, I learned a lot of skills from surfing, but for swimmers who’ve never surfed the duck dive is mysterious, it’s hard to remember but it doesn’t doesn’t naturally or easily to most surfers either as I recall.

  2. Good advice on diving under the lip just before they crash. I learned an extreme form of this while body surfing in La Jolla a few years back. While waiting for waves in the line-up, I noticed one of the body surfers coming back out after a ride. After one particularly large wave went by me I lost sight of the body surfer until he exploded out of the back of the wave like a dolphin…all but his ankles were fully out of the water and then he splashed down and swam up next to me. It was quite a sight. The guy explained that its a pretty easy thing to do…just find the sweet spot: a quick moving belt of water just below the face of the wave. Once you’re in it swim up the face(under water) and exit as close to the lip as possible just as it starts to peel. After a few tries I was able to harness some of the power and get half of my body out of the water in these lip launches. Lots of fun!

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