Analysis: The long and the short of long period swells

Craig Brokensha
Swellnet Analysis

When East Coast surfers plan their week around the surf forecast, most simply base their decision on the swell direction, size, and local winds.

Under most scenarios this is enough information to get a grasp on what the region is doing, however when the East Coast receives long-period swell energy things get decidedly more complicated.

The East Coast usually receives swells between 5-11 seconds which offer a fairly even distribution in size across locations open to the incoming direction. For example, a 3-4ft mid-period south swell in Sydney will come in around that size across all south-facing beaches, and a 3-4ft mid-period east swell, the same across all east-facing locations.

Yet when the period ramps up to, say, 15-16 seconds, and a proper groundswell hits the coast, then that pattern breaks down and similarly exposed spots can vary considerably in size. One south facing beach may be 6ft, while the other just up the coast struggles to reach 2ft.

The reason for this is that long-period swells 'feel' the ocean floor and its various irregularities, be that canyons or sea mounts, well before the swell reaches the coast.

Long-period swells are produced by strong, prolonged storms, and their kinetic energy travels well below the ocean surface.

Swells start to feel the ocean floor at a depth equal to half their wavelength, and this wavelength can be calculated by taking the swell period, squaring it and multiplying by 1.56.

Wavelength = 1.56*period^2

So for a 10 second swell it only starts to feel the ocean bottom at a depth of 78 metres.

When looking at a 16 second swell though, it starts to feel the ocean floor at 200 metres.

And a 20 second swell will start feeling the ocean floor at an incredible depth of 312 metres.

If you've ever had a close look at the bottom topography (know as bathymetry) around the world, or even at your local surf break, you'll see that the ocean floor rises up from depths of kilometres to the continental shelf which sits at about 100-150m before then grading slower to the land.

Small features like canyons from ancient river flows lead towards certain regions and these are what start to influence long-period swells well before we see them approaching the nearshore zone.

With long-period swells 'feeling' the ocean floor at depths to 200 to 300m one can easily visualise how a canyon at this depth would focus a swell towards one part of the coast, while taking the energy away from other locations.

This is why Nazare is such a large and unique wave. North-west swells follow a deep canyon that cuts 700-1000m deep through the much shallower surrounding shelf, ending at Praia da Nazare. Besides allowing less swell dissipation due to bottom friction it also sees long-period swells bend in on themselves as the deepwater energy travels faster than that on the shelf, creating monstrous wedges.

We don't have quite as pronounced setups on the East Coast, but there are certain locations that perform very well on these long-period swells compared to other similarly oriented zones.

Now that we've got you thinking about what's happening below the ocean surface, there's one more thing to consider.

Not all long-period swells follow the same path. If a 16 scond swell feels the bottom at 200m, and a 20 second swell at 312m, they're going to be steered by different bathymetric features. The best way to visualise this is to imagine the depth contours as highways; a 20 second dead south swell will be diverted to a slightly different region than a 16 second dead south swell as it will feel the ocean floor earlier.

Throw in a tweak in direction and it complicates the matter even further.

This is why under these long-period swell events we see the coast doing funky things, and we simply don't get enough of these swells to ascertain the patterns. Without running hi-res modelling going through each iteration of swell period and direction, it's still nearly impossible to pin down where the go to zones are.

There are a few reliable locations, but as we've seen before, they can also completely miss the swell, while another location just up the coast magnifies all the swell energy. This doesn't just apply to south swells running perpendicular to the East Coast (though these are where we see the greatest effects), straight east groundswells from New Zealand are known to focus north and south of what would be perceived as a perfect bullseye right in the middle. One such east groundswell years ago was 6ft in Newcastle and Wollongong, but Sydney, smack bang in the middle, only reached 2-3ft.

Over the coming week the East Coast will receive multiple long-period southerly swell pulses, each from different directions and with different periods, so it will be great to watch and see this happening in real time, especially with tomorrow afternoon's swell, coming in with peak periods of 18 seconds.

And if there's a take home message from this aricle, it's to keep an open mind and be prepared to travel.

Bathymetry datasets courtesy of Geoscience Australia and Navionics

Comments

Nick Bone's picture
Nick Bone's picture
Nick Bone commented Thursday, 17 May 2018 at 5:15pm

Where you get those bathymetry maps. I can never any?

Craig's picture
Craig's picture
Craig commented Thursday, 17 May 2018 at 5:17pm

Just added links to the bottom.

Nick Bone's picture
Nick Bone's picture
Nick Bone commented Thursday, 17 May 2018 at 5:32pm

I tried geoscience once but I got so lost. Downloaded some big thing and it didn't include any maps haha. Chances are I kooked it hard. Cheers!

Nick Bone's picture
Nick Bone's picture
Nick Bone commented Thursday, 17 May 2018 at 5:18pm

P.S. Good read and this definitely backs up my vicco threoy

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Thursday, 17 May 2018 at 5:38pm

Well don't be shy, spill the beans on your Vicco theory.

Ada gula, ada semut!

Nick Bone's picture
Nick Bone's picture
Nick Bone commented Thursday, 17 May 2018 at 6:09pm

I brang it up once in forecaster notes, people had a problem..

pittsy's picture
pittsy's picture
pittsy commented Thursday, 17 May 2018 at 8:45pm

people will have a whinge about anything, do share

geek's picture
geek's picture
geek commented Friday, 18 May 2018 at 10:35am

No, please don't Nick. Work it out yourself pittsy

pittsy's picture
pittsy's picture
pittsy commented Friday, 18 May 2018 at 3:16pm

OMG vicco surf secrets!!!! Some of you blokes get way too caught up in that stuff...

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Thursday, 17 May 2018 at 5:37pm

Interesting read another good article

Ada gula, ada semut!

simba's picture
simba's picture
simba commented Thursday, 17 May 2018 at 5:38pm

yep nice one Craig ,just shows to go that you learn something every day...or should try to.
Throws a new light on why big ground swells do weird shit at times.Although its pretty amazing that a big 20 sec swell can feel the ocean floor at 312 meters....

simba

crabs's picture
crabs's picture
crabs commented Thursday, 17 May 2018 at 8:04pm

Ah, sorry all those canyons are down slope of the 1000m contour and well below wave base even for our longest period swells (except for tsunamis).
I would note though, most of the bathy on the shelf (in the area of interest for wave modelling) is not that good which doesn't help on the modelling front.

Craig's picture
Craig's picture
Craig commented Friday, 18 May 2018 at 8:07am

You're correct. These features were easiest to point out for the purpose of the article so people would get a better understanding of other influences below the ocean surface.

Some actual features can be seen here off Jervis Bay and inshore irregularities heading towards Sydney and the Northern Beaches.

freeride76's picture
freeride76's picture
freeride76 commented Thursday, 17 May 2018 at 9:43pm

what are the depth contours on the mid-tasman ridge?

Craig's picture
Craig's picture
Craig commented Friday, 18 May 2018 at 8:11am

It rises to about 1000m from 4000m.

truebluebasher's picture
truebluebasher's picture
truebluebasher commented Thursday, 17 May 2018 at 10:57pm

Insightful Craig! Eerily suspenseful as well.
Thanks also Nick for "Navionics" Map request.(Score)
Dr No target range finder with Super Ministry spy zoom. (Worn it out already!)

garyg1412's picture
garyg1412's picture
garyg1412 commented Friday, 18 May 2018 at 7:58am

Now all we need to do is nuke King Island and get some of those 20 sec swells into Bass Straight.

radiationrules's picture
radiationrules's picture
radiationrules commented Friday, 18 May 2018 at 10:45am

Hi Craig > awesome explanation and links add to capacity to explore. Two questions:

1. That map of Sydney shows 4km offshore depth; and the canyons that trap the swell are obviously shallower. Is there a common ratio that makes this better for big wave spots? That is, to make ratio of offshore depth: canyon depth and/or the ratio of the length that gradient has to change to force a ledge to jack-up in certain way, fro example like Peahi does?

2. Without wanting to break any cone-of-silence, where is the best example of big-wave bathymetry in Australia? I want to go and look closer on the mapping links you have sent to get a feel for the ratio of the components: offshore depth, canyon depth and ledge depth, etc

Many thanks

Craig's picture
Craig's picture
Craig commented Friday, 18 May 2018 at 11:21am

Not sure on the ratio but what you want is the shelf break as close as it can get to the spot, and as steep a gradient as you can get leading to the spot, before backing off a bit before the wave so that it doesn't full slab.

Have a look around Esperance way where the granite reefs come straight up out of the ocean with the shelf not that far away at all. All mostly unsurfable slabs but very interesting.

radiationrules's picture
radiationrules's picture
radiationrules commented Friday, 18 May 2018 at 11:41am

craig > will do; many thanks. will have a look and revert.

stunet's picture
stunet's picture
stunet commented Friday, 18 May 2018 at 12:07pm

@RR,

I don't think it's as simple as finding an ideal ratio as there are more factors that go into creating a great big wave spot then gradient. Also, some of those factors would seem to contradict each other.

For instance, as Craig's mentioned, Nazare is the product of a deep canyon that cuts through the shelf. The waves are big because less energy is lost via bottom friction and the interplay between the deep channel and the shallow shelf makes for wedges that amplify the true height of the swell. Puerto Escondido is another beneficiary of a deep canyon breeching the shelf.

Conversely you have big wave spots such as Mavericks and Pipe that dont have deep water offshore but comparitively shallower water offshore. Mavs has a seamount, Pipe a string of reefs, and each serves to focus the incoming swell before aiming it towards the inner reef where it hits with greater size and force.

On a grander scale, I've often wondered if this is the reason the Margs region gets so much more swell than Mandurah. Both are fully exposed to the southwest swell engine so should theoretically get similar swells, except they don't - Mandurah is always much smaller. Is that because the Naturaliste Plateau, which lies offshore from Margs, directs the swell towards Margs and away from Mandurah?

Craig's picture
Craig's picture
Craig commented Friday, 18 May 2018 at 12:17pm

Having a look at Mandurah, there's a shallow fringing reef/shelf extending 10km+ out to sea of only 10-15m depth, so this would absorb a lot of swell.

Compare it to Margs which goes from 50m to shallow right on the coast..

radiationrules's picture
radiationrules's picture
radiationrules commented Friday, 18 May 2018 at 12:53pm

Hi Stu > thanks for that; what you've written adds to some of the thought's I've been having. It's an endlessly fascinating topic for me. The only reason I got onto the ratio idea, is I saw Craig's formula - Wavelength = 1.56*period^2 - and thought there may have been some other equation that I'd never heard off? Even that I might have stumbled on the golden mean of surf prediction for a minute of so - ha!
In real life, I love looking a waves approaching you at a somewhat predictable line-up like Tombstones, trying to catch "the one", the one which varies by 5 degrees and hugs the ledge in a completely different way. My enquiry with Craig, is an extension of that thought process. It's awesome knowledge that you guys share on this level, greatly appreciated.

I did an art work about the Cow Bommie's bathymetry in 2016; based on oral history from Camel, Antman and Damon Eastaugh - I can send you/Craig a few pics if you're interested.

stunet's picture
stunet's picture
stunet commented Friday, 18 May 2018 at 12:55pm

Yep, very interested. Feel free to email when you get a chance.

thedrip's picture
thedrip's picture
thedrip commented Sunday, 20 May 2018 at 9:51pm

I’ve been out there and sounded around. It’s an almost vertical cliff face that faces pretty into any SW swell. It jumps from 50m-ish (it was a few years ago) to 12m depth in the space of a couple of metres then tapers of slowly back to 50m over a couple of hundred metres. Think a big wedge.

I’ve also been out there on a tow day. It’s frightening and my tow partner and I wanted nothing to do with it.

radiationrules's picture
radiationrules's picture
radiationrules commented Monday, 21 May 2018 at 10:14am

hey drip > thanks for that info; and your honest too (about the frightening)!

Gary G's picture
Gary G's picture
Gary G commented Friday, 18 May 2018 at 11:11am

Gary's no scientist, so he wants to make sure he understands what you're putting out there, Craig.

Say you have a short period swell, who we'll name Craig, and a longer period swell, who we'll name Gary.

What you're saying is that because of Gary's extra girth he'll hit all sorts of out of the way places on the canyon (compared to Craig) and this could be uncomfortable if you haven't experienced many Garys in the past as you won't know which positions will reap the best rewards?

But if you've paid attention to what works in the past, you'll have a wet and wild time to remember?

We
are
fa-mi-ly:
I got all my Gary's and me

Craig's picture
Craig's picture
Craig commented Friday, 18 May 2018 at 11:22am

Haha, that takes if for the best post!

Nick Bone's picture
Nick Bone's picture
Nick Bone commented Friday, 18 May 2018 at 5:51pm

I love how this guy’s mind works.

Clam's picture
Clam's picture
Clam commented Friday, 18 May 2018 at 11:53am

"And a 20 second swell will start feeling the ocean floor at an incredible depth of 312 metres."

Craig is there a difference if the 20s swell is 1m ? To , for example 4m ?
Is the depth 312m any different ?

Craig's picture
Craig's picture
Craig commented Friday, 18 May 2018 at 11:55am

Good question, but no, it's just that there'll be a little more energy stored at that depth.

strictlybizness's picture
strictlybizness's picture
strictlybizness commented Friday, 18 May 2018 at 1:47pm

Unreal post as always Craig. Thanks for sharing.

billie's picture
billie's picture
billie commented Friday, 18 May 2018 at 9:08pm

Wow. That tickles my intellect for the evening. Thanks a lot all contributors.

Billie

trolio's picture
trolio's picture
trolio commented Friday, 18 May 2018 at 10:51pm

35 years on the sea and I still love looking out the window
Great to know how and why and pass on to the young fella
Good stuff Craig
Cheers

Gaz1799's picture
Gaz1799's picture
Gaz1799 commented Saturday, 19 May 2018 at 10:03am

Great article, whats the significance of 1.56?

sdizzle80's picture
sdizzle80's picture
sdizzle80 commented Monday, 21 May 2018 at 11:42am

1.56 = gravity divided by 2 x Pi
just a quick way of doing the calculation

Craig's picture
Craig's picture
Craig commented Monday, 21 May 2018 at 11:54am

Yep, nailed it.

batfink's picture
batfink's picture
batfink commented Sunday, 20 May 2018 at 2:27pm

Great article Craig, but trying to work things out based on period, direction and bathymetry takes you into chaos theory. 180+ degrees of swell direction, and even slight variations in direction and period will lead to large variations in outcomes. Observation over time and plenty of hindcasting will give a feel for how a swell will play out.

Then there are the issues of the local banks at the time. Then there are those mystery spots that always out-perform every other wave in that region, most with no established explanation.

Best left as a mystery to be revealed with each iteration, but don't let me discourage anyone from trying. Always good for the mind to try to imagine what's happening down there.

gearoid's picture
gearoid's picture
gearoid commented Tuesday, 22 May 2018 at 8:18am

Hello Craig,

what is the general rule for wave refraction vs. period/wavelength? Does a long period swell refract more or less than a short period swell?

Also, for waves of the same size, do you get more refraction in shallow water or deep water (ie low vs. high tide)?

Apologies if answered somewhere else on the thread and thanks.

Craig's picture
Craig's picture
Craig commented Tuesday, 22 May 2018 at 1:53pm

Firstly long period swells refract more as they feel the bottom earlier.

And the shallower the water the more the refraction as the wave slows down and bends as it 'feels' the bottom.

gearoid's picture
gearoid's picture
gearoid commented Tuesday, 22 May 2018 at 6:25pm

Thanks Craig,
So is that a major factor when it comes to Victorian conditions or do these things have a minor effect in real world conditions? I'm thinking a double bonus effect in lower tide / long period swell?

mowgli's picture
mowgli's picture
mowgli commented Tuesday, 22 May 2018 at 10:39am

Excellent article. Pretty civilised comment thread too for a change :P

Felt compelled to comment (normally all I ever do is lurk, lurk, lurk, lurk).

Not exactly sure where Shipsterns is but it looks like one great big shelf off southern Tassie.

“Life is a long lesson in humility.”

mowgli's picture
mowgli's picture
mowgli commented Tuesday, 22 May 2018 at 10:37am

For those boffins on the sunny coast, there was this report commissioned by the State a little while back.

Go here: https://qldgov.softlinkhosting.com.au/liberty/libraryHome.do
Copy & paste this into the search field: Queensland Coastal Risk and Bathymetric LiDAR
Click on the report link, and click on link to full text for Appendix 1.

The relevant images are on pages 17 and 18. The Sunshine and Sunrise are clearly visible. As well as a few shallow features offshore I had no idea about north of Stumers Ck, off Maroochy R. mouth. Didn't realise the headland at Coolum extends so far out either.

“Life is a long lesson in humility.”

thermalben's picture
thermalben's picture
thermalben commented Tuesday, 22 May 2018 at 11:49am

Excellent, thanks.

dangerouskook2000's picture
dangerouskook2000's picture
dangerouskook2000 commented Wednesday, 23 May 2018 at 6:36pm

Hey Craig,
Is that why we don't get periods bigger than 18 seconds? Because at 18 seconds its wavelength is 505m and the ocean floor at the shelf is only 200m? So technically it hits the ocean floor at 250m (half it's wavelength) so it loses heaps of energy? Or am I way off track

Craig's picture
Craig's picture
Craig commented Wednesday, 23 May 2018 at 7:21pm

You talking East Coast? No main reason is because we're not exposed to storms with enough strength to generate noticeable size at these periods.

Swells do lose energy once they hit the shallower water, but still make their way to shore, and if there's enough size, it'll show in wave height.

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