Waves Misbehavin': Why the size varied during the East Coast swell
The weekends incredible swell event on the East Coast was a striking example of how swell period effects wave size.
Most surfers already know how the period - the time between consecutive wave crests - effects the power and size of a swell. In short: the greater the period, the more energy the swell train carries and the more powerful the swell is. The visible manifestation is long, well-defined swell lines that increase greatly in size when they 'feel' the shoreline. They also have the ability to refract (bending: as in around a headland or point) a greater amount when they feel the bottom then lesser period swells.
This is due to the fact that long-period swells carry much of their energy deep below the ocean surface and, as they approach shallow water, they feel the ocean bottom earlier than short-period swells.
The swell that hit the East Coast had its genesis in a storm near the Antarctic ice shelf. By the time it reached the East Coast it had travelled over 3000 kilometres and the period had drawn out over 15 seconds. Periods of this magnitude are very rarely seen on the East Coast and thus the confounding nature of the swell caught some people by surprise.
On Friday afternoon reports of twelve foot plus waves came in from certain Central Coast, Cronulla and Shellharbour-region waves. See todays Wave of the Day (and stay tuned for tomorrows!) for proof of this. Yet, during the same period beaches that often get swell, and faced the same direction as those listed above, barely got over six feet. What was happening?
Well, here's an analogy: If you've ever been to, or seen photos of, Sunset Beach in Hawaii you may have noticed that the peak at Sunset can be ten feet while the reef at Kammie's - not 200 metres away - can be half that size, and the channel seperating them can have no waves except a chest-high shorebreak. The size difference in a short stretch of coast with equal access to swell is startling.
What is happening is that the long-period swells hitting the North Shore of Hawaii feel the offshore reefs beyond the wave zones and the swell train refracts and focuses its energy toward different parts of the coast. The result is a magnification of swell energy in some areas and a paucity of it in others. In this case, Sunset Beach receives an abundance of energy while Kammie's doesn't. It all depends on the shape and depth of the reefs offshore.
Meanwhile, back home, we had a swell of Hawaiian-style period hitting the coast and behaving in a way we were unfamiliar with. The waves were feeling the ocean floor further out than is usual. For those people wanting to make use of large waves on the East Coast there were valuable lessons to be learnt, and it would serve those people well to remember how their local big wave spot behaved during this swell.
Yet there were also lessons to be learnt for the average surfer. Since the weekend there have been a few comments appearing on websites and Facebook pages that the swell was a hoax. One can only assume that the people making those claims visited beaches that didn't receive the full extent of the swell. It's important to understand that the forecast had to cover the whole coast, yet, as I've stated above, certain regions behaved very differently.
Obviously this swell wasn't a hoax and Swellnet forecaster, Craig Brokensha, got the numbers and timing correct.
Also, there was much media hype over the 18.4 metre wave that passed the Cape Sorell wave buoy on Thursday. Because that wave and the East Coast swell had a common source there was an expectation that we would see a far larger swell. It's important to note the waves that passed Cape Sorell were in a very different stage of evolution when that wave was recorded. Cape Sorell was very close to the source and completely exposed to the full extent of the storm. On the other hand, the swell that hit us had two days to calm down, draw out the period, and depreciate in size. And, very importantly, the swell was heading north-easterly up the Tasman Sea toward Fiji. The waves we saw had spread out radially toward the East Coast and lost some of their energy in the process.
If anyone has any questions about the swell or the characteristics that were on display (refraction, period, radial spread etc.) please type them below and our East Coast forecaster, Craig Brokensha, will answer them for you.//STUART NETTLE
Frontpage image for this story taken by Matt Burgess
Comments
What? Swell? On the East Coast? Wonders will never cease!
It weren't a hoax hodad.
T'was the real damm thing.
While the exact science between bathymetry and swell period is still being worked out swells like this one and the Post-Wati south swell gave very valuable clues.
The article probably skimmed over the other big reason for the variation in wave height. That was the swell direction. It was due southerly in direction, so while Merewether near Newcastle was 8-10ft on Saturday, Manly was 3-4ft. A quick look at the shape of the coastline on a map will make it clear that beaches that have a southerly aspect picked up far more size. Obviously the wave period came into play but you could see the swell steaming past Fairy Bower, almost parallel too the coast. The other big problem is that the forecaster is originally from Adelaide, so cannot be trusted!
To an extent that is true Alex, and that's why I included the line: "faced the same direction as those listed above". Thing is though, dead south swells are not uncommon on our coast. In fact, we get them quite often, yet the size variation last weekend was unlike anything I can remember, even during other dead south swells.
It would be instructive to get some bathymetry maps of the east coast and correlate the contours and canyons with the amount of swell exposure.
This sort of knowledge is more advanced in the Northern Hemi where long period, long range groundswells are more common.
Your comments are correct Alex, but we still saw large variations in wave heights between different locations which were open to same amount of southerly swell, or were aligned exactly in a same orientation.
The Bathymetry has a big part to play with various canyons and ridges focussing wave energy into some locations, while steering it away from others, and that's the big lesson here. Trying to see which period swells hit certain regions of the coast compared to lesser or greater period swells of the same size.
Would love to get hold of some bathymetry maps Steve. Pore over those bastards and see why breaks do what they do. That would be great.
Just to clarify Alex. Three big wave spots on the south coast of NSW that need south swell to get going: No Toes, Red Sands and the one in todays WOTD. All are sheltered from south swell to the same extent as, say, Winki at Manly (Go Ogle Earth to see). Yet while Winki struggles in a south swell the other spots fire.
The reason you were seeing the south swell running at 90 degrees to the coast is, I assume, because there aren't any offshore reefs to focus the swell in toward Deadies, Winki and The Bower while at the three spots listed above there are.
So how deep, or shallow does it have to be for bottom contours to come into play in a swell like that?
reall interesting shit..pitty i live in a place where wer're lucky to see 2 foot waves off a south swell all over the coast haha
Some time ago, and after I had a few weeks down the south coast - and watched this spot doing the exact same thing when other places around the place were only half the size - I went into the city and into a government building in Bridge street that sold maps of the Continental shelf off the east coast of Australia.
- and there it was, a gap in the shelf off shore the spot pictured here - that's why.
Well it isn't just gaps mister.
It's also offshore shoals and reefs which refract swells back into the coast, focusing it on certain bombies, headlands or both.
Sometimes it is a combination of both, as when a large river system has gouged a canyon in the shelf .......but now we have said too much.
A few thoughts and examples on the effect of bathymetry on effective wave height here: http://www.surfingatlas.com/articles/teahupoo_anatomy_of_a_monster
Mavericks is one of the best examples of the effective of shallow water canyoning on wave height. At the end of the canyon is a large solid ramp of basalt that stretches from about 100 metres deep to 10 metres. Everything passes into the canyon and up the ramp
hey craig, why was angourie so shit on saturday?
Benski, waves start to feel the bottom when they are in water shallower than half their wave length.
So if you have a 15 second period swell the wavelength (distance between 2 consecutive crests or troughs) is approximately 350m in deep water. So this swell will start feeling depth contours at 175m.
I think those numbers are correct, but you get the basic idea of when swells feel the bottom.
Thanks craig.
Kookus, could you please elaborate..
Ie, was the swell really inconsistent, small, fat, were winds onshore etc?
South swells aren't especially great at Angas as they push wide on the reef and are usually fat.
A couple of simple questions, not quite on topic, but trying to clarify the language of the discussion:
1) What exactly is wave height? A 2 mt Hmax swell on the Manly buoy, with decent period, will be head high at Curly, seen on the face, and be called a 2 ft surf. Errr ... what? Surely somewhere in chapter 1 of the Surf Predictor's Handbook is a definition of these terms, which I'd interested to hear.
2) It is well known that wave velocity increases with wave height with a very strong relationship. This makes big swells doubly energetic, the waves have more water and they travel much faster. So a 2m Hmax wave with a 10 second period will be well-formed but small, but a 5m Hmax wave with a 10 sec period will be mushy. So is there a relationship between height and period, such that waves below the line (i.e. short period for their height) will be lousy, but above will be good?
Thanks for a really great website & discussion !
Psingle,
1) If you're talking wave heights that the wave buoy's measure then Significant Wave Height (Hsig) is the average of the highest 1/3rd of all recorded wave heights during a fixed time. So this gives a good average of the size of the swell hitting the coast.
The Maximum Wave Height (Hmax) is the highest wave measurement during the period and can be as you've seen, more than twice the Significant Wave Height. You usually see a greater difference between Hsig and Hmax when there is a lot of windswell contamination as you can get both the swell and seas created by the winds mixing too a higher than normal wave height. (This is why we saw a Hmax of 18.4m of Tassie last week, as there was a lot of windswell in the mix with the groundswell).
Regarding wave size calling by me in the forecasts, I generally stick by the rule that just below shoulder high on the face is 2ft, head high is 3ft, double overhead is 6ft, triple over head is 8ft and so on. Obviously this changes from person to person, but if you follow the forecasts closely you'll pick up on my scaling of waves.
Equating the Hsig or Hmax to wave heights on the beach depends on numerous factors such as the period of the swell, it's direction, the actual nature of the beach and so many other little things that you can only use the buoy's as a general guide as to what to expect at the beach.
I would try and steer away from using Hmax though as your guide and keep an eye on the Hsig to calibrate yourself with your beach, as Hmax can change considerably depending on the local sea state around the buoy.
2) The period is directly related to the wind strength, so the greater the winds within a storm, the greater the swell periods will be once the swell moves away from the storm. There is a maximum period in which a swell can reach depending on the wind strength, but generally the further the swell travels away from it's source, the larger the period grows.
Now larger period swells travel faster than lower period swells and they also carry more energy. If we have a strong frontal system pushing past the southern Tasman Sea 1,000km's south of Sydney between Tasmania and New Zealand (as we have been seeing over the last week or so) we can expect to see a S'ly groundswell arriving 1.5-2 days later but with a strong period of 14-15seconds depending on the strength of the winds. This will be seen at the beach as long and evenly spaced lines stretching right across beaches and reefs if local winds are offshore.
If a low forms right off our doorstep and produces a burst of 40kt winds up our South Coast, the period doesn't get a chance to draw out to 14-15 seconds and the swell is not as well organised and you may see a 5m swell at 10seconds on the buoy and it will appear more raw and closely-spaced (Also there's probably a lot of local windswell added into the mix).
As this swell travels further up the coast and away from the storm it will gradually become more organised and the period will grow bigger, and beaches receiving this swell to our north will then see a more spaced out swell and it will also feel stronger.
I hope this answers some of your questions.
Just a note, these numbers are just examples and every swell is different.
Cheers, Craig
Yeah Craig, your numbers look pretty spot on for depth of swell below the surface. Here's a couple more:
14sec=160m
17sec=230m
20s=320m
Pretty impressive depths when you think about it!!!
Yeah, very impressive.
I remember a very large "stormy" event on the SA Mid Coast about 2.5 years ago where a well known left point got to 3-5ft, and this is in Gulf St Vincent (GSV).
My mate dives and said that a shipwreck which they frequent often within the gulf often had been moved some distance from it's previous position which it had been in for decades. I think something in the order of 30-50m's. The average depth of GSV is 21m, but there are area's where it reaches 40m or so. So this wouldn't need a very strong swell to start stirring up the bottom and moving objects around.
Keeping in mind that the shelf is around 200m deep we can see that swells over 15 seconds will interact with various canyons and ridges, while as we move closer into shore, the effects become much greater and lesser period swells start interacting with the sea floor bathymetry as well.
a lot of people dont realise that them lines on the weather map dont indicate the direction of swell. its actually about 20 degrees off kilter from the isobars, pointing inward towards the low pressure centre.
Craig, you used a lot of words to say:
1. Your forecasts and reports are half the wave face height.
And you left out:
2. Your reports get tweaked +/- 1 to 2 feet to match the prophesies.
Actually boussinesq, Craig doesn't provide a daily surf report for Swellnet - he merely filled in for me for a couple of weeks while I was overseas. However, I can guarantee you that none of our surf reports are 'tweaked' to match its corresponding forecast as you've suggested.
Boussinesq, I used a lot of words because people have asked various questions and I wanted to give a comprehensive answer.
I'm sorry my love and interest in the topic made me ramble on...
Secondly, I did the Manly surf report for 3 weeks while Ben was away (this ended 2 Friday's ago).
At no stage did I ever tweak the report up or down to match my forecasts. I went to the beach, watched the surf, and got images to back up my size reporting and posted to the site.