River deep, mountain high: Explaining Nazare
Over the next 72 hours the eyes of the surfing world will be focussed on a small stretch of ocean offshore from Praia de Norte, Nazare.
On Friday (local time) the Big Wave Tour will kick off the new season with the Nazare Challenge, and then on Sunday the real challenge will arrive when an exceptionally large swell sweeps in from the North Atlantic.
Various forecasts have been thrown around, yet wave size at Nazare is notoriously hard to pin down. Despite the difficulty, Praia de Norte is widely regarded as having the largest rideable waves in the world.
And the reason for the extraordinary size? Whether it be WSL commentators or surf journalists, we’re told it’s because of the Nazare Canyon - a deep trench cutting through the adjacent continental shelf. The incoming swell supposedly travels up the canyon striking Praia de Norte without losing any of its energy to the shallow continental shelf.
Yet when we look at the location of Nazare Canyon, particularly as it relates to Praia de Norte, doubt is thrown on that theory.
The predominant swell direction for big Nazare is from the northwestern quadrant. The big winter swells sweep down from the North Atlantic striking the Portuguese coast at an angle. Both Friday and Sunday’s swells will hit at 290°, or west-northwest. Yet Nazare Canyon lies to the southwest of Praia de Norte.
Coming from the west-northwest, much of the energy in those big winter swells won’t even pass over Nazare Canyon, let alone travel along it.
So then, what’s happening at Praia de Norte? If the swells don’t pass over the canyon how do they get so damn big?
Winter swells come from the northwest, yet Nazare Canyon is to the southwest of Praia de Norte
It’s important to note that, despite Nazare Canyon being located to the southwest of Praia de Norte, the head of the canyon comes to within a kilometre of where the surfers ride the waves.
Detailed bathymetric charts show that the takeoff zone at Praia de Norte is between 15 and 20 metres deep, yet immediately to the south, at the canyon head, the depth is 125 metres. In fact, at one location, the seabed rises 70 vertical metres in just 140 horizontal metres. That’s a rise of 1:2, which is extremely rare on the ocean floor.
It’s this extreme difference in water depth that creates the extraordinary waves at Nazare.
Despite Nazare Canyon being to the southwest, its head reaches to within a kilometre of Praia de Norte (Instituto Hidrográfico)
Before continuing, we need to point out that Nazare breaks unlike other big waves such as Jaws or Mavericks. Rather than having a single line of swell approaching the coast, Nazare is characterised by swell lines that run at angles. It achieves its extraordinary height by a process called constructive interference: those swell lines move toward each other and where they meet - i.e where they interfere with each other - the size increases.
Outside of the science lab we call it a wedge.
The typical set up for a wedge is a groyne or headland reflecting a swell back into the lineup. Yet a similar thing can happen when swells refract around an underwater rise. At Praia de Norte, northwest swells slow upon approaching the coast, yet just to the south the same swell retains it's open ocean speed due to the depth at the canyon head. This causes the swell line to refract towards the shallow area.
Swell from the northwest slows over the shallow shelf, yet to the south where the water is deeper the swell moves faster, thereby bending it towards the shallow water
The result is a swell that originally came from a northwest angle starts to bend and move towards the northeast across the Praia de Norte surf zone. And where the refracted swell interferes with the incoming northwest swell is where the wedges, ridiculously oversized wedges, occur.
November 8th, 2017, and the Nazare wedge is in full effect
Therefore, it's not swell travelling up the canyon that creates the size at Nazare, but the sharp change in depth immediately south of the headland.
And it also explains why wave size can be hard be so hard to forecast at Nazare. It’s not one swell the forecasters are concerned with, but the collision of two swells. Add to that the myriad angles of refraction and interference and you have the potential for a vast range of swell heights.