Different swell types "Primary" "Secondary" "Tertiary"

indo-dreaming's picture
indo-dreaming started the topic in Saturday, 16 Nov 2013 at 7:23pm

What is the difference between "Primary swell" "Secondary swell" and "Tertiary swell"?

How is it decided which is which?

And can there be more than three swells in the mix?

Ive just assumed primary swell is largest followed by secondary swell then tertiary, but ive noticed at times the Secondary swell can be a bit bigger than the Primary swell for example Phillip Islands forecast for 20/11 primary swell= 1m @ 10.8 sec with a secondary swell @ 1.2 and a very sad 5.8 sec

Also noticed the Primary swell doesn't always have the highest period either.

Although have noticed the primary swell is the most consistent in height and period.

thermalben's picture
thermalben's picture
thermalben commented Saturday, 16 Nov 2013 at 8:21pm

Simplest way to explain is that the primary swell is the swell train which theoretically produces the largest 'surf size'. Our surf size calculation uses size, period and direction within a proprietary algorithm. We then order the swell trains based on a decreasing order of surf size.

The wave model actually outputs up to six swell trains however it's very rare for more than three swell trains to be concurrently influencing a single surf spot. So, for aesthetic reasons we have limited our main website displays to the top three swell trains.

indo-dreaming's picture
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indo-dreaming commented Saturday, 16 Nov 2013 at 9:45pm

Cheers, quite interesting this stuff, going to have to re read some of those articles you guys did on swell trains.

thermalben's picture
thermalben's picture
thermalben commented Sunday, 17 Nov 2013 at 7:24am

And here's a classic example of where the model sometimes falls over. In the example below (Central Coast, today), the model has split the swell trains because of the unusual synoptic pattern, which is creating a broad spread of swell directions as the swell size and period increases from a local source.

See how the 'surf size' goes from 2-3ft at 6am, to 2ft at 12pm and 4-5ft at 6pm? The 'drop' to 2ft at 12pm is because the model has split the 6am [email protected] S/SE swell train (which is building, under this synoptic pattern) into two swell trains: one out of the S ([email protected]) and one out of the E/SE ([email protected]). From this information, our model then picks the 'largest' swell train to use as a reference for the overall surf size.

Then at 6pm, both swell trains have merged back into a single swell train from the SE ([email protected]).

What SHOULD have happened was the 1.7m 6am swell forecast build to (say) 2.1m at 12pm and 2.4m at 6pm, which would have translated to a 2-3ft 6am surf height building to 3-4ft at 12pm, then building to 4-5ft at 6pm. That would be more inline with the expected swell trend today.

Unfortuately, we do see this splitting of the swell train every now and then, and there's not much we can do about it - it's an inherent characteristic of the WW3 model. We have theorised a couple of possible solutions to this but they're still under development.

Anyway, just a little more insight as to how our surf forecast model works and interprets the data.

indo-dreaming's picture
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indo-dreaming commented Sunday, 17 Nov 2013 at 3:31pm

For anyone else reading this thread, heres a good read on "secondary swells" http://www.bom.gov.au/marine/about/forecast/second-swell.shtml

I never really thought about it all before, but it makes so much sense, i always knew there was more than one swell happening at times, but until seeing the new forecast i never realised how common it actually was.

Ideally for the best conditions at most reefs or beaches there would no secondary swell or tertiary swell just a nice size long period swell with an offshore.

I always thought that when it went offshore for a day or two when there was a good swell running that it was just the wind cleaning up the one swell, but really the reason things become more organised is most likely got more to do with the fact that any other smaller short period swells that are in the mix (secondary or tertiary swells) are either dropping off or totally gone, not because the wind kills them but because there produced closer to shore than a long period ground swell so have less chance of being produced under those conditions.

All makes sense..cheers.

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indo-dreaming commented Sunday, 17 Nov 2013 at 3:41pm

Also and in regards to set ups that wrap into spots that normally result in more organised/lined up the swell (rather than an open beach) maybe it isn't getting more organised because it has to wrap in, its getting more organised because in most cases the second or third swell will have a different direction (that needs to wrap more) or doesn't have the size or power to wrap in hence you end up with the primary swell only, that is off course unless the spot is more open to the secondary or third swell in which case conditions may still be junky.

wellymon's picture
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wellymon commented Sunday, 17 Nov 2013 at 6:11pm

Indo-dreaming thats a good read in the link above, cheers.
Interesting rules BOM use when forecasting secondary swell train "When the primary swell is less than 4 metres, second swell is included if it is greater than 1 metre and from a different direction." etc etc.
Its pretty good that SN show any second or third swell train, no matter what size, interesting as the swell up this ways is tiny but from 3 different directions and a beach I check out which is usually pretty poor had fun waist to chest high peaks running everywhere.
It will be interesting for you to use SN's swell forecasts, all 3 swell trains and keep an eye on the different little reefs and beaches down there, some correlation might be real useful in the long run...?

Our brains are too small at the moment to comprehend the reality of what's happening in our forests . We're only just waking up so to speak . The big problem is we think we know everything, we are specks of dust on a timeline and we know nothing .

donweather's picture
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donweather commented Tuesday, 19 Nov 2013 at 6:06pm

Craig/Ben, just wondering if you could provide some insight into the image below which was taken from one of your forecasts earlier today. Why does Friday predict 2ft and Saturday 2-3ft and yet the primary swells of [email protected] seconds appear in both Friday and Saturday forecast swell trains?

Craig's picture
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Craig commented Tuesday, 19 Nov 2013 at 6:42pm

Don, the difference is the direction swinging 15 degrees and this has bumped the swell just over our 2-3ft threshold.

If you looked at the graph for the same period you would see that the jump in size was way less noticeable with a slight climb in height Saturday from Friday (more so than a jump from 2ft to 2.5ft).

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davywayback commented Sunday, 29 Nov 2015 at 11:18am

thermalben wrote:

Simplest way to explain is that the primary swell is the swell train which theoretically produces the largest 'surf size'. Our surf size calculation uses size, period and direction within a proprietary algorithm. We then order the swell trains based on a decreasing order of surf size.

The wave model actually outputs up to six swell trains however it's very rare for more than three swell trains to be concurrently influencing a single surf spot. So, for aesthetic reasons we have limited our main website displays to the top three swell trains.

10/10 for that one thermalben ! Isn't it weird though, I remember one beautiful summer afternoon about 6 years ago. I'll never forget it. I was feeling great, there was one of those sultry, hazy light on shore breezes coming in at my local regular spot. I looked out - no waves apart from a tiny well shaped inner reef " wave " lapping the edge. Damn it, with no one out there, I threw on the boardies for a paddle and a swim. Without exaggeration - from the time I paddled out to where I'd hoped I just might be lucky - I most certainly was. From that moment on I had the summer session of my life. A 1ft to 2 ft fun right - literally EVERY time. I surfed better than ever, and with every wave, the moment I got back out to the take off point , another wave would come along. It was literally one wave after the other, hardly any wait time for the whole session. There must have been eyes on me for an hour because the spot ended up with about 8 guys out there. Karma ? Who knows.

donweather's picture
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donweather commented Friday, 4 Dec 2015 at 1:56pm

I have a question. What happens when two swells from different directions in deep open ocean pass each. Will there be an instantaneous increase in swell when the two swells coincide? So for example, way out in the South Pacific, you have a E'ly swell and a S'ly swell passing through each other. Just as one E'ly swell passes through the S'ly swell, will the combined instantaneous swell be bigger than each of the individual swells. Kinda like when backwash comes back at a wave approaching the shoreline, the back wask pushes the wave approaching the shore up. So based on this I think I have answered my own question but was just curious to know for open ocean swells.

Craig's picture
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Craig commented Friday, 4 Dec 2015 at 2:00pm

Yes, that's right Don.

donweather's picture
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donweather commented Friday, 4 Dec 2015 at 4:09pm

Thought so. Thanks Craig.

thermalben's picture
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thermalben commented Friday, 4 Dec 2015 at 4:12pm

That's essentially what a rogue wave is.

caml's picture
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caml commented Friday, 4 Dec 2015 at 11:27pm

Seeing we are on the subject can anyone explain what happens when a swell from the north collides with a swell from the south . Whatever direction I just said nth / sth as example )

thermalben's picture
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thermalben commented Saturday, 5 Dec 2015 at 6:19am

Nothing really, they pass through each other without any major effects. 

caml's picture
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caml commented Saturday, 5 Dec 2015 at 10:18am

Really , ben do u think there would be any loss of energy ?

groundswell's picture
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groundswell commented Saturday, 5 Dec 2015 at 3:35pm

What are the major factors in consistent swells with frequent sets?
It seems ealy in some swells they can be inconsistant but other times consistant early on or something else.
Do wind gusts and things like that have anything to do with sets and the frequency or is it just more intense storms produce more frequent sets?
(besides long period swells i mean- like how indo is often slower than swWA but similar period)

caml's picture
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caml commented Saturday, 5 Dec 2015 at 3:51pm

Period would be the major factor I think , also if you are getting point blank aimed swell or just the side / shoulder batch . most surfers dont seem to understand period & the correlation between close or distant swell source it seems . And why would they . Not sayin u groundy but I mean beware , theres so many misunderstandings that it can be confusing to try & understand what u hear . Much better to learn the science facts .

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goofyfoot commented Saturday, 5 Dec 2015 at 6:47pm

How did you learn caml? Mainly just from reading and studying weather maps? I've learnt so much from this website. I have to read things a few times so they sink in but it's been great for learning new things.

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caml commented Sunday, 6 Dec 2015 at 12:42am

Oh From old legoinds at their local breaks

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caml commented Sunday, 6 Dec 2015 at 12:49am

Swellnet has rich knowledge indeed

thermalben's picture
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thermalben commented Sunday, 6 Dec 2015 at 6:38am

Camel - the literature and research states that there is negligible loss of energy when swell trains of opposing directions "pass through" each other. From my personal observations, I think this is broadly true - there have been occasions when it seems that swells have lost size or energy due to their interaction with secondary swell trains, but in truth there could have been a wide range of contributing factors leading to the apparent smaller surf.

However, the vast majority of research around swell energy has been done without a focus on breaking waves, and their surfability. In my opinion, there is still an enormous body of research work yet to be done on various parameters in and around the surf zone and deep water ocean. I have a lot of personal theories that I'd love to spend some time researching, but simply don't have enough time to do so.

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penmister commented Sunday, 6 Dec 2015 at 11:44am

Big fat A frame

caml's picture
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caml commented Sunday, 6 Dec 2015 at 12:08pm

Thanks ben , interesting

wellymon's picture
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wellymon commented Monday, 18 Jan 2016 at 8:35am

Was interested in the Whakatane vs the Mahia Peninsula forecast?
Whaka has 4-6ft NE swell solid green bar, green arrow pointing NE..
Mahia has 4-6ft NE swell half green and half yellow with both arrows yellow and green pointing NE?
Why is this, just curious.

Our brains are too small at the moment to comprehend the reality of what's happening in our forests . We're only just waking up so to speak . The big problem is we think we know everything, we are specks of dust on a timeline and we know nothing .

Craig's picture
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Craig commented Monday, 18 Jan 2016 at 2:12pm

That's because Whakatane faces NE, so gets the full brunt of the NE swell energy.

But we've aligned Mahia Peninsula to the SE, so it's giving the forecast for open beaches (facing SE) in the green, with yellow showing there'll be more size at north-east facing beaches.

You can click Mahia, and that'll show similar forecasts to Whakatane because it's in the bay facing NE.

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Craig commented Monday, 18 Jan 2016 at 2:16pm

Also in regards to consistency.

The further away from the swell source you are, and as Cam said, if it's not aimed towards your coast and you're getting side-band energy then the less consistent it is.

This usually means longer-period swells from distant storms are very inconsistent, but that isn't always the case.

If the storm is real strong and right off the coast, like the last Hawaii swell you'll get large long periods but also high consistency.

wellymon's picture
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wellymon commented Monday, 18 Jan 2016 at 2:41pm

Ok cool as Craig, I didn't realise you had Mahia Peninsula as the SE coast, makes sense now;)

Be some nice waves around that region this week.

Our brains are too small at the moment to comprehend the reality of what's happening in our forests . We're only just waking up so to speak . The big problem is we think we know everything, we are specks of dust on a timeline and we know nothing .

Craig's picture
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Craig commented Monday, 18 Jan 2016 at 2:49pm

Yeah, it's gonna cook for a week!