Double-Ups: What Are They and Where Do They Come From?

Tony Butt

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Updated 16d ago

Double-ups have a habit of turning a nice, smooth take-off into a bumpy, near-impossible staircase. Depending on your point of view, they can be an interesting challenge or a damn nuisance. But where do they come from?

Even with quite a long period swell, you sometimes get two or more waves coming in towards the shore really close behind each other; a double-up. When a double-up breaks, those waves combine to give you an unpredictable, lurching wave with a lumpy face.

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Ben Gravy is the king of novelty waves, and of course, double-ups.

The explanation behind double-ups is not obvious. I always thought it was something to do with two different swells, or perhaps waves of different speeds within the same swell, combining together. But I noticed that they always seemed to form quite close to the shore, as if one or more ‘auxiliary waves’ mysteriously appeared in front of the main wave. Perhaps they weren’t always the result of different swells combining – perhaps there was more of a localised thing going on.

In fact, if you watch carefully, you can see the main wave ‘giving birth’ to these auxiliary waves in front of it. The whole family then continues on towards the shore, all linked together, a bit like a mother duck and her babies, except with the ducklings out in front instead of behind. It wasn’t until I came across a scientific paper explaining double-ups that I found out how it really works. I would have never guessed

It wasn’t until I came across a scientific paper explaining double-ups that I found out how it really works. I would have never guessed. The author was a non-surfing colleague of mine and he didn’t think his study was very useful. As far as he was concerned, it was a solution waiting for problem.

Basically, double-ups form because of an outer sandbar or reef, and a principle called the Fourier Transform. Here’s how it works. More or less; Consider a simple scenario, a fairly steep beach with a sandbar just offshore, and a good-quality, lined-up swell approaching. The sandbar is long, sausage-shaped and runs parallel to the shore. A wave is just approaching the sandbar from offshore. Before it gets to the sandbar, the wave retains the shape it had in deep water. That shape is not far off a sinusoid. As the wave starts to propagate over the shallow water of the bar, it starts to shoal or ‘feel the bottom’. The slowing down of the wave due to the shallow water means that the top tries to overtake the bottom. The wave changes from a sinusoid to a ‘pitched forward’ shape. Crucially, it doesn’t quite break.

Now this is the really important part; A wave that is distorted like this can be mathematically decomposed into several sinusoids added together – a concept originally discovered by the French mathematician Jean Baptiste Joseph Fourier (1768-1830). According to Fourier’s theory, any wave, no matter how distorted, can be broken down into a fundamental of the same wavelength as the original wave, plus several harmonics, of different sizes and wavelengths. Essentially, when some physical process (like shoaling) distorts a wave, harmonics are generated.

If you find that a bit tricky to understand, think of the process in reverse. Draw a sinewave on a piece of paper. Then, below it, draw another sinewave, this time smaller and with half the wavelength of the original. Then draw another one below that, smaller again and with a quarter the wavelength of the original. Now carefully add those three waves together and draw the result at the bottom. You should end up with a distorted wave with the same wavelength as the top one.

Example of a sinewave, follow the above instructions to really map out double-ups.

Example of a sinewave, follow the above instructions to really map out double-ups.

So we can now think of the pitched-forward wave consisting of several sine waves – namely a fundamental and harmonics – all added up together.

The wave now begins to propagate over the other side of the bar, once again into deeper water. This is called de-shoaling. If the physical action of shoaling generates harmonics, the action of de-shoaling ought to get rid of those harmonics and restore the wave back to the original sinusoid before it hit the bar.

However, the harmonics that were generated during the shoaling process cannot just ‘un-generate’ themselves. They are there to stay. Why? Well, it has something to do with the fact that some physical processes just cannot be reversed – a principle called entropy.

Therefore, the distorted wave has no choice but to ‘shed its harmonics’, so that they become free-travelling waves of their own right. This leaves the fundamental and the harmonics all travelling as separate waves.

On its day, Nias is a great example of a double-up.

Finally, the wave train starts to get closer to the beach. Now, instead of just one wave, there is a whole family of waves steepening and pitching forward all at the same time. As the whole family gets closer to the beach, the big one at the back catches up with the smaller ones at the front; the mother duck effectively gobbling up her own ducklings. The final result is a lumpy, pitching wave that is like trying to take off on an upwards-moving escalator.

If you like double-ups, or indeed, if you want to avoid them, you’ll want to know when they are most likely to happen. Firstly, you need a long sandbar or reef somewhere not too far from the beach, preferably running parallel to the shore. The water over the bar needs to be just shallow enough for the waves to become distorted but not shallow enough to make them break. This will depend on the wave height, the tidal stage and the size of the shoal itself. Finally, the wave period must be quite long, and the swell must be of good quality. That's it. Time to hunt them down, or not, whichever you prefer.

An animation showing how a wave turns into a double up when it reaches the beach.

An animation showing how a wave turns into a double up when it reaches the beach.

Cover shot: Mark Matthews chasing Taj Burrow down at The Right in Oz. As you can see, the wave is birthing another and is going to be all kinds of nasty. Pic by Russell Ord.