Wave heights

[Roberto]

Does anyone know of a graph giving expected average wave height / length in a wind against current situation ?
Or by how much a given sea state is modified by this ?

There are some giving wave characteristics with various wind speeds and fetches, but obviously in the same direction, so maybe there are some for tide against wind.

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[AndrewB]

Simple guide.

Click <A target="_blank" HREF=http://marine.cwb.gov.tw/CWBMMC/windwaveE.html>HERE</A> for a simple guide to wave height at given wind speeds. But it varies with a lot of other factors: currents, rising or falling wind, cross-swell, depth, weather etc.

 

[Roberto]

Re: Simple guide.

Thanks Andrew, I think that is for waves generated by wind, in a stable situation they would be in the same direction. There are also graphs on Bowditch giving similar results.

What I was really interested in is the "theoretical" wind against tidal current effect: for ex. a 4kt current setting exactly opposite a 20kt wind, what kind of sea state can one expect?

I can figure out a steeper, maybe shorter wave, but by how much ? What wind and current opposed speeds would give an average breaking wave ? Or else, if a given sea state (wave height and length equal to a and b) goes suddenly against an opposed currentm what happens to wave geometry?

Obviously talking about a perfect world, perfectly theoretical..

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[AndrewB]

Literature search?

Sorry if my response was simplistic. My knowledge extends no further than the equation developed by Bretschneider (Bretschneider, C.L. and E.E. Tamage, Hurricane wind and wave forecasting techniques, Proceedings 15th International Coastal Engineering Conference, pp.202-237, 1976), which provides an empirically based formula for wave height as a function of the key factors: mean wind speed, duration, fetch, gravity, depth, but not current. (The formula is given THIS of any use?

 

[Paul_H]

Talking of wave heights. You want to see the Tsunami that forms over the mud when the Barfleur ferry goes in and out of Poole Harbour. In the channel there's only a few inches of wash but this piles up to a breaking 4ft wave over the mud. Great for the jetskiers but not for us going to our mooring in the dinghy.

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[Roberto]

wow!

my question was way more simplistic than that!

I just wanted to have a rough idea of what to expect in wind against tide, I have an idea based upon my limited experience, but then with all this talking about rogue waves one begins to wonder : what if a SW 30kt winds goes against an agulhas current with 3-4-5m high waves ? or more significantly nearer to where I can sail now at cap de la hague or ouessant..

thanks

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[Aeolus_IV]

Rogue Wave

I seem to remember that reciently there was a documentary (in the UK) about Rogue waves, and that there seems to be evidence suggesting that wave height contains an element of statistical probability in the exactly the same way that quantum mechanics does. Effectively the program said that it is quite possilble to have waves much larger than current models predict, as it is possible for energy to be transfered between waves so resulting in a series of smaller waves then a single over size wave. They had data from North Sea Oil Rigs (measuring wave height below the platform) which fitted rather well with their alternate theory.

Very interesting viewing, Jeff.

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[Ruslynr]

Hi,

A nice little book with that info in it is
"Handling small boats in heavy weather"
by
Frank Robb
Granada Publishing
ISBN 0 229 97492 9

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[richardandtracy]

Re: Rogue Wave

Yeah, and pressures of 100 tonnes per square metre if it falls on your boat.
My steel yacht can just take 30 tonnes per square metre with severe damage and distortions of up to a foot (done a non-linear finite element stress analysis on it).
Help.

Regards

Richard


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[MainlySteam]

Re: Rogue Wave

Out of interest, in determining the pressure from a falling wave (100 Tonne/sq.m), what constraints did put on the vessel's freedom to move in reaction?

John

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[AndrewB]

Re: Rogue Wave

The yacht will be able give way, so maybe 30 tonnes per sq meter (2.9 x atmosphic pressure sounds so much less impressive) would be sufficient. I wondered whether a watertight yacht might actually be safer than a ship in a rogue wave.

Interesting programme, even if the quantum theory bit was pretty absurd.

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[qsiv]

Re: wow!

In one sense the theoretical models still work - but modify the windspeed by the component of the tidal vector in the direction of the wind.

eg: Wind 15 Kt
Tide 5 knots ..
Wind with tide we have an effective windspeed of 10 knots
Wind over tide we have an effective windspeed of 20 knots - not surprisingly the latter is much rougher.

As well as that there is some theoretical work (publicised after that Hobart race), which goes into the physics of the wave faces being held up by a weather going tide, which accounts for the fact that the waves are materially steeper in such circumstances, and therefore more prone to breaking. I'll see if I can find the source of this and post a link.

To add inslt to injury, we tend to find that both the wind and tidal speeds are prone to being increased around exposed headlands, and by the nature of such headlands there are often underwater features that cause overfalls.

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[Roberto]

Re: wow!

Makes sense. For steepness, maybe (?) the fact that the circular movement of water particles that creates a wave, (i) in a wind with tide situation is amplified until sea state corresponds to wind force and a balance is found, (ii) in a wind against tide situation the same circular movement induced by the wind is opposed by the tidal current, giving it a more vertical elliptical shape, hence steeper wave ? just guessing..

if you can find the link about modifications of wave steepness please post it

thanks qsiv

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[qsiv]

Re: wow!

I'll search when I get home this evening. I know it was published in YW, some time after the Hobart tragedy, and I seem to remember it originated from a scientist in the west of Uk (not Southampton or Wolfson, but maybe Bristol or Wales). nyway, I think I cut the article out, and I may be able to track it down from there.

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[tome]

Re: wow!

There's an article in the July/August Navigation News (magazine of Royal Institute of Navigation) about Freak Waves written by Professor Douglas Faulkner. Prof Faulkner is at the Dept of Naval Architecture at the University of Glasgow so you might find some info on their site.

RIN website is <A target="_blank" HREF=http://www.rin.org.uk/>http://www.rin.org.uk/</A> - you can download Navigation News but the current issue isn't yet posted so afraid you'll have to borrow a copy or wait a few months for this.

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[Roberto]

thanks tome...err to you (nm)

.

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[Aramas]

I'd take any correlation between wind speed and wave height with a pinch of salt. There are just too many variables involved, and most sea conditions are made up of several wave components, none of which are caused by the present wind conditions unless it has been blowing consistently for quite some time.

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[MainlySteam]

"...most sea conditions are made up of several wave components, none of which are caused by the present wind conditions unless it has been blowing consistently for quite some time."

This is contrary to simple observation, and is incorrect. The predominant driving force for waves is wind and current wind conditions are the predominant cause of current sea conditions. For example, how frequently have you seen waves (seas, not large swells from distant or decayed severe weather) travelling in opposition to the wind? Sometimes, but only in special situations and usually only for very short periods of time.

In the case of a lessening of wind speed the wave height decays, usually very quickly for waves of low energy, and that change is entirely due to the current wind condition. Even big seas of high energy abate very quickly to less threatening swells when the wind speed is reduced to low levels. In the case of an increase in wind speed the change in the sea state is again due to the current wind condition and can be seen almost immediately.

If the direction of the wind changes, there is a short time of confusion, but again the predominant cause of the confusion is the current wind condition including both its speed and direction.

Waves are not influenced at all by historical events, they have a state set by historical events but the only influence on them is what is happening at the present moment. They will always be immediately influenced by current wind conditions at an extent according to the strength of that wind.

John

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[richardandtracy]

Re: Rogue Wave

I think the 100T/m^2 assessment assumed that the vessel was big enough not to be pushed out of the way quickly enough (like a bulk cargo vessel) - so was assumed to be rigidly fixed. Consequently, a comparitively small yacht may well be much safer - particularly one with a shallow keel & less grip on the water. I dread to think of the damage to the crew if not strapped in when the wave strikes though!

The safest thing to do when faced with a wave like that (other than transporting immediately to Starship Enterprise) would be to try to submarine the vessel in the front face of the wave before it could break. Then the pressure would be down to the 30T/m^2 mark. However, who could guarantee to think straight in those conditions? I'm pretty sure I couldn't.

Regards

Richard.



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[Aramas]

""...most sea conditions are made up of several wave components, none of which are caused by the present wind conditions unless it has been blowing consistently for quite some time."

This is contrary to simple observation, and is incorrect. The predominant driving force for waves is wind and current wind conditions are the predominant cause of current sea conditions."

Bzzt...wrong.
Try doing a couple of years of oceanography. Simple observation is ok for simple situations. Your simple observations would eventually apply to an infinitely wide and deep, uniform body of water that starts out calm, but the ocean is entirely different. Most wave components observed at sea are the result of wind/current/tide and other effects at another time and place. This is especially true at bottlenecks like the English Channel, Bass Strait, Cook Strait etc., where two oceanic masses meet.
Add in the effects of Eichman transports, geostrophic currents, reflection, refraction, salinity/density/temperature/depth gradients and migrating ocean masses (eg gulf stream) and your simple observations become just plain silly. For the current weather conditions to create the main component of current sea conditions it would have to blow at the same speed and direction for a sustained period of time - usually days. Ever seen waves turn around and head back out to sea when the wind goes from onshore to offshore? Didn't think so. If the wind on a beach is blowing against the waves, then where do the waves come from? Another time and place perhaps? Maybe there just aren't a lot of waves in your area, so it might seem that a storm makes waves right away. However, a storm actually brings the waves with it from elsewhere. A strong wind can kick up a nasty chop in no time, but a real swell takes quite a while.
So the question should be:
"If a wind of x knots blows in direction y on a flat calm sea for z hours, what would the wave height be?"
The answer is, of course, "who cares?"



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