Dispelling the Underwater Canyon Myth

Tony Butt

by on

Updated 110d ago

Southwest France has some very special surf. The most famous spot, La Graviere at Hossegor, is well-known for sucking in most of the swell, sometimes getting perfect six-to-eight-foot A-frames when, at the same time, just down the coast at Capbreton, you could be looking at three feet or less.

There must be something that shovels up the energy and directs it away from Capbreton and towards Hossegor. If you look at the bathymetric charts, or even on Google Earth, you can see it: Le Fosse de Capreton, a giant deep-water trench that exists just off the coast.

Capbreton during the summer months.

Capbreton during the summer months.

© 2018 - OUTOGRAFIE

Most people agree that the trench has something to do with the waves. But many people fall into the same trap and say something like: “Swells are able to pass through the deep water of the trench with very little resistance while being compressed at the same time as the trench narrows towards the coast.”

You might be forgiven for thinking this is logical. After all, since the trench 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 trench doesn’t actually point towards La Graviere. It points towards Capbreton to the south where the wave height is smaller. So, contrary to what many people think, wave height is bigger to one side of the canyon and smaller in the middle.

The ‘funnelling’ effect that people talk about just doesn’t work with normal ocean swells because ocean swells don’t transport large masses of water from one place to another. The water beneath ocean swells moves at scales far smaller than the size of the Capbreton Trench. For that funnelling effect to work you would have to have a much larger water movement like a current or tide.

On the left, you can see the trench point directly at Capbreton. And the right, the trench's vastness as evidenced by Google Earth.

On the left, you can see the trench point directly at Capbreton. And the right, the trench's vastness as evidenced by Google Earth.

The real answer is refraction, or bending of the waves as they propagate over different depths of water. To be more precise, bathymetric focusing. Approaching swells slow down either side of the trench where the water is shallow, but not in the middle where the water is deep.

The swells are steered away from the trench, resulting in smaller waves directly opposite, but a concentration of energy, and therefore bigger waves, to one side. Usually this steering is more intense on one side of the trench; in the case of Hossegor, most of the swell is directed towards the north.

The A-frames are produced as the steered waves – that are now coming in at an oblique angle – interact with those that miss the trench and are coming straight in towards the coast.

Of course, just down the road at Nazaré there is also an underwater trench, called the Nazaré Canyon, which works in a very similar way. I’ll talk about that in more detail in a future article.

Cover shot by Damian Davila