Swell Period: Here's Why It's So Important

Tony Butt

by on

Updated 64d ago

Size matters – we all know that. But when we are talking about swell, the period matters almost just as much.

The last few big swells in the North Atlantic have arrived with periods of up to 19 or 20 secs, which is pretty long for this part of the world. The long periods gave the waves an extra punch at many places, and, at some spots, a lot more size than you would normally expect.

This is your MSW forecast, the numbers in the centre column are the swell period, 14s, or, 14 seconds.

This is your MSW forecast, the numbers in the centre column are the swell period, 14s, or, 14 seconds.

In deep water, long-period waves travel faster than short-period ones. So when they hit shallow water they slow down more abruptly and break with more punch. Long-period waves are also thicker from front to back, so more volume of water is shifted when they break. The combination of those two factors makes longer-period waves more powerful than shorter-period ones.

But period really comes into its own at spots that have a significant refraction effect. Refraction is where part of a swell line hits shallow water while the other part is still over deep water, so one part slows down and the other part doesn’t, and the swell line ends up bending towards the shallow water. This can cause many different things to happen to the waves before they break.

In fact, the way the waves interact with the shape of the sea floor just offshore of a surf spot is the most important factor for determining the characteristics of that spot. At spots where refraction causes the waves to bend in on themselves and increase their height (bathymetric focusing), period is really important. The magnification factor is highly dependent on the period. At some spots, for example, the breaking wave height can be the same as the open-ocean height one day, but four times as big the next day when a new, long-period swell arrives.

Ever seen the film Chasing Maverick's? Gerard Butler helps recount the tale of Jay Moriarity. The film opens with Jay counting the seconds between waves, figuring out that the longer the period, the larger the waves. Big waves need more than just swell, it's a combo of many factors with period being one of the more prominent.

So what is the connection between refraction and period? Well, the water particles beneath an unbroken wave travel in circles that diminish in size with distance below the surface. In deep water these orbits diminish to nothing before they reach the bed, so the wave is unaffected by the bed. In shallow water the orbits touch the bed, so the wave is slowed down.

Now, a longer-period wave is thicker from front to back, so those orbits are bigger under longer-period waves than under shorter-period ones. This means that the orbits ‘reach down’ further into the water column. As a result, a longer-period wave approaching the coast will feel the bottom in deeper water, further out, and will start to refract earlier.

If the waves start to refract earlier, any effects such as bathymetric focusing will be more pronounced by the time the wave reaches the break point. This is why bathymetric focusing is period-dependent, and is what makes wave heights at some spots increase so dramatically with long-period swells.

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.

Cover shot: Nazare by Helio Antonio