A Deeper Look Into Swell Period

Craig Brokensha picture
Craig Brokensha (Craig)
Swellnet Analysis

Recently, Swellnet received an email asking the question: "If a wave buoy is reading 2 metres, why does the surf come in smaller than 2 metres if the period is five seconds, and why do larger period swells provide more size than that 2 metre reading”.

The answer to this and a deeper look into wave period is explained below.

A surfer's view of the ocean is usually two-dimensional. Sitting in the lineup, boardriders identify lumps of swell rising and falling in the vertical plane (up/down) while they approach in the horizontal plane (forward/backward/sidewards).

Not much thought is given to what lies beneath the surface, yet this is where the bulk of the swell energy is stored. Such is the importance of this energy, it makes or breaks a session on your local beach, reef, or point break.

We’re talking about swell period, the energy hidden below the ocean surface which only becomes apparent when the swell moves into shallower water, on approach to the coast.

Swell period is the time (measured in seconds) between two successive wave crests (peaks), and is best observed when watching an incoming set passing a stationary object such as a buoy or fixed pylon. If it takes ten seconds between wave crests then the swell period is 10 seconds. The larger the wave period, the greater the distance between two wave crests will be and this is known as wavelength.

Swell period is the time between successive wave crests passing a fixed point such as the outside take-off at the Bells Bowl

Swellnet has previously discussed swell period and its effect on your local beach or reef. On the East Coast, lower period swells are preferred, as they generally provide peaky A-frames which are ideal for the beaches. As the swell period increases, the lines become more drawn out, longer-lined and tend to close-out beachbreaks. This favours reef and point breaks which focus the incoming lines into peeling waves.

Surfers in the southern states wouldn't get out of bed for periods under 11-12 seconds and for the reef breaks to properly fire, long-period swells of 15 seconds or more are a must. 

Regardless of how peaky or straight a swell comes in, questions like the one at the start of the article arise in respect to why similar size swells out to sea can create differing wave heights when they break.

To answer it we must go back to first principles.

Tsunami aside, every wave begins its life as wind blowing across the ocean surface. The wind transfers energy into the water column which initially shows as tiny capillary waves. As the wind continues to blow, non-linear (read: non-uniform) wave-wave interactions cause the energy to be transformed into longer waves, that influence deeper into the water column. The ocean sea state will continue to grow as long as the wind input is greater than the dissipation due to wave breaking and white-capping.

Wind blowing over a stretch of ocean is known as fetch, and there comes a point where the longer waves escape the fetch and travel onwards, or the fetch dissipates allowing the organised waves to spread out and continue on their way.

The greater the wind energy that's transferred into the ocean, the longer the waves and hence the time between successive peaks and troughs. This is where swell period comes from and generally the stronger the storm, the greater the swell period it produces.

Swell generation due to wind blowing over an area of ocean (fetch). Source: University of South Florida

Relatively speaking, longer waves are created by strong winds transferring energy into the water column over a substantial amount of time. Just as an increasing swell period stretches out the wavelength, so too does its energy stretch down into the water column under each wave crest - this is the three-dimensional aspect of wave energy.

Therefore the larger the swell period, the greater the amount of energy that is stored below the sea surface.

To visualise this, we see the majority of the swell energy stored in the upper part of the water column, decreasing in a cone like fashion the deeper you go.

To work out how deep in the water column the energy sits in regards to period, you can use this formula:

The wavelength of a swell (gap between each crest or trough) = 1.56*(swell period)^2.

So for a 16s groundswell, the wavelength is 1.56*16^2 = 1.56*16*16 = 399.36m. Meaning there is about 400m distance between each wave crest in a 16s swell out in the open ocean.

The depth to which the energy is stored in the water column is half of the wavelength, so for a 16s swell, we see energy down to 200m below the sea surface.

This is shown in the below diagram with the orbital energy associated with each wave decreasing the deeper into the water column you go.

Cross-section of the ocean showing decreasing wave energy (orbital motion) with depth. Source: Physical Geology - Steven Earle

Having a 16s swell carry energy 200m deep into the water column is quite a distance and this is why you can see long-period swells 'feeling' the ocean floor (bathymetry) way offshore while also being steered by certain underwater features, be they canyons, sea mounts, or other features.

This steering/focussing of swell is most pronounced across the East Coast when observing long-period southerly groundswells tracking northwards. We often see these groundswells providing a wide variation in size across regions with similar exposure to the incoming southerly energy. It's not unusal for one spot to be 6-8ft while nearby another spot is only 2-3ft.

The same effect can be see at Sunset Beach, Hawaii, where waves can be twice the size of Kammieland, though it lies just the other side of the channel.

The effect of bathymetry steering or focussing is also the magic behind Nazare. When swells arrive from the north-west they refract around a deep canyon that leads all the way to the coast, in effect causing the swell to wrap back in on itself, generating super-sized wedges.

This hidden energy and the way it interacts with the ocean floor is why a 2 metre swell at 5 seconds performs much differently to a 2 metre swell at 16 seconds.

A swell of 5 seconds has a comparitively short wavelength of 39 metres, and it only feels the bottom at 19.5 metres. Compare that to the aforementioned 16 second swell which feels the ocean floor at a depth of 200 metres.

Think also of how much more energy is present in the water column. That energy will be compressed upwards as the wave approaches the shore.

With all things being equal, as the swell moves into shallower water the wavelength reduces and the amplitude (height) increases.

If both swells are 2 metres, the 5 second energy will hardly rise (perhaps even coming in smaller owing to bottom friction) compared to the 16 second swell which will rise quite significantly in size due to the extra energy in the water column being squeezed and having nowhere to go but up.

Rough estimates would be the 5 second swell coming in at 2-3 feet (waist-shoulder high) while the 16 second swell will rise right up and break in that 6 feet+ range (double overhead +).

In the next article we'll look at how to read and identify the wave period of the buoys around the country so you can better understand the expected size and conditions on your local beach or reef.

Comments

sangsta's picture
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sangsta Wednesday, 1 Dec 2021 at 1:14pm

Thanks Craig. Now even a simpleton like myself can understand how the magic happens.

mpeachy's picture
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mpeachy Wednesday, 1 Dec 2021 at 2:19pm

How far off the east coast would it get to 39 metres deep and 200 metres deep? Ie. if there's a 5 second versus 16 second swell how far offshore does the seafloor become relevant?

Craig's picture
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Craig Wednesday, 1 Dec 2021 at 2:29pm

Good question. The shelf-break is where the 200m depth contour sits and is about 30km off Sydney while the 39m contour is around 1km or less.

Open these images in a new window to see the depths.

Nick Bone's picture
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Nick Bone Wednesday, 1 Dec 2021 at 6:11pm

What this site Craigoss?

Patate's picture
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Patate Wednesday, 1 Dec 2021 at 9:58pm

Been looking for this data in an easily accessible form for a while, thx for sharing,

Craig's picture
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Craig Thursday, 2 Dec 2021 at 7:21am

Yeah a great app!

Eggman's picture
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Eggman Saturday, 4 Dec 2021 at 10:23am

Wow. i just lost 4 hours exploring that website...

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Craig Monday, 6 Dec 2021 at 9:51am

Haha right! I could spend days on there.

Pat Hollingworth's picture
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Pat Hollingworth Wednesday, 1 Dec 2021 at 3:06pm

Thanks Craig. A poorly phrased maths question if i may. In terms of the 'amount' of energy in each wave, does increased wave size scale linearly but increased period scale non-linearly?

So for example, imagine a 6 ft wave at 14 secs and an 8ft wave at 14 secs both part of the same swell event, would the 8ft wave contain a proportionate (i.e. additive) increase (say 25%) in energy?

And then imagine a 6 ft wave at 14 seconds and a 6ft wave at 20 seconds (from different swells obviously), would the larger period 6 ft wave have a non-proportionate (i.e. multiplicative) increase in energy amount compared to the shorter period wave?

Craig's picture
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Craig Wednesday, 1 Dec 2021 at 3:36pm

Yep you've nailed it Pat.

And the key to that is the (period)^2 factor in calculating the wavelength and hence energy extending deeper into the water column, which then gets compressed and shows as larger inshore surf.

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willibutler Wednesday, 1 Dec 2021 at 4:59pm

As waves get taller they also get wider so, so even at the same period it isn’t linear as it grows in more than one direction. This is why they say it’s so hard to build bigger waves in wave pools, because the increase in energy usage is exponential.

Pat Hollingworth's picture
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Pat Hollingworth Wednesday, 1 Dec 2021 at 8:44pm

Thanks Willi. Of course, width increases too.

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Craig Thursday, 2 Dec 2021 at 9:11am

I wouldn't say widen Willi.

When the waves come into shallow water the wavelength reduces, the amplitude(height) increases so there is actually a thinning if you take a cross-section of the incoming swell. Not a widening.

Conservation of energy.

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Pat Hollingworth Thursday, 2 Dec 2021 at 12:04pm

ok, so, if you were flying above the ocean in a helicopter looking down upon (a) a 20 sec 6ft swell and (b) a 10 sec foot swell, i was thinking that the 20 sec swells would appear to be 'wider'. But now that i think further, i guess they wouldn't appear wider, but just less frequent (i.e. the 10 sec swell would just be spaced much closer together). Love this stuff, thanks Craig.

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Craig Thursday, 2 Dec 2021 at 12:17pm

Depends what you mean by wider ha. The 20s swell is going to be way more drawn out and be wider at sea level either side of the crest compared to the 10s swell which will be thinner.

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Craig Thursday, 2 Dec 2021 at 3:49pm

Aghh, you mean more drawn out across the bay wider, not wider in profile view?

Lanky Dean's picture
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Lanky Dean Thursday, 2 Dec 2021 at 3:46pm

Nice one Willy. no wonder they always come in around the same height at the pools.
great explanation.

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Pat Hollingworth Wednesday, 1 Dec 2021 at 8:43pm

Awesome thanks Craig

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Johknee Wednesday, 1 Dec 2021 at 3:27pm

Love this stuff! Looking forward to the next article.

Makes me feel smrt when I'm learnding!!

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indo-dreaming Wednesday, 1 Dec 2021 at 5:41pm

@Craig

In regard to sand movement beyond the wave breaking line, do higher period swells move sand from offshore deeper water towards shore better than short period swells? (as they feel the bottom deeper)

Is there much movement of sand beyond the wave breaking line? (something ive never thought about or read about)

In Victoria at my local beach Cape Woolamai we seem to go through different periods of the banks being good or crap etc the whole beach, ive often thought about the effects of different size swells, tides, winds ect but never thought much about the movement of sand being pushed from deeper water beyond wave breaking line to shore.

Definitely seems we get our best banks after periods of small long period swells and lots of offshores, always thought it was just small swells grooming the banls, but maybe small long period swells also move sand from deeper water to shallower water gradually piling the sand up.

Obviously big winter swells also do the same but they also often rip the banks apart and take sand away as so much water being pushed around..

Craig's picture
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Craig Thursday, 2 Dec 2021 at 9:00am

Yep the longer period, deeper reaching swells mobilise sediment across the shelf-break. So do the tides!

Where the deposition/erosion zones are, depends on the bathymetry and also sediment inputs.

From this study in Bass Strait: https://www.sciencedirect.com/science/article/abs/pii/S0025322704001628

"Using observed grain size data, mobilisation from swell waves occurred on ∼31% and tidal currents on ∼41% of the continental shelf.

Swell wave energy is sufficient to mobilise fine sand (0.1 mm diameter) to a water depth of 142 m on the Otway Shelf near the western entrance to Bass Strait.

Tidal currents in King Sound (northwest shelf) are capable of mobilising large areas of medium sand (0.35 mm diameter) 100% of the time.

Superimposing the distribution of threshold exceedance by wave and tidal currents indicates that there are areas on the shelf where either wave-induced or tidal currents dominate, some areas where waves and tides are of relatively equal importance and still other areas where neither is significant. "

For your local region I can't comment with any certainty but this Australian wide chart of continental bed shear stress shows that the Strait is one of the more active regions for sediment suspension (along with the Spencer Gulf and Perth/Margs regions).

Continental shelf (~20 to 300 m depth) bed shear stress (unit of measure: Pascal, Pa) covering the period March 1997 to February 2008 (inclusive)..

tango's picture
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tango Thursday, 2 Dec 2021 at 1:26pm

And this is part of the reason that long-term (ie much greater than annual) coastal impacts of climate change have a few question marks over them.

indo-dreaming's picture
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indo-dreaming Thursday, 2 Dec 2021 at 3:40pm

Thanks Craig interesting.

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Jamyardy Sunday, 5 Dec 2021 at 3:54pm

"Using observed grain size data, mobilisation from swell waves occurred on ∼31% and tidal currents on ∼41% of the continental shelf."

Interesting stat Craig. Would tidal currents pretty much have a net zero affect (given they travel in both directions daily), where as swell waves actually move sand from area A to area B with each event ?

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Thegrowingtrend.com Wednesday, 1 Dec 2021 at 6:05pm

how do waves break

mattlock's picture
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mattlock Wednesday, 1 Dec 2021 at 8:35pm

Their wives usualy break them as per normal.

Craig's picture
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Craig Thursday, 2 Dec 2021 at 9:27am

Due to bottom friction. That energy hidden under the surface starts dragging in shallow water and the wave crest starts moving ahead of it, spilling over due to gravity.

The MIDdleman.'s picture
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The MIDdleman. Thursday, 2 Dec 2021 at 4:40pm

Really cool.
It's trippy trying to visualise what would have a cyclonic shape yet has vortices spinning on the lateral relatively speaking.

And then grabbing the arse of it as it travels by to tip it over.

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gsco Wednesday, 1 Dec 2021 at 7:09pm

Craig you said:

"The larger the wave period, the greater the distance between two wave crests will be and this is known as wavelength."

I was wondering where does wave velocity/speed come into the picture?

Could a greater (smaller) wave period also mean the waves are travelling slower (faster)?

Or do water waves of a given height/amplitude have the same speed?

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bluediamond Wednesday, 1 Dec 2021 at 7:38pm

I know FR76 and myself and a couple of others had a fairly robust debate a while ago about how on certain long period straight S swells, the Tweed Coast can both get the swell before Ballina coast, and also at times be bigger than the Ballina coast, particularly between Pottsville and Kingscliff.
The argument against was that Ballina sticks out further, is closer to the swell source, hence must recieve the swell earlier, and also more of the swell, but i personally have done trips on the same day and seen a marked difference in swell size.
It made me wonder so i had a good look at the navigation charts, and it seems they both have similar looking canyons running straight from East to West out off the main shelf, however the one out off the Tweed, straight out off HP seems to have a more gunbarrel look to it, staying deeper, longer in a straighter line than the one off Ballina. Not sure if this would make a difference when a big South swell is running straight up the coast and then funnels into both of these canyons, but maybe Craig if you have an inkling? Cheers.
http://fishing-app.gpsnauticalcharts.com/i-boating-fishing-web-app/fishi...
Thinking about it, it's probably way too deep out there still to make any real difference but in the biggest south swells...

Nick Bone's picture
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Nick Bone Wednesday, 1 Dec 2021 at 7:38pm

199-D41-F8-60-E7-418-A-AAAC-479-ADB3-C90-E0

I’ve asked a few times before but here seems like a good place to get peoples opinions.

How can you get any other swell direction bar SW if a 11+ second swell starts feeling the bottom when it hits the shelf and is turned SW regardless of how W or S it’s original nature was?

Im sure there’s more too it but yeah, looks pretty straightforward haha.

greyhound's picture
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greyhound Wednesday, 1 Dec 2021 at 8:02pm

Would say a west swell remain more west the further south the shore it hits is.. almost getting stuck on cape Otway as it passes by.. eg.. staying more of a true west swell at the prom and say wsw on the surf coast. But I guess a swell is always described by its origins rather than it’s angle at destination point..

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Craig Thursday, 2 Dec 2021 at 10:28am

The thousands of different permutations between swell period, size, angle and hence the amount of energy 'feeling' that bottom bathymetry would still not see every swell coming in at a similar direction or acting the same once it hits the beach as you'd know. I've surfed both west and south swells on the Surf Coast and there's a noticeable difference in the quality and wall pushing more in towards you. Also the size difference as well.

Also there are finer details like this south west of Cape Otway that come into play.

goofyfoot's picture
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goofyfoot Thursday, 2 Dec 2021 at 9:01pm

That Bravenes Rock out from Johanna, how deep is it out there?
Imagine some of the swells that have stood up over it if its shallow enough!

udo's picture
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udo Thursday, 2 Dec 2021 at 9:57pm

9.8 m
Surrounding is around 65 m

goofyfoot's picture
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goofyfoot Friday, 3 Dec 2021 at 6:20am

Ah is that a 9.8 on there, I couldn’t make it out on my phone. Thanks udo. Wonder if it breaks?

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Craig Friday, 3 Dec 2021 at 7:32am

Yeah it would for sure!

Hibidybibdy's picture
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Hibidybibdy Thursday, 2 Dec 2021 at 9:58pm

I’ve heard as the period increases so does refraction ? It gets into those hidden corners more.

Craig's picture
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Craig Friday, 3 Dec 2021 at 6:17am

Yep, owing to the swell feeling that ocean floor (bathymetry) to a much greater extent than lower period swell.s

Troppo's picture
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Troppo Wednesday, 1 Dec 2021 at 7:58pm

Great explanation, thanks Craig!

Random maths question - in the equation to calculate wavelength, the number is 1.56*period^2.

Where does the 1.56 come from?

Craig's picture
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Craig Thursday, 2 Dec 2021 at 10:40am

It comes from linear wave theory.

Wavelength = gravity*period^(2)/(2*pi)
Wavelength = 9.8*period^(2)/(2*3.14)
Wavelength = 9.8*period^(2)/(6.28)
Wavelength = 1.56*period^(2)

Solitude's picture
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Solitude Wednesday, 1 Dec 2021 at 8:51pm

Trying to think of how a wave like sunset works. I’ve never surfed a ‘wider’ wave. I know it’s got something to do with the take off potentially being in deeper water but anyone hazard a guess as to why the ‘wave crest’ is so thick (as well as tall).
Anyone who’s surfed it could attest to the length of board required to keep pace with these thick beasts (pro freaks aside - how they surf 10’ sunset on a 6’4 is beyond me)..........????

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bluediamond Wednesday, 1 Dec 2021 at 9:05pm

Haven't surfed it but have heard it breaks on a finger(s) of reef? So as the swell hits that reef on it's approach the part that hits the reef first would stand up(rear up) and to an extent slow down where as the shoulder of it running in such deep water would still be moving at pace,(but alot closer to the breaking part of he wave than normal) as opposed to a reef that has a more gradual bathymetry? Does that deep water onto shallow finger of reef also have some kind of convergence of the slower peak and the thicker faster moving shoulder enhancing the energy? Guessing way more to it but that's one possibility. Lucky bugger surfing there!!

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Solitude Thursday, 2 Dec 2021 at 12:00pm

Thanks for the considered reply mate. I like the theory. You really do see what you're saying out there, big peak (possible wave hitting more shallow finger of reef) with a fairly shouldery section next to it (deep bit). Far from a ruler edge wall as you would see at a more consistent bottom at other point breaks (Lennox, Honolua, etc).

Such a complex, intriguing and challenging wave. Can see why some people devote so much time to it. Would love to surf it again.

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bluediamond Thursday, 2 Dec 2021 at 7:37pm

Sweet Solitude. Good on you for getting amongst it over there. Sunsets always been the top of my surfing bucket list since i was a grom. Love watching the comp there every year too....on the big days not much seems to have changed in the approach....big board, big lines, big drops, big bottom turns. You get a few sick ones out there??

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Solitude Friday, 3 Dec 2021 at 8:07am

Yeah had a great time. One of those spots that has that air of something special when you paddle out.

When I got there it had been a really wet period so most of the north shore east of Waimea was putrid brown and far from what I had imagined it would be like for all of those years.

After a couple of days of umming/ahhing due to the conditions and the fact locals avoid brown water for fear of getting pretty crook, we thought fuck it and paddle out alone on a 6’ day. Was a really nice warm up for what was to come and with no crowd we were in brown Disney land.

It was late season so had quite a few surfs in that 6-8’ realm. A couple of sessions it got really serious (for me) at 10-12 - say triple over head. Was very tough to get into them with what I was riding.

What they say is true about the west peak. The moment you feel a little comfortable sitting where you think you’ll be sweet for the next set, something hideous will manifest out of the channel and swat you.

Had a great time up and down that coast. It gets busy, yes, but you can zig and zag. Highlight for me were some absolute magic afternoon sessions at rocky rights.

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bluediamond Friday, 3 Dec 2021 at 9:53pm

Great description Solitude. Thanks for that. Sounds unreal, and to get it with hardly any crew out. Whoa, 10-12ft Sunset is proper!! Good on ya mate.
Yeah that's crazy thinking about that West peak. I've talked to a few crew that have surfed there and that seems to be a feature of getting the wind put up them.
Sounds unreal over there and maybe late season not so hectic with the crowds....well maybe not this year with all the comps moved. haha.
Cheers mate. Could listen to stories of Sunset til the cows come home.

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mibs-oner Wednesday, 1 Dec 2021 at 9:11pm

Love the explanation Craig.

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Craig Wednesday, 1 Dec 2021 at 9:16pm

Some great questions above and I'll get back to all of the tomorrow.

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Tooold2bakook Wednesday, 1 Dec 2021 at 9:20pm

Great read. Thanks Craig

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channel-bottom Wednesday, 1 Dec 2021 at 10:08pm

Great article Craig.

Is there an upper limit of how long the swell period can get? I’ve read somewhere the mega Jaws swells are around 25 seconds, do they get over that level much?

And at what point does swell or the period begin to decay? If a swell began travelling at 2 metres at 10 seconds, on a constant bottom depth so no increased friction, and no land mass to get in the way, is the decrease a linear scale?

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Craig Thursday, 2 Dec 2021 at 11:38am

There is a limit but that only depends on the strength of the winds, fetch length, and duration of time.

Swell trains continue to sort themselves out and grow in period as the travel away from the storm but reach a maximum theoretical and observed period depending on those factors listed above.

That's why with a trade-swell of persistent 25-30kt easterly winds for days on end, we say it's a fully developed sea state. Ie it's reached maturity and hence periods coming out of that are likely to be in the vicinity of 10s but not get any larger, no matter how far away you are from the swell source.

Even if that fetch persisted for a month you'd never get 15s swell out of it.

So when you up the wind strength, fetch length and duration that's where you get the larger swell periods developing. But again only to a maximum depending on this values.

Very significant, long lasting storms do produce swells of 23s+ but these are generally only realised thousands of kilometres down the line once they've continued to organised themselves out into those long-period sets.

Onto your second question, because waves travel in groups and are always cycling over each other they conserve energy very well in the deep ocean.

It's non-linear though.

Half of the energy is usually lost in the first 400-600km of travel but after that it only halves again after 1,800-3,000km. So there's longevity in the tail of the swell.

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Jamyardy Sunday, 5 Dec 2021 at 4:03pm

Good article by the way Craig.

"So when you up the wind strength, fetch length and duration that's where you get the larger swell periods developing. But again only to a maximum depending on this values."

OK this is how the swell period is increased, how exactly is the swell height increased or is this just a function of distance from the swell source or a combination thereof ?

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stanfrance Wednesday, 1 Dec 2021 at 10:24pm

Craig, is a wave with an 18 second period moodier than one from a shorter period swell?

Craig's picture
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Craig Thursday, 2 Dec 2021 at 6:06am

Haha.

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ShaneK Wednesday, 1 Dec 2021 at 10:28pm

Craig, a question, please - perhaps for a future article. What is the relationship between swell period and consistency? There are two issues here - consistency of sets and number of waves per set. I remember being in Peru and having long period waves with 10+ waves in a set on a SW swell. It seemed very different to Vic which seems to only get more consistency with more medium wave periods and rarely seems to have such long sets...

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Craig Thursday, 2 Dec 2021 at 11:57am

Yeah this is complicated.

Consistency depends on how close the swell generating storm is to the coast, but that's just the start.

If the break needs longer-period energy and the storm only just reached a certain strength, you can see lots of mid-period sets not hitting properly or missing and then the odd better long-period set swinging in.

Also bathymetry might focus that long-period energy away creating less consistent sets while mid-period energy slips in more consistently as it's not 'feeling' the bottom as much.

We've found after forecasting the Mid Coast for decades that mid-period swells perform much better than long-period swells due to the energy pushing in with less bottom interaction, hence providing more consistent, and slightly bigger surf.

The less bottom irregularities that a swell can run into on its way into a certain spot will helps its consistency and also seeing those stacked, long-period sets with so many waves. It seems the further into a bay or shallow shelf you get the less waves in a set and also the less consistent the surf becomes.

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PeteWebb Thursday, 2 Dec 2021 at 6:08am

@solitude surfline.com/surf-news/mechanics-sunset-beach/59778

Solitude's picture
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Solitude Thursday, 2 Dec 2021 at 12:02pm

Cheers Pete, will revisit it, see if it helps explain the wave's breadth.

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harrycoopr Thursday, 2 Dec 2021 at 7:43am

Whew! Just go for a surf...

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Merrick10 Thursday, 2 Dec 2021 at 10:31am

Thanks Craig, an excellent article. I assumed (being a Vic surfer) a higher SP was always better but this provided a great insight in to how ocean energy works elsewhere, thanks.

Craig's picture
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Craig Thursday, 2 Dec 2021 at 11:58am

Thanks for the kind words all and keep firing off the questions.

wallpaper's picture
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wallpaper Thursday, 2 Dec 2021 at 12:12pm

There's something wrong here. Swell period is not the time that is taken for anything to pass a certain point.

It's the time that elapses between the arrival of a wave crest at a certain point and the arrival of the next wave crest at that point.

(edit: technically it doesn't have to be the crest, just any point and the same one on the next wave, though crests are a good reference point for ocean waves of course)

Craig's picture
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Craig Thursday, 2 Dec 2021 at 12:20pm

Isn't that what I've written..

"Swell period is the time (measured in seconds) between two successive wave crests (peaks), and is best observed when watching an incoming set passing a stationary object such as a buoy or fixed pylon".

And yes crest/trough, which ever part of the wave you want to choose.

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wallpaper Thursday, 2 Dec 2021 at 8:09pm

No. that wasn't what you wrote. in big letters on the photo is written 'TIME TAKEN FOR SUCCESSIVE WAVES CRESTS TO PASS A CERTAIN POINT', which frankly is garbage.

and

"Swell period is the time (measured in seconds) between two successive wave crests (peaks), and is best observed when watching an incoming set passing a stationary object such as a buoy or fixed pylon".

isn't really very clear either.

however I know you understand how it works, you just weren't able to express it clearly, but if you read what I wrote and think about it and why yours doesn't quite make sense then you will be welcome to use mine in future when you're writing for Nature,

yours pedantically

W

Craig's picture
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Craig Sunday, 5 Dec 2021 at 1:50pm

Hmm, I'm still not getting your point sorry. It is a tricky one to describe in words though and would be best explained with a video.

I'm guessing most understand what we're talking about though. IE look at a stationary buoy, see a wave crest pass it and then observe the time it takes for the next crest to move through. That's the period.

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Solitude Sunday, 5 Dec 2021 at 2:16pm

I reckon it’s pretty clear, we all get it Craig. Thanks.

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goofyfoot Sunday, 5 Dec 2021 at 7:53pm

Wtf are you on about. It couldnt be explained any clearer could it?
It’s been expressed very clearly, you just weren’t able to understand it.

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tango Thursday, 2 Dec 2021 at 1:54pm

Good article, Craig.

There's something of an analogy to draw between swell-generating storms and the availability of information. I'll admit, I'm a glutton for every source of information I can get my hands on, but I wonder where the logical end point is with our collective never-ending thirst for surf forecasting knowledge and understanding.

I liken the increasing availability of information/interpretation to the continuum of a a swell being generated on the open ocean. We started off with a light breeze of interest and not much info aside from the weather maps resulting in a hit-and-miss approach to surf forecasting where those in the know scored. As the weather predictions got better, the interest picked up over 20kts to a strong breeze - a bit of trade swell was obvious to the average punter and a few out-of-the-way spots started getting busy. In the 2000s we saw things really kick off with the modelling hitting new levels and the wind of interest hitting over 40kts with some proper swell becoming visible, but if you had your stripes you could still sneak in the odd epic and uncrowded day. Now we seem to be approaching the surf equivalent of constant hurricane strength interest and the lines of info are non-stop and everyone is tuned to all the data - the uncrowded sessions are getting fewer and further between, and the hype around each swell is now days out and bordering on absurd. I'm actually at the point where part of me dreads the A+ swells because everywhere is likely to be packed.

I'm not suggesting information not be made available, but I really think discretion is the better part of valour with this. I hope that can be taken into account in the next article which will look at the buoys. A part of me wishes that the sender of that email had just received a couple of links to journal articles and a suggestion to read them, but that wouldn't get a discussion going. It's such a double-edged sword.

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Craig Thursday, 2 Dec 2021 at 2:16pm

Appreciated the feedback Tango but more than giving away any trade-secrets here I thought this article was more along the lines of getting surfers to think about swell and the waves from a different perspective.

I find it fascinating and love passing on that inquisitive look at the world.

tango's picture
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tango Thursday, 2 Dec 2021 at 2:54pm

Yes, can't argue with that, Craig.

By the way, that can't possibly be the Bells bowl - there are two 3ft waves in a row with nobody on them.

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freeride76 Thursday, 2 Dec 2021 at 3:30pm

Tango, I still think you can use this info to duck and weave and escape the crowds.

Different factors have different levels of importance for different coasts and surf spots.

I know this is an unpopular view for Southern State surfers but I rate period of secondary importance for a lot of the East Coast, particularly above Seal Rocks (which Craig did allude too).

Swell direction and bathymetry are far more important factors here.

Not to mention the interplay between different swell trains which tend to overlap on the very wide East Coast swell window.

I'll say no more.

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tango Thursday, 2 Dec 2021 at 6:57pm

Yep, FR, loose lips and all that stuff, and a number of us do use the info to duck and weave. Every now and then it hits pay dirt.

There's no question the east coast and the southern coasts have different bells and whistles - after having the east coast pretty well dialled in it took me at least 10 years to start to understand the southern ocean.

I'll say nothing of the influence of period here, but I will note that discretion is of the utmost importance to ensure a bit of ducking and weaving is available for those who have put the time in.

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Solitude Saturday, 4 Dec 2021 at 12:04pm

Whilst I reckon too much info regarding specific swells is wrong, I don’t feel as though the ‘average surfer’ looks into it much more than what some of these forecast sites are predicting on a bar graph. . Could be wrong.

On the east coast it can be very much about chasing grains of sand about and knowing what tides to get it on. The beauty of this is that information required to score gets reset every couple of weeks or even days.

I think this article is lovely learning and good food for thought for those that already or are interested at looking into the surfing experience at a deeper (pun not intended) level

tango's picture
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tango Wednesday, 8 Dec 2021 at 9:29pm

Can't argue too much with any of that Solitude. Sometimes I feel like one of those stone statues guarding the entrance to a secret Inca tomb in an Indiana Jones movie.....there's something sacred about it all that requires some form of protection from the "uninitiated". A position which seems increasingly impossible.

Redmond Clement's picture
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Redmond Clement Thursday, 2 Dec 2021 at 5:09pm

I'm going to link my alarm clock to the Swell Buoy and set it at 15 seconds.

Standingleft's picture
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Standingleft Thursday, 2 Dec 2021 at 11:06pm

Wave dorks like me finding this utterly fascinating.

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Sheep go to heaven Friday, 3 Dec 2021 at 7:30am

I remember very clearly surfing a 23 second swell in south Sumatra many years ago . Fark, did it have some grunt in it ! You barely needed to paddle as it felt like the wave just had so much energy, it just picked you up . great article Craig :)

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Craig Friday, 3 Dec 2021 at 7:35am

After surfing 'soft' beach breaks and lower period swells on the East Coast It always catches me out first session when going back home and surfing the Southern Ocean reefs, how much quicker the longer-period swell comes in and breaks. There's a marked difference.

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petedj Friday, 3 Dec 2021 at 1:47pm

How do you determine the wave speed using swell height and period? I think that would be more useful information. Then you can figure out what board to bring to the beach factoring in drag and shape of the wave. I mean you have to use gravity to help you get up to speed, and of course a bigger board will help you get into the wave sooner, but no one actually likes big boards, but if the board is too short and lacking volume there is probably some limit where it is just impossible to get into the wave without chipping on the white water or free falling into a miracle landing under the lip. That would be the killer surf app, that gives advice on what board to bring based on swell info, your weight and the bathymetry of the break.

Craig's picture
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Craig Friday, 3 Dec 2021 at 2:08pm

Wave speed in the open ocean can be calculated using this equation.. 2.81*period. So a 16s swell travels at 45km/h and a 10s swell at 28km/h.

This changes when the wave moves into shallower water and starts to slow.

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petedj Friday, 3 Dec 2021 at 7:06pm

So the height of the swell doesn't matter if we are just looking at the speed? The taller swell will have more momentum p=mv because the taller wave has more mass. I'm trying to figure out if it's purely a drag issue that prevents heavier surfers from catching waves as easy as lighter surfers. Or if it's more to do with momentum and inertia. I mean you can adjust for drag with adding volume to increase buoyancy, but it still seems like smaller people can get into waves faster. My guess is there is less inertia a smaller person has to deal with to get up to plaining speed. Inertia is less of problem for bigger surfers when the wave has more momentum. I don't know, it's so confusing.

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Jamyardy Sunday, 5 Dec 2021 at 3:50pm

" but no one actually likes big boards,"
You should surf margs sometime !

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mpeachy Friday, 3 Dec 2021 at 4:13pm

Craziest example I've seen was when it was 2 foot down near Gerroa, 4-5 foot on Sydney's beaches, and 15 - 20 foot faces at the outer bombie at Long Reef (German Bank?). Would that random patch of shallow water off Longy have bent a SSW swell back towards the coast just enough for that spot?

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groundswell Friday, 3 Dec 2021 at 10:22pm

maybe the swell kicked in on your drive home.
I once went to aussie pipe and it was flat then on the way home checked stoney creek which started pumping but was deadlier than cape solander...then checked gerroa reefs which were pumping...im sure aussie pipe started pumping on my way home.

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nextswell Friday, 3 Dec 2021 at 9:55pm

Craig, really appreciate these articles. Thought I knew a little about weather and what drives our swell systems on the east coast but the depth of your articles put me in my place. That said after 15 yrs of regularly surfing the superbank, we moved south 3 yrs ago and I only surf my empty local despite the wind, swell period, direction and tide. I’ll take the average empty peaks over crowded perfection. Keep the articles coming.

udo's picture
udo's picture
udo Saturday, 4 Dec 2021 at 3:29pm

Shit there's some deep water between Bali and Java
Gosung Ratu rises to 9 mt...from 350 mt ..with 500 mt not far away
Would break in a Big Swell...?

Spuddups's picture
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Spuddups Monday, 6 Dec 2021 at 7:39am

Yep, there's some crazy shit going on between all those islands. Particularly the Lombok strait. There'll be waves in the most unlikely places depending on the tide. I've seen the North Coast of Sumbawa at 2-3ft. I still can't figure out how that is possible.

udo's picture
udo's picture
udo Monday, 6 Dec 2021 at 8:21am

I have seen pics of Nth coast Bali at 4-5 ft in Wet season.

Spuddups's picture
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Spuddups Monday, 6 Dec 2021 at 7:41am

...Some weird shit going on in Timor as well.