The Canyon Effect

On 20 December 2016, the circus came to town at Nazaré in Portugal. This time it was in the form of a big-wave contest, held in large, lumpy shifting peaks. Even though surfing competitions don’t interest me in the slightest, I do like big waves and I do find it fascinating how each spot works in a different way. So, in this article I thought I’d talk a little about how those giant peaks at Nazaré actually come about.

Highlights of the Nazare Challenge Final

Since the towsurfing exploits of Garrett McNamara, and more recently with people like Shane Dorian saying that Nazaré is right up there with Jaws, Cortes Bank and Maverick's, many people have been asking me how a beachbreak can hold waves so big without closing out. And, indeed, how it can get so big in the first place, when, just down the coast, the waves are half the size.

There must be something that steers all the wave energy towards that one beach (Praia do Norte) and away from the adjacent beach (Praia da Nazaré) even though both beaches face the same direction. Then, just before the waves break, there must be something that shovels up the wave energy from either side and pushes it into the middle – converting what would be a closeout into a peak.

So, what is it? Well, if you have a quick look at the bathymetric charts, or even on Google Earth, you can see it: a giant deep-water canyon that exists just offshore of both of these places. Most people agree on that. But then, when asked to explain further, most people fall into the same trap. For example:

“Swells are able to pass through the deep water of the canyon with very little resistance while being compressed at the same time as the canyon narrows towards the coast”

Beneath Nazare

You might be forgiven for thinking this is logical. After all, since the canyon gets narrower towards the coast, this must squeeze all that energy into a smaller area, making the waves bigger, right?
Well, not quite. For a start, the canyon doesn’t actually point towards Praia do Norte. It points towards an area just to the south where the wave height is much, much lower. Praia do Norte is located to one side of the canyon. So, contrary to what many people think, the swell energy is increased and the waves are bigger in the shallow water either side of the canyon; and the swell energy is decreased and the waves are smaller in the deep water in the middle.

The ‘funnelling’ effect that many people try to use to explain Nazaré does not work with normal ocean swells. Ocean swells are not like constant streams of water – the water is not displaced from one place to the other; it just moves in closed circles below the waves. Ocean waves transport energy, not water, from one place to another. You have to have a transport of fluid, not just energy, for that funnelling effect to work.

The real answer is because of refraction, or the bending of wave fronts as they encounter shallow water. When ocean waves start to propagate in shallow water, their speed becomes dependent on the water depth: the shallower the water, the slower the wave. Waves approaching the coast near Nazaré slow down either side of the canyon where the water is shallow, but not in the middle where the water is deep. Incoming swells are steered away from the canyon, resulting in a reduction in wave height directly opposite the canyon itself. The energy that is steered away from the middle is therefore concentrated at the sides of the canyon, making the waves bigger.

Depending on the orientation of the bathymetry in relation to the prevailing swell direction, the effect is usually more pronounced on one side of the canyon than on the other. In the case of Nazaré, the most marked effect is to the north.

The wave height is also increased due to another effect. The waves steered away from the middle of the canyon turn and start to propagate at an oblique angle to the coast. These waves interact with straight-incoming waves that didn’t pass through the canyon. The interaction between the steered waves and those approaching head-on is called linear superposition or constructive and destructive interference. Depending on the relative phases of two waves when they meet, they will either add together or they will cancel out. The overall effect is to create a ‘lattice’ of alternating ups and downs in the water surface. The result is those large A-frame peaks so emblematic of Nazaré.

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