fast waves

[daryl]

http://www.surfline.com/surfology/surfo ... ecast2.cfm

individual waves actually move twice as fast as the wave train or the swell, and a single wave's speed can be calculated by multiplying the swell period times three. So individual waves with a period of 20 seconds travel at 60 knots in deep water. Again, think of the wave train like a rotating conveyor belt that is also moving forward.

Halving this monster, does this mean that in typical heavy conditions we paddle onto a wave that is doing well over 30k

[mical]

individual waves actually move twice as fast as the wave train or the swell, and a single wave's speed can be calculated by multiplying the swell period times three. So individual waves with a period of 20 seconds travel at 60 knots in deep water. Again, think of the wave train like a rotating conveyor belt that is also moving forward.

Halving this monster, does this mean that in typical heavy conditions we paddle onto a wave that is doing well over 30k

This explantion of deep water change anyhting?

Deep Water
Water deep enough so that surface waves are not affected by the bathymetry on the ocean bottom. Generally, water more than 1,000 feet, or at least deeper than one-half the wavelength of the existing waves is considered deep water.

[daryl]

individual waves actually move twice as fast as the wave train or the swell, and a single wave's speed can be calculated by multiplying the swell period times three. So individual waves with a period of 20 seconds travel at 60 knots in deep water. Again, think of the wave train like a rotating conveyor belt that is also moving forward.

Halving this monster, does this mean that in typical heavy conditions we paddle onto a wave that is doing well over 30k

This explantion of deep water change anyhting?

Deep Water
Water deep enough so that surface waves are not affected by the bathymetry on the ocean bottom. Generally, water more than 1,000 feet, or at least deeper than one-half the wavelength of the existing waves is considered deep water.

Um, no that wave is doing 111k.

Like yeah they slow down, but

How fast are they in our zone, since the math is differnt? They've felt like 80-90 sometime.

I suppose it's the wind increasing the energy of the wave, or so-called

wind wave, or sea,

as well as the swell.

A 20s deep water wave is going to slow down from friction, and everything else, there are a lot of factors, that increase and decrease it's celerity i.e. speed, so by the time it's become (for example) 10s near shore, for all those reasons, but because of being shallow, I'm guessing for that reason, it's still moving fast?

I'm just wondering what's a ball-park figure?

[LONGINUS]

Hey Guys,

That surfline description is a little too simplified to make much sense. I wrote some of the content on the subject on wikipedia which expands it a little more if you are interested. You can take breaking wave speed calculations to a complex level with very shallow water and Boussinesq equations but heres a really useful simple formula you can do in your head. Its good because you can do this for any break in the world. All you need to know is roughly what depth the wave is breaking in and a little about the bathymetry.

breaking wave speed in knots = sq root of (10 x water depth)

10 is the gravity acceleration co-efficient, use 9.8 if you want to be more precise.

When you calculate depth you need to use an average depth that the entire wavelength will fit into. So over about 300 metres is fine. For beachbreaks, depth slopes fairly uniformly so you can just add 30% to the depth the actual wave is breaking in and call that your average.

Example: Manly
The main peaks at Manly break in about 10 metres of water, so adding 30% we come up with an average depth of 13m
10 x 13 = 130
sq root of 130 = 11.5kts

The thing to keep in mind is that once a swell becomes a shallow wave, period works a little differently in effecting speeds and it becomes more pronounced on reefs or waves with very deep water behind them. Think of it like this - the atmosphere is a fluid just like the ocean and sound travels within it as energy the same way waves do in shallow water. Someone is standing next to a jet engine and you are 100 metres away when they shout and the jet engine is turned on, the sound from both sources travel at exactly the same speed. The reason you only hear the jet engine is that its amplitude (wave height) is so much greater than the human voice. Frequency (wave period) has nothing to do with it at all. But sound waves don't 'break', so period works uniquely in ocean waves as it effects the avergae depth of the breaking wave, making it effectively deeper and hence faster.

So back to our breaking wave, this means that a 10 second period 1 metre wave breaks at roughly the same speed as a 5 second period 1 metre wave. The reason bigger waves break faster than smaller ones is due to the fact that they break in deeper water. It is only when periods become extreme such as in Tsunamis that they have a marked effect on speed. Because the actual height of Tsunmai waves (amplitude) are generaly so small ( 6 inches) though they break in very shallow water, usually only a metre. This brings their speed down substantially. The long period though reaches back into a few hundred metres of water so the average depth of even a small tsunami is over 100 metres. So using the formula:

speed = sq root of (10 x 100)
sq root of 1000 = 32kts.

But instead of this wave breaking way out the back, its now on the shore doing 60kms per hour, and because the wavelength is so long, hundreds of metres of ocean are at the same height travelling at the same speed right on the shore line.

A 20 second period wave has a wavelength of around 600 metres, so your average depth calculation needs to stretch back that far past the spot the wave is actually breaking in to work out effect depth for speed calculations.

So a 10 second 2 metre wave at Manly has an effective break speed of 11.5 kts, a 20 second wave = sq rt of (10 x 20)
= 14kts so pretty close

Lets make it 5 metre swell, so Queenscliff Bombie will be breaking. Knowing that there is about 15 metres of water on the bombie we can say the wave is breaking in 15 metres of water. Because the bombie is 700 m from the shore though, 300 metres past that puts you into 50 metres of water, so the effective depth for our calculation is more like 35 metres.

Therefore speed = sq rt (10 x 35)
= 19 kts

Reefs and open ocean breaks will always break faster than beachbreaks because that depth gradient is so much larger than a beach break. Add a long period swell to that and the effect will be even further pronouced due to stretching back into deeper water.

Teahupoo is a perfect extreme to demonstrate the effect on reefs and the whole theory in general. Whilst the wave effectively breaks in only a few metres of water, its a fall to about 300 metres once you head seaward from the break, so the effective depth for our calculation is about 150 metres.

Therefore speed = sq rt (10x150)
=38kts

Big, long period Teahupoo breaks in effective depth of around 200 metres

Therefore speed = sq rt (10 x 200)
= 45kts

This is the reason the wave breaks below sea level, the trough is drawing back off the reef the same way a tsunami does and the appearance of the rest of the ocean being the same height as the crest of the wave is because its travelling so close to the speed it would in the open ocean. Teahupoo is the closest thing you will see to a real life example of tsunami dynamics other than the real thing. Now imagine the classic shots of Teahupoo, this will be a 5 metres swell at 20 seconds and Laird Hamilton will cry after towing into it. Now imagine that same wave only 2 feet high but with a period of an hour; an an incredible amount of water passing the same point.

This theory breaks down of course when there is an ASP event and it will be 14 knot 2 foot slop

Remember though that these speeds we are talking about are only valid for a surfer at takeoff. You will be able to move much faster than this going across the face of the wave. The advancing front moves at this speed initially but as the wave / foam moves shorewards it continues to slow. If you rode the wave straight to the shore like they do at Bondi then you would slow as the depth shallowed.

All swell / wave trains slow as they approach a shoaling coast but, the reduction in speed is translated to an increase in height. What causes the wave to 'break' is the ratio of steepness in height compared to period. When this ratio reaches .8, the wave breaks as by now the top of the wave is travelling faster than the bottom which continues to slow. The big factor in a shallow water equation is the steepness of the shore, so a long continental shelf (Northern Beaches) will attenuate more energy whilst a thin shelf (Margaret River, Hawaii) will attenuate far less. Open Ocean reefs have had almost no time to slow their waves down as those waves do break a lot faster than almost any others.

[daryl]

thought Iwas with you until this switch to using period

So a 10 second 2 metre wave at Manly has an effective break speed of 11.5 kts, a 20 second wave = sq rt of (10 x 20)
= 14kts so pretty close

and before that you're using gravity, 9.8 or 10

[LONGINUS]

Hey Daryl, I should have used different depths to prevent confusion. The numbers in the equations are depths in metres, they just happen to be the same numbers as the period in these examples. I used 10 instead of 9.8 for gravity in these examples to keep the maths easy. The other factor you may hear is water density. Waves travel quicker through a denser medium ( more saline) so waves in the tropics travel a little faster than those past 50 north / south but the difference is only around half a knot in the extreme.

[SAsurfa]

Waves travel quicker through a denser medium ( more saline) so waves in the tropics travel a little faster.

I would think that tropical waters are less dense than in the Southern Ocean due to the higher sea temperatures and high rainfall (lower salinity)

[puurri]

From Longinus' post then we're in for a mass emigration from the northern Beaches.

[Nick Carroll]

Yes, I imagine everyone will be rigid with terror at the thought of being approached by a North Steyne cleanup set at 11.5 knots

[LONGINUS]

Waves travel quicker through a denser medium ( more saline) so waves in the tropics travel a little faster.

I would think that tropical waters are less dense than in the Southern Ocean due to the higher sea temperatures and high rainfall (lower salinity)

Yeah I know what you mean, you look at the charts though and it is all over the place with the Atlantic being the most saline ocean, I think it has something to do with rain shadowing meaning east coast tropics get less rainfall and then depending on where the area is, the runoff from minerals in rocks being eroded increases local salinity. Maybe turbidity is a better term, ie all the crap in the water making it more like a solid, wonder what the speed of a wave through a tampon or coke bottle is. Blahn mute point I guess. Rasta will have all that cleaned up with his little boat soon. By the way, Mick Fanning wears the first rip curl recycled wetsuit in Mundaka shortly after becoming clean up day ambassador. Did I call that or what!

edited for typos

[puurri]

Yes, I imagine everyone will be rigid with terror at the thought of being approached by a North Steyne cleanup set at 11.5 knots

Nah, I thought he alluded in part to the pussy slap of NSB breaks.

[LONGINUS]

anyone try this speed formula out on your local? Did if make sense?

[black duck]

Example: Manly
The main peaks at Manly break in about 10 metres of water, so adding 30% we come up with an average depth of 13m
10 x 13 = 130
sq root of 130 = 11.5kts

What? I don't get it. You saying a 3ft wave breaks in 10 metres of water? What are "the main peaks"? Assuming you're correct that's almost 33 ft of water for a 3ft wave (for the feet and inches people). That's just seems ... well... not right?. When it's 3-4 ft at a beachbreak, the waves are breaking in approx 1.5 to 2 times their own height (according to the science). When I dive off my board and do handstands on the sand bottom the depth seems about 3 metres. Am i not understanding something here Longi? WTF?

[SAsurfa]

Yeah don't know what you're referring to when you say the main peaks at manly break in 10m of water, that is at least 3x over what it should be :?

[LONGINUS]

yeah, its the problem of trying to simplify the shallow water wave equation too much. For this simplified formula to work you need to average a lot of things out. The main one being that that you need to consider a wave as being as long as its wavelength, so for a 10 second period wave, 150 metres long or so, then lay that onto your bathy profile, so your depth will always increase. 10 metres is about where the waves are feeling the bottom and the major slowing / increase in height stability breakdown is commencing. If anything though, hopefully it illustrates why waves with a deeper drop off of water behind the often break faster and with more power than your beachie.

[SAsurfa]

Longinus, I just saw your article posted to Bansai and Surfing Atlas and it's basically this thread re-hashed..

I have to say there are so many errors in this thing that people will become worse off for reading it.

Firstly the deep water wave equation wave speed = squareroot(depth x gravity) is in m/s not knots!

Secondly what's this talk about waves at Manly breaking in 10m of water, I don't think even the bombie would be half of that!

From there on all the calculation are off as you can't use deep water wave equations to account for shallow water dynamics.

And to top it off.. this quote "Big, long period Teahupoo breaks in effective depth of around 200 metres" is what should ring alarm bells in any knowledgable surfers head.

I hope you accidentally added two 0's to the end of that because Chopes breaks in only a few feet of water. No wave that I know of breaks in 200m's of water unless running into a strong counter current.

Please make sure you get your facts right before posting this kind of stuff all over the internet

[Kunji]

Oh wow! Longy got owned.

[LONGINUS]

Okay...

I just saw your article posted to Bansai

thats http://www.banzai.com.au - your source of the finest collection of surf and water based action on the web. Take me, I was a 90 pound weakling before I stared clicking on http://www.banzai.com.au once per day, now - I cant' keep the girls away!

I have to say there are so many errors in this thing that people will become worse off for reading it

Smash it up your arse

As I pointed out in the beginning of the article. This was designed to simplify what is a pretty complex science. If you simplify deep water wave equations into one formula that covers every break in the world as a rule of thumb you need to accept about a 30%-50% figure for the variation in bathymetry over about a 300 metre wavelength.

Firstly the deep water wave equation wave speed = squareroot(depth x gravity) is in m/s not knots!

Speed given in m/s is about 50% slower than the same number in knots and it saves people having to know their 60 times table to work it out.

Of course if you know the frequency of the swell its easier as speed in m/s = wavelength / period, is this what you were thinking of?

Secondly what's this talk about waves at Manly breaking in 10m of water, I don't think even the bombie would be half of that!

Remember, we are talking about when a wave 'starts to break' so when its starts to slow in shallowing water and becomes unstable - chart of manly/queenscliff below:



So the bombie is officially known as Manly Rock, if you lay a 300 yard wavelength over the 3.9m feature you can see the wave begins breaking in around 20 metres of water. Check out the 5m contour on the chart, thats where your usually up and riding but again, overlay a 300 metre wave length on that and the wave begins breaking past the 10 metres contour.


From there on all the calculation are off as you can't use deep water wave equations to account for shallow water dynamics.

As discussed, we are looking at a rule of thumb here, not a unified theory of particle physics. If your the kind of guy that heads to the beach with a set of haversine tables and a scientific calculator then you go for it big fella - work it out by first principles. As for me, I'd rather be out there rippin the bag out of it, flipping people the bird and saying faaark and frofing like Nick says in Surfing Life.

And to top it off.. this quote "Big, long period Teahupoo breaks in effective depth of around 200 metres" is what should ring alarm bells in any knowledgable surfers head.

The word 'effectively' should have rung alarm bells in your head. Again, the steep bathmetry at Teahupoo means that when you overlay a 300 metre wavelength over the break, your into 200 metres of water as opposed to 10 at Manly or 20 at the bombie...thats why those three waves look different...dig?

Please make sure you get your facts right before posting this kind of stuff all over the internet

Again, smash it up your arse