The waves you've never heard of

Craig Brokensha
Swellnet Analysis

It came as a surge, pushing inshore and at a rate to what I'd expect a small scale tsunami to move. Paddling against it was impossible, the rock wall only 15 metres away being a great reference point - I was losing ground despite my efforts. In the end I had to jump off the board and anchor myself, wait for the surge to stop, and then resume paddling as it retreated back out to sea about thirty seconds later.

While surfing South Steyne during the peak of last week's south swell an interesting phenomena was taking place. One that I've experienced before but not enough for me to look further into. The surge in the sea was due to the presence of 'infragravity waves', something different to the normal swell waves we usually surf. In scientific circles swell waves are called gravity waves, so infragravity refers to waves that occur within gravity waves.

We know that wind creates waves, initially tiny capillary waves, growing bigger and becoming more organised into windswell and then groundswell with increasing strength, length and longevity.

As these swell waves travel away from their source they sort themselves into wave groups - what you and I call sets - that contain individual waves of similar size and period.

If we imagine these sets travelling through the deep ocean you'll have an area of increased wave amplitude (wave height), maximised in the middle of the set and then immediately ahead and behind this a low point in amplitude.

Because the larger waves carry more momentum with them, there's a slight decrease in the water level under the larger waves and an increase under the small waves.

This in turn creates a long wave spanning the peaks and troughs between wave groups [see image below], with periods upwards of twenty five to thirty seconds though they can reach up to five minutes. These are known as infragravity waves, and they're bound (attached) to the wave groups they are travelling with.

In the open ocean, the height of infragravity waves are only in the order of centimetres but as they approach shallower water they increase in height much like swell waves increase in size just before breaking.

This effect - which is also known as 'surf beat' - is most apparent on slowly sloping beaches, and can reach up to 1 metre in height under big swells.

However, instead of breaking like swell waves, infragravity waves surge up the beach, and this is what I experienced multiple times during last Tuesday's swell event. During large swell events, storm-surges and coastal inundation beyond the normal high tide line are usually the result of infragravity waves.

My personal experience last Tuesday was the most noticeable in my surfing life and I'd put that down to the very dynamic and incredible swell event that was created directly off the coast, with wave heights rising to 7-14 metres offshore. With such a large, energetic sea state and South Steyne having the gradual sloping sea floor, the infragravity waves would have been in the order of 1 metre once inshore, surging in and out towards the beach at a rate you can't fight against, even on a board.

While infragravity waves were discovered by the late Walter Munk in the 1950's the way they seperate away from swell waves once these themselves break inshore is still not fully understood. Sometimes they reflect back out to sea and at other times on steeper shores they dissipate much quicker.

The implications of infragravity waves regarding the transportation of sand has been researched more, and observations show that they influence the temporal behaviour of rip currents, fostering the detaching of eddies which than transport sand alongshore.

Next time you're struggling to push out across that sand bar that seems to be constantly surging you inshore, have a think back to this article and maybe wait a minute or so and try and push out as the infragravity wave recedes.

Comments

freeride76's picture
freeride76's picture
freeride76 commented Tuesday, 11 Jun 2019 at 2:42pm

thats really interesting Craig.

I think we experience a similar phenomenon here where during the heavy S to SE swells it seems for a time the whole ocean moves and shunts you northwards.
I think that is an infragravity wave running obliquely to the coast.

Craig's picture
Craig's picture
Craig commented Tuesday, 11 Jun 2019 at 2:58pm

Yeah totally, I've experienced it as well quite a lot when surfing bigger point break or reef surf.

From the literature in regards to what they call 'edge waves'.. "but with obliquely incident waves and the breaking-induced alongshore current an asymmetric distribution in dominant propagation direction is created."

IE oblique infragravity waves propogate down the point/reef.

tidak_bagus's picture
tidak_bagus's picture
tidak_bagus commented Tuesday, 11 Jun 2019 at 3:09pm

anyone who goes worming on the beach should be well versed in infragravity waves, even if they were unsure of the name.
I was under the impression that the bound energy is released in the surf zone as you say but energy is reflected between the shore and back of the surf zone creating a standing wave, similar to sloshing water in a bath tub back and forth. the result creates nodes and antinodes which has a big impact on sandbar development.

Craig's picture
Craig's picture
Craig commented Tuesday, 11 Jun 2019 at 3:11pm

Yep you're correct, this also happens, there are many scenarios that can take place, including as you said standing waves, harbour seiches etc.

alexz's picture
alexz's picture
alexz commented Tuesday, 11 Jun 2019 at 3:18pm

These long waves are particularly problematic in Harbours for moored vessels. My colleagues at MetOcean Solutions have developed a forecast product for Ports that models these lesser known waves - https://www.metocean.co.nz/long-waves

A.Z.

Craig's picture
Craig's picture
Craig commented Tuesday, 11 Jun 2019 at 3:22pm

Nice Alex and great video in your link..

thermalben's picture
thermalben's picture
thermalben commented Tuesday, 11 Jun 2019 at 3:25pm

'Surf Beat' is a great name for a New Wave band, I reckon.

I'll get my jacket.

surfstarved's picture
surfstarved's picture
surfstarved commented Tuesday, 11 Jun 2019 at 4:05pm

That ship looked like it was jiggling to a catchy surf beat.

Fascinating stuff Craig, on a subject I've never heard even whispers about until now. It's interesting how the infragravity waves seem to peak when the regular sets ease off into a lull, then dip when the next set reaches its peak. The universe is an endlessly fascinating place.

Don't let the bastards grind you down

squigz's picture
squigz's picture
squigz commented Tuesday, 11 Jun 2019 at 4:22pm

Interesting phenomenon. From an audio engineers perspective it makes sense that a number of set waves could have an underlying fundamental frequency. Each set wave is briefly in alignment/ perfect phase leading to a large output of energy.

Thegrowingtrend.com's picture
Thegrowingtrend.com's picture
Thegrowingtrend.com commented Tuesday, 11 Jun 2019 at 5:11pm

Craig, i also experienced this paddeling out at south styene. same time..

The only other time iv experienced this was up the coast while on a 'trip' and ended up saving 3 poms. within a seconds the flash rip was gone and was back to getting barred.
crazy experience.

Nicko

Craig's picture
Craig's picture
Craig commented Tuesday, 11 Jun 2019 at 7:20pm

Yeah how insane was it. I’ve had a few messages in Instagram from others at other breaks experiencing the same thing. Oh and just when the swell was starting to kick proper Tim, Max’s dad from Queensy was out and said the water started moving and swirling weirdly. This would of been the same phenomenon. 

Thegrowingtrend.com's picture
Thegrowingtrend.com's picture
Thegrowingtrend.com commented Tuesday, 11 Jun 2019 at 8:40pm

Mean while Max caught the biggest barrel iv ever seen...
Quantum

Nicko

Max Wax's picture
Max Wax's picture
Max Wax commented Wednesday, 12 Jun 2019 at 1:34pm

Can confirm both of the above haha

Craig's picture
Craig's picture
Craig commented Wednesday, 12 Jun 2019 at 1:35pm

Soo... tell us, anyone grab photos of it?

Max Wax's picture
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Max Wax commented Wednesday, 12 Jun 2019 at 1:38pm

Cant confirm that unfortunately, it was earlier in the day around 12. Not so many cameras out then maybe

glennod's picture
glennod's picture
glennod commented Tuesday, 11 Jun 2019 at 5:43pm

Tony Butt in his excellent book Surf Science talks about infragravity waves. Wonder what effect they would have on Greg Webbers proposed V wall

Sam Mozaffari's picture
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Sam Mozaffari commented Tuesday, 11 Jun 2019 at 5:48pm

This looks like one of those waves close to the bommie that were coming through completely sideways, crazy!

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Tuesday, 11 Jun 2019 at 5:56pm

Another good article.

Chaz's picture
Chaz's picture
Chaz commented Tuesday, 11 Jun 2019 at 7:11pm

so,is the bounce off a rock wall or point an infragravity wave or just a bounce of normal swell?

Craig's picture
Craig's picture
Craig commented Tuesday, 11 Jun 2019 at 7:18pm

Nah, that’s just the swell waves bouncing and reflecting as you’ve said. 

jimbrown's picture
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jimbrown commented Tuesday, 11 Jun 2019 at 8:03pm

So fascinating. Could I have some help with this aspect: "Because the larger waves carry more momentum with them, there's a slight decrease in the water level under the larger waves and an increase under the small waves". Momentum is the mass and velocity of an object (the gravity waves, in this case). How does the greater mass and velocity of the larger waves necessarily lead to the slight decrease in water level and the opposite for small waves?

Craig's picture
Craig's picture
Craig commented Tuesday, 11 Jun 2019 at 8:08pm

Don't quote me on this but I believer it's because these larger waves displace more water, hence decreasing the ocean level more as a whole.

Laurie McGinness's picture
Laurie McGinness's picture
Laurie McGinness commented Tuesday, 11 Jun 2019 at 8:24pm

Larger waves have more momentum than smaller waves travelling at the same speed simply because they have a greater mass. I'm thinking that because water molecules cohere quite strongly, larger waves actually carry water forward leaving a trough in the infra-gravity wave behind them ......... but I could be wrong.

jayet-010's picture
jayet-010's picture
jayet-010 commented Tuesday, 11 Jun 2019 at 8:20pm

Hi Craig,

What's the effect called when you're surfing in a (relatively) small bay and the water starts sucking out of the bay ahead of a set (I'm thinking Blowholes in Albany here)?

marcus's picture
marcus's picture
marcus commented Tuesday, 11 Jun 2019 at 8:21pm

Noticed this a few times. One recently a 6ft day a mate and i were walking in waist deep water with my daughter at the cowrie hole and the ocean between sets all of a sudden flooded in knocking my daughter off her feet.

Had something out an island off tge central coast. If you get caught inside near the island cave cliff you wait till it all builds up and rushes back out. But that wouldnt be infragravity. I guess its just delayed backwash trapped between the cliffs and a bombie

i remember the internet when it was just for inteligent people but.

frog's picture
frog's picture
frog commented Tuesday, 11 Jun 2019 at 9:25pm

Not sure if this is the same phenomena but year"s ago i noticed that the sea level falls noticeably well before a set arrives. On tiny days if you are standing on the sand the water might drop about 6 inches. On bigger days you need a reference point such as a visible reef bottom or water colour change due to sun reaching the bottom more easily to pick this up.

What this does is give you advance warning of a set approaching often well before you see it approach. It is not really a current of water so much as a mysterious water level drop. This can be very useful if you are sitting close in and need to move out ready for a set and can give you an edge on the crowd on some days.

Gave away my secret there.... oh well.... a few less set waves coming my way but all the swellnet forumites can send me $10 to compensate.

Frogg

southey's picture
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southey commented Wednesday, 12 Jun 2019 at 12:54am

I,ve been relying on this my entire surfing life . Rarely caught inside at Heavy off shore reef , breaks often exposed to sneaker sets .
Without visual indicators of water depth , boils , eddies and actually being pushed towards the shoreline off your markers can be tell tales of impending sets . often minutes in advance and sometimes well before " indicators " breaking further down the coast show and alert the pack .
Launching tinnies and Jetskis off the beach inside protected reefs , this sort of knowledge can be do or die for your beloved machines .

" SA's Reserve Capacity "

jez's picture
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jez commented Wednesday, 12 Jun 2019 at 8:00am

My understanding was that sea level is elevated where you have the wave group (set). The longwaves form in-between the sets to maintain isostatic equilibrium. Because wave height is the main factor for the depth which waves break, the sets slow down near shore and cut the longwaves loose, so the longwaves should arrive before the set they were originally behind in a normal beach scenario. One of the cool things we were taught at uni was that the long waves reflect if they come straight in but if they hit the shore at an angle they can refract and reflect so sharply as they hit the shore that they can actually propagate up the beach kink of like a slinky going down stairs. The nodes of the longwave pattern as they refract/reflect up the beach then helps form the bar and rip pattern on some beaches. I think this might be more significant on super protected beaches where you see prominent rhythmic burms at the shoreline, I noticed this at Bundeena a while back, (I am speculating a bit perhaps someone that knows better can jump in...)

Jezdawg

Craig's picture
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Craig commented Wednesday, 12 Jun 2019 at 8:27am

I thought similar Jez, but no it's counter intuitive and lowest where the set is. It's 180 degrees out of phase with the sets.

Those other points are correct though and it should be noted that when impacting the shoreline the infragravity wave takes a phase shift away from 180 degrees, slightly lagging the incoming set. So it usually follows just behind the set.

jez's picture
jez's picture
jez commented Wednesday, 12 Jun 2019 at 9:14am

Thanks Craig, I think I explained myself poorly. I know that the peak of the longwave is in between the sets but my understanding was that the longwaves exist to maintain isostatic equilibrium between the sets because the sets 'would have' a relatively raised sea level relative to the area between the set if not for the presence of the longwave. Is that correct?

Jezdawg

Craig's picture
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Craig commented Wednesday, 12 Jun 2019 at 9:19am

Ah yes, not sure on that but you're thinking seems to be correct. There's a great paper here if you've got the time.. FIELD OBSERVATIONS OF INFRAGRAVITY WAVE RESPONSE TO VARIABLE SEA-SWELL WAVE FORCING

Craig's picture
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Craig commented Wednesday, 12 Jun 2019 at 8:28am

Also here's a great example of infragravity waves surfing up the beach, unstoppable.

No need to watch the rest of the video, just from 40s.

uncle_leroy's picture
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uncle_leroy commented Wednesday, 12 Jun 2019 at 9:02am

I always just assumed and referred to it as a 'storm surge', set waves pushing in extra water, thanks Craig for giving it an official title.
Another interesting one is the displacement of water caused by cargo/tankers (this could also be similar to a low pressure event passing through the ocean). Plenty of interesting youtube that shows water levels being lowered as ships pass prior to the wake arriving. https://www.youtube.com/watch?v=LonGit7Jjdg

Craig's picture
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Craig commented Wednesday, 12 Jun 2019 at 9:08am

Thanks UL, I guess the main difference is that these infra-gravity waves occur even without large stormy swells, it's just that they're more significant during active swell events.

One classic example that we see during the Pipeline CT events during large swells is the infragravity waves surging up the sand and taking out the spectators and cameras, destroying all their equipment with it.

truebluebasher's picture
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truebluebasher commented Wednesday, 12 Jun 2019 at 12:35pm

Great story Craig...(It's those little waves that bowl you over!)
Bodybasherz can't escape this world as we can't paddle nor can tbb swim well.
Point breaks!
Standing in 3ft water the first set wave rips by with a deeper trailing wave.
2nd wave breaks in 4ft of water with trailing wave sweeping more mass & speed.
3rd set wave swamps 5 ft of water trailing another low size wave of massive force.
4th set trailing wave skips a flood over whole bank washing all inside down the point.
So awesome that only those wide of the line-up escape it.
Note skip waves 3,4,5 feed off their host & may even consume their waning host.
Trail waves may be small but each build in volume to travel faster & further
This happens more often on bays & points where such momentum can carry.

Note: Set waves mostly back off to last whereas each skip wave gains power to last.

tbb is seldom washed off the bank by a larger wave but Craig's waves run like rivers.
These all encompassing waves sweep one out to sea or be flooded 200m ashore.
A more telling gauge of the oceans sustained power for sure.

As said here...same waves again consume the set waves heading back out.
This is when swimmers get dragged of the Back Break & caught in undertows.
When bodysurfing for long sessions you must time them our exhaust yourself.

Note: Craig's waves carry far greater volume & force hidden beneath.
Similar to still water runs deep saying. Even dams have massive wind Current.

Body basherz utilize this wave force to porpoise ashore beneath the rip.
We also neutralize this equal wave energy for no stroke/kick takeoffs!
With experience this wave force is harnessed for reverse or forward Barrel Rolls.
Bodysurfers source this force to marry rotation manoeuvres otherwise arduous.

Meaning a bodysurfer must mould shape to harness skip waves hidden power.
Be that running beneath or combing over, one's body atunes to nature of the wave.
By same nature a Bodysurfer gets ragdolled as 2m nearby a learner sits & smiles.

Craig & readers might wanna explore this alternate modern twist with similar effect...
Credit: NSP Surfboards - Wave Pools in 2019
[What you never read or see in the videos]
https://www.nspsurfboards.com/surf-wave-pools-in-2019/

"Quote is fully credited to NSP surfboards from Wave pools 2019 review."
*Current
"You don't really see this in the videos,but a wave pool running at full capacity will oscillate.Once engineers managed to generate sizeable waves they quickly discovered that their biggest hurdle in creating consecutive waves, is the current these waves generate.Several visitors report their initial surprise at just (how much effort it takes to stay at the right spot)and you intuitively look for something to hold onto.
To counter this,existing wave pools are being retrofitted with drainage rosters and newer installations are designed with complex software that helps dissipation of the kinetic wave energy!"

tbb's Rip Roaring Bonus...
"The Wave Pool you never heard of (Shh!)...tbb is caught in the Rip Zone!"
1936 Portobello (X Large) Heated Wave Pool also harboured a massive current.
{ Contribution from Mr A. Lightbody D.I.S.R.M }
"Due to the dimension of the pool (100m x 50m) and heavy bating loads and in order to make the waves reach the shallow end of the pool the waves had to be very high at the deep end (3ft+) resulting in backwash from the shelving beach making a millrace type of current returning down the middle of the pool.This created a dangerous situation where non swimmers could and were washed out of their depths.
Safety Ropes had to be provided across the pool mid-way and during wave periods,pool attendants were kept busy rescuing the bathers swept off their feet." (Off the record...Sean Connery was one of the said Life Guards)
https://www.bathsandwashhouses.co.uk/archive/your-local-buildings/edinbu...

Read between the lines! Portobello pushed out a 3ft shapely right & left for a full 100m!
Other photos show waves slamming the shallow end. Yes! Pool allowed Surfboards!

the_b's picture
the_b's picture
the_b commented Wednesday, 12 Jun 2019 at 12:10pm

Isnt it just the set waves breaking turning wave energy into a volume of water trapped in the surf zone. Which forms the peak infragravity wave between sets and which then escapes via flash rips etc

Craig's picture
Craig's picture
Craig commented Wednesday, 12 Jun 2019 at 12:15pm

Nah different phenomena. Proven by doing experiments with offshore bars, and the intra-gravity waves travelling onwards to shore.

Laurie McGinness's picture
Laurie McGinness's picture
Laurie McGinness commented Wednesday, 12 Jun 2019 at 12:37pm

Ha ha still can't work it out at the fundamental level. Either it is more complex than I am allowing for or I am missing something. Off to Professor Google for a while!

Craig's picture
Craig's picture
Craig commented Wednesday, 12 Jun 2019 at 12:39pm

Yeah def more complex, check this image for how complex it can get if you want to go deep..

Full paper here: 

Infragravity Wave Processes - Recent Experience in New Zealand and Australia

Laurie McGinness's picture
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Laurie McGinness commented Wednesday, 12 Jun 2019 at 1:56pm

Thanks Craig. Managed to fit in some decent research on ordinary gravity waves this am.

dawnperiscope's picture
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dawnperiscope commented Wednesday, 12 Jun 2019 at 12:34pm

Thanks, great article.
Now I'll be able to get it right next time i'm telling my daughters the water is on its way to fill our moat! Always wondered why the sets ended in disappointment and didn't find their way up the beach.
Thinking back, it seems that on small days with frequent sets the longer 5 minute period would be on the money.

Ride on

truebluebasher's picture
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truebluebasher commented Wednesday, 12 Jun 2019 at 10:24pm

My apology Craig! I do now understand what you described to be an actual thing!
Just going by what you experienced when paddling + the science + Vids.
When I think hard it's not always set wash that drives me back out drags me out.
I do now understand what you are describing more so by feel, than science papers.

I'm guessing the surf beat plays it's own beat, dancing between the set waves.
So the gravitational wave can interweave before,during,after or between set waves.
I'm also guessing coastline & depth dictate how these waves may or not sequence.
One study paper insists the lull period needs better focus because of said waves.

I also often recall many beach breaks that have exhibited this strong force as well.
Usually these waves sweep arrears on Point sets but advance the lull on Beachies.
Also exaggerated on banks with drop off gutters pinning surfers with little swell.

At these times a bodysurfer feels like giving in as it's impossible to swim against.
I feel these as massive deep walls that push inbound stronger than tides that rip out.
Again I don't feel the wave face as such more so the considerable moving force.

Many ask if one can surf a Gravitational Wave?
https://cdn3.theinertia.com/wp-content/uploads/2016/02/Shaka-Einstein.png

Oops! Try this Board...
https://www.facebook.com/9NewsGoldCoast/videos/a-byron-bay-surfer-is-mak...

Thanks again Craig for sorting out our wild blue yonder!
tbb should have no problem bashin' the gravity outta this big bully wavelength!

Thingo's picture
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Thingo commented Thursday, 13 Jun 2019 at 6:28am

Hey Craig, love these articles. I didn't know the name of this wave type at the time but managed to capture some video of it in the harbour last week:

The funnel shape of the coastline mixed with the harbour wall and the surge of the water creates a weird little wave every minute or so. There were no shorter period waves observable at this spot. Water quality looked a little dubious so I passed up a novelty surf.

thermalben's picture
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thermalben commented Thursday, 13 Jun 2019 at 6:32am

Fantastic! Well captured.

Craig's picture
Craig's picture
Craig commented Thursday, 13 Jun 2019 at 7:09am

That's the best example I've seen Thingo, great capture and 10/10. This also proves that the infra-gravity waves aren't made from breaking surf. A real pure example. Thanks for sharing.

thermalben's picture
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thermalben commented Thursday, 13 Jun 2019 at 7:26am

Wonder how often they’re confused for small tsunamis?

Spuddups's picture
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Spuddups commented Monday, 22 Jul 2019 at 5:08am

I witnessed a series of small tsumamis after the 7.8 Kaikoura earthquake here a couple of years back. They were more like a rapid rise in the water level rather than a wave I could identify. Somewhere in the area of 200-300mm. high. This was nine hours after the earthquake, so I guess the ocean was sloshing round like a bath-tub in Cook Strait.

thermalben's picture
thermalben's picture
thermalben commented Thursday, 13 Jun 2019 at 6:37am

Heard a couple of local stories on the Tweed Coast about older crew 'surfing up the creek' on longboards at Mooball many decades ago. It's now obvious what kind of waves they were riding.

Whenever I'm fishing there, I occasionally see infragravity waves push a reasonable distance up the creek, even on small swell days - and that's with the mouth of the creek sometimes no more than a metre or so deep.

Flogonalog's picture
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Flogonalog commented Thursday, 13 Jun 2019 at 3:53pm

I’ve heard those stories too, always thought they were tall tails until I was working at a farm property backing onto the creek way up near the Tweed gallery south of murbah during Oma and witnessed it first hand. It was relatively still on the creek but then a 1ft wave came cruising up and sloshed its way inland. Too pronounced to be tidal. Was a spooky sight and sound

truebluebasher's picture
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truebluebasher commented Sunday, 21 Jul 2019 at 9:17pm

Thanks Craig, came across a nest of these massive sweeping sleepers from Oregon.
Seems the Locals get off playing chicken with them...

*Everyone laughs off big wave then a 1 ft high wave @ 40km/hr comes at them. RUN.
https://www.youtube.com/watch?v=RPypT9dOvSY

*Love the jet roar of this one...Like a concrete mixer it is!
https://www.youtube.com/watch?v=87NlwSVnevI

*This silent one really paints a vivid picture...poetry in motion.
https://www.youtube.com/watch?v=-JNjKLdDNQ8

*Final one! What can you say! That's an awesome display of power (hit refresh)
This clip puts out...slow to massive rush...Loamy Left in the soup, then Rogue right.
https://www.youtube.com/watch?v=ou7A1ybSuNE