How it Works: The XXL Waves of Nazare

Tony Butt

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Updated 782d ago

Little more than ten years ago, if you said that the biggest surfable waves in the world were in Europe, not many people would have believed you. If you then told them that they were at a beachbreak, people would have thought you were crazy.

At the time, most of us thought that you couldn’t get really big breaking waves unless you had big open-ocean swells in the first place. The local focusing effect of reefs such as Maverick’s and Pe'ahi were thought of as secondary, not really the main factor. And nobody realised that an underwater trench near the coastline could magnify average-sized open-ocean swells to truly gargantuan proportions. Until Nazaré.

Ever wondered how these monsters are created? Read on.

Nowadays, everybody knows about Nazaré. Most people understand that those giant A-frame peaks have something to do with a narrow finger of deep water called the Nazaré Canyon, that reaches right in to a few hundred metres from the coast. Somehow, the Nazaré Canyon shovels up the energy, pulls it away from Praia Sul and pushes it towards Praia Norte, not much more than a kilometre away. But exactly how it works, is something a lot of people still don’t fully understand.

The most common explanation, at least around the time when Nazaré was first being discovered, was something like this: “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 trench narrows towards the coast”.

Sounds logical. After all, the trench gets narrower towards the coast, so this must squeeze the energy into a smaller area and make that waves bigger. Right?

Well, not quite. If you look carefully at the depth contours (the bathymetric chart) you can see that the Canyon doesn’t actually point towards Praia Norte. It points towards Praia Sul. Those giant A-frame peaks are not in front of the Canyon; they are to one side of it.

Nazaré bathymetry with a NW swell showing in a simplified way how refracted and non-refracted swells interact.

Nazaré bathymetry with a NW swell showing in a simplified way how refracted and non-refracted swells interact.

The real answer is related to refraction – the bending of the waves as they propagate over different depths of water. Once ocean waves get nearer the coast and start propagating over shallower water, their speed begins to be affected by the water depth. The shallower the water, the slower the wave. If a wave travels over an area with unequal depths, it will be steered away from the deeper water and towards the shallow water. At Nazaré, the effect of the Canyon is to bend the waves away from the deeper water in the middle and towards the shallower water either side.

At Nazaré, the orientation and scale of the Canyon is such that the swells refracted towards the north of the Canyon are the ones that produce those giant peaks straight off the beach. The swells that arrive at Praia Norte from out of the Canyon approach the coast at an oblique angle, more or less from the west or southwest.

At the same time, Praia Norte also receives swells that never passed through the Canyon; swells that come straight in from the northwest. The two swells meet at an oblique angle just offshore, resulting in a criss-cross interference pattern. The energy of both swells then combines together to create those giant peaks.

To find out a bit more about how the refraction works at Nazaré, we need to know at what depth the bed starts to affect the speed of the waves. Hence, at what point they start to refract. This isn’t the same for all waves: longer waves ‘reach down’ further into the water column than shorter ones, so they ‘feel bottom’ in deeper water than shorter ones. Therefore, they begin to slow down earlier.

A longer-period wave (left) feels the bottom, but a shorter-period wave in the same depth of water (right) doesn’t.

A longer-period wave (left) feels the bottom, but a shorter-period wave in the same depth of water (right) doesn’t.

The threshold depth at which the waves begin to slow down – and hence begin to be refracted – is directly related to the period. It is 0.8 times the period squared, with the period measured in seconds and the depth in metres (more about this in my article HERE).

Now, good swells that reach the west coast of Portugal have periods typically ranging from about 12 to 18 secs. From the relationship above, the threshold depth at which the waves first start to refract is between about 115 and 260 m. If you check the bathymetric chart you will see that these depths are found at the edge of the Canyon running from northwest to southeast, about five or so kilometres from the coast. A northwest swell (travelling towards the southeast) of anywhere between about 12 and 18 secs hitting these depth contours would be significantly steered to the left, out of the Canyon and towards Praia Norte.

Swells from other directions do not behave in the same way. For example, west swells are known to not produce such classic A-frames because the refracted and unrefracted swells don’t combine in quite the same way.

In summary, the best swells for producing those giant peaks so emblematic of Nazaré are from the northwest, long period and not mixed with any other swells. And because of the intense magnifying effect, you don’t need very big open-ocean swells to produce healthy-sized waves at Nazaré.

So, in theory, you might even get big waves in the middle of summer. True, but the only problem is that Praia Norte is directly exposed to a persistent, strong northerly tradewind called the ‘Nortada’. The Nortada blows a lot during spring and summer, making conditions anything from uncomfortable to suicidal. Best bet is to hit Nazaré in autumn and winter.