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Adam Wright
by
(Thursday) 6.21.12

Characteristics of Long-Period Swells

categories: Solspot Q&A

Jason Asked:

What kind of storms produce long period swells with a relatively small swell height, 3ft @ 19sec for instance, is that possible? And would something like that translate to some ledgy overhead surf when it reaches a coast?”

Adam Wright – Solspot Forecaster:

This definitely follows the idea of swell decay and travel distance…it is actually a pretty common occurrence to get a swell with “small wave heights” but very long periods. In fact almost all of Southern California’s S-SW swells arrive with similar properties. There are a couple of things to keep in mind when you start thinking about long-period swells.

1. Longer swell periods equal more swell energy. For example a 3-foot swell at 19-seconds is made up of considerably more energy than a 3-foot swell at 10-seconds.

2. Longer swell periods extend deeper into the ocean. An 18-19 second swell will actually start to “feel” (the correct term is shoaling) the ocean floor at nearly 1000-feet of water depth. As the swell periods shorten up the swell becomes shallower, to use the 10-seconds as an example again, it would start to feel the sea floor around 80-100 feet of water depth.

3. Longer period swells travel faster than short period ones. A 19-second swell travels around 23-25 knots per hour in deep water (meaning it isn’t feeling the sea-floor in any way) while a 10-second swell is closer to 10-15 knots.

4. As swells travel away from the storm or wind fetch that generated them, they slowly drop in swell-height size, this is the swell decay that I mentioned before. The decay is caused by a tiny bit of friction that occurs as the swell moves through the ocean. Now here is where things get sort of counter-intuitive…the shorter the swell period the more friction it feels and the faster it decays. This decay is exponential so it is very possible for a swell (even a long-period one) to lose almost all of its energy if it has to travel over a certain distance. The actual range of a given swell is determined by both the swell-height and the swell period.

5. The further a swell has to travel the more “organized” the swell train becomes. Storms generate a whole spectrum of swell-periods in the core of the storm…all of which is heading different directions, piled on top of each other, generally making a total mess of the ocean (we call this an “active sea-state”). I would say it is sort of like dropping big-ass rock into a pond, but that is too simple…it is more like putting an orbital sander into the pond then turning it on while no one is holding it…anyway I think you get what I mean. Eventually the swells move out from under the storm’s influence, with the majority of the energy travelling along the path of the storms strongest winds (hopefully aimed at your location). As they do, the longer period swells will out-distance the shorter period and will create a more organized group with the longest periods as the leaders of the swell train and then progressively smaller swell periods further back.

OK…sorry to nerd out there for a second, but those are the core concepts you have to wrap your head around in order to get to the heart of your questions.

1. “What kind of storms produce long period swells with a relatively small swell height…3ft @ 19sec for instance…is that possible?”

Yes it definitely is possible. To actually get a pure 3-foot swell @ 19 seconds you would have to have a pretty solid storm that is at least 4500-5000 (or so) miles away. The storm would need to have an area of fetch aimed your way with some 40-50 knots of wind, and sufficient time to produce about 35- to 40-foot seas…a good portion of which would need to be about 25-feet of energy in that 19-20 second period range. (The more swell it produces in those long-period ranges the further away the storm can be and still reach us with the 3-feet of swell @ 19 seconds)

Eventually what would happen is the swell would leave the storm area, with the 19-second energy moving out to the front, and since a swell with periods that long decays away about 1/3 of it size about every 800 miles it travels…(so the 25-foot initial swell would eventually decay to the 3-foot mark after travelling 5600 miles).

2. “And would something like that translate to some ledgy overhead surf when it reaches a coast?”

The answer to that is yes as well…but it is determined a lot by the nearshore water depths, the beach slope, and host of other factors. Having the swell periods at that length definitely helps, because the “beneath the surface” swell energy, which if you recall is going down nearly 1000+ feet starts to be compressed as the swell moves into shallower water. That energy does a couple of things as it is compressed, it slows the overall speed of the swell and if the coastal shelf isn’t uniform it will refract portions of the swell different directions…but more importantly it starts to push the energy up, causing that 3-foot @ 19 second deep-water wave to gain size.

To get the ledgy overhead shape you want a beach that has very deep water just offshore, so the swell/wave doesn’t feel too much friction until it starts to hit the surf break…this is one of the reasons a lot of the more hollow waves are located around submarine canyons…the deeper water lets the full swell energy make it almost all the way to the beach before suddenly forcing it into a much shallower water depth. It also is why reefs that drop off to deep water, like Teahupoo or Cloudbreak, behave the way they do…in fact their transition from deep to shallow water is so quick that they don’t need to have excessively long-periods to create the hollowness. Surf spots like those, or even some that have rocks or structures that cause the swells energy to fold back on top of itself act as multipliers when it comes to taking deep-water waves and turning them into breaking waves…some may only double the size of the deepwater height, while others may triple it (or go even bigger)…so while a lot depends on the actual break to create the final wave shape, having very long-period swell, which are loaded with lots of extra energy, definitely help the process.

Hopefully that shed some light on the subject without anyone snoring (I think I nodded off a couple of times while typing…)

  • Jason

    Thank you!

  • Zenki

    Adam you are the best.

    • http://www.solspot.com/ Adam Wright

      Thanks Zenki!

  • ed

    seriously, adam, this is so above and beyond! excellent stuff!

  • Cyclcone

    Very interesting. I have a 4.5 hour drive to my ultimate surf spot. I’ve been going once a month when “its on” – I’ll be using some on this in my planning. Thx Adam


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