by Tony Butt
A double-up is two or more waves either breaking on the shore in very close succession, or combining into a single irregular-shaped wave. It is formed quite close to the shore and, contrary to what you might think at first, its particular characteristics do not come from the swell out in the open ocean.
If you watch a potential double-up coming in towards the shore, you will see several smaller waves appearing almost imperceptibly in front of the original one. Out there somewhere, the main wave gives birth to these auxiliary waves. When the whole family of waves reaches the shore, the main one will begin to catch up the others as if it were trying to gobble them up. When the main wave eventually breaks, the auxiliary waves either break in front of it or combine with it, making the face of the wave uneven and lumpy. Trying to surf this type of wave can be a bit like trying to surf an upward-moving escalator.
The generation of a double-up is linked with the existence of an outer sandbar or reef, and works in more or less the following way:
- Initially, a wave is just approaching the sandbar from offshore. At this stage, the wave still retains the shape that it had in deep water. The shape is not far off a ‘regular-wave’ shape or, in technical terms, a sinusoid.
- Once the wave starts to propagate over the sandbar, the depth of water and the height of the wave are such that the wave doesn’t quite break. It ‘rears up’ and nearly breaks but then rears back down again.
- As the wave starts to propagate over the shallow water of the bar, it starts to feel the bottom. The slowing down of the wave due to the shallow water, and the fact that the bottom part of the wave slows down more than the top, means that the wave is transformed into a pitched-forward, ‘sawtooth’ shape.
- Now, a wave that is distorted like this can be decomposed mathematically into several different regular-shaped waves, all added together – a concept originally discovered by the French mathematician Jean Baptiste Joseph Fourier (1768-1830). According to Fourier’s theory, our wave now consists of a fundamental of the same wavelength as the original wave, plus several harmonics, of different sizes and wavelengths, all amalgamated together.
- After the wave has propagated over the top of the bar, it retains its distorted shape, but once again begins to propagate into deeper water. It cannot maintain that distorted shape and therefore tries to revert back to the regular, sinusoidal shape it had before it hit the bar. However, the laws of physics forbid that to happen – the extra waves generated during the shoaling process cannot just reconstitute themselves back into the original wave. The only solution is for the main wave to ‘shed’ the auxiliary waves, so that they become free-travelling waves of their own right. This leaves the fundamental and the harmonics all travelling as separate waves. Like a family of ducks, but with the ducklings in front of the mother duck instead of behind her.
- Finally, we go almost full-circle. The wave train has propagated over the deep water shoreward of the bar and is starting to travel towards the beach. Once again, it is propagating in water that is getting shallower. Now there is a whole family of waves being distorted as they are feeling the bottom, slowing down, steepening and pitching forward all at the same time. As the whole family gets closer to the beach, the smaller ones in front slow down more than the big one at the back, making the big one catch up with the smaller ones. As the big one starts to break, the small ones either break at the same time or become lumps in the front face of the big one. The final result is a lumpy, ugly wave with a sucking, double or triple-staircase take-off.
If you like double-ups, or indeed, if you want to avoid them, it might be useful to get an idea of the conditions required for their generation. Firstly, you need a sandbar or reef somewhere not too far offshore: preferably a long, sausage shape, running parallel to the shore. The water over the shoal needs to be just shallow enough for the waves to become distorted as they propagate over it, but no so shallow that the waves break. The wave period must be quite long and the swell must be fairly unidirectional, long-lined and not peaky. In other words, it must be a good-quality swell. Depending on your point of view, the fact that double-ups tend to occur mostly in good-quality swells could either be a damn nuisance or an added fun-factor.
Photo: Sean Davey
Tony Butt is a big wave surfer, scientist, writer, environmentalist and the founder of Surf Science. He believes that the more we interfere with the coastline the more problems it will cause us.
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