Wind against tide - physical reasons why this is dangerous ?

[franksingleton]

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Reading the thread, I was struck by the difference between A: waves entering a current, and B: waves generated in a current – the latter emphasized by lw395 in #42 and later posts, e.g. #97.
………….

Others have, I think, covered this. Perhaps more succincty, is it not just that waves being propagated into a (tidal) current will be swell waves with long wave lengths compared to waves generated within a current? The latter will be wind waves with short wavelengths.

 

[Hydrozoan]

Others have, I think, covered this ...

Yes, the difference was noted by some, as I said in my reply to LittleSister.

But nobody AFAICS came up with a paper which, in a similarly simple and clear way to Lapworth’s, explained fully, and gave data on the extent of, amplitude enhancement occurring when a wind generates waves ab initio against an opposing current rather than on still water.

A number of papers have addressed that – a couple are cited in the introduction to the Suh et al. paper (and more in my Refs 2 and 3). But they are behind paywalls. If you know of a good available account I’d very much like to see it.

 

[guernseyman]

I'm finding it difficult to see that there is any difference between waves generated in still water by wind with velocity V, and waves generated by a wind with a velocity V relative to a current. Assuming of course that the water is deep.
So long as you are moving with the current, how can there be any difference?

 

[lw395]

I'm finding it difficult to see that there is any difference between waves generated in still water by wind with velocity V, and waves generated by a wind with a velocity V relative to a current. Assuming of course that the water is deep.
So long as you are moving with the current, how can there be any difference?

I think mostly the water is not deep enough for it to be this simple.
I hate to guess exactly how deep the water needs to be, but 10m clearly isn't enough in the small scale case of our harbour.

 

[guernseyman]

I think mostly the water is not deep enough for it to be this simple.
I hate to guess exactly how deep the water needs to be, but 10m clearly isn't enough in the small scale case of our harbour.

So it's just a shallow water effect?

 

[Hydrozoan]

I'm finding it difficult to see that there is any difference between waves generated in still water by wind with velocity V, and waves generated by a wind with a velocity V relative to a current. Assuming of course that the water is deep. ...

Back at #32, dt4134 said: ‘…a 16 knot wind against a 2 knot tide, would generate the same waves as an 18 knot wind in slack water, and conversely when the wind is with the tide it'll generate waves as though it were a 14 knot wind.’

That was contested strongly by lw395 at #35 (and subsequently) who said: ‘You will observe much bigger waves in 12 knots of wind against 2 knots of tide than you will in 20+ knots of wind with two knots of tide. You can observe this in a Northerly wind in the harbours around here (Particularly Langstone), so it is not just a matter of waves being generated in deeper water being amplified by wind over tide.’

Yes, I cannot see the difference in deep water, but I was looking for more information on the effect described by lw395.

PS I see after posting that lw395 beat me to it!

 

[guernseyman]

I think mostly the water is not deep enough for it to be this simple.
I hate to guess exactly how deep the water needs to be, but 10m clearly isn't enough in the small scale case of our harbour.

Deep water is normally taken to be more than twice the wavelength.

 

[franksingleton]

Yes, the difference was noted by some, as I said in my reply to LittleSister.

But nobody AFAICS came up with a paper which, in a similarly simple and clear way to Lapworth’s, explained fully, and gave data on the extent of, amplitude enhancement occurring when a wind generates waves ab initio against an opposing current rather than on still water.

A number of papers have addressed that – a couple are cited in the introduction to the Suh et al. paper (and more in my Refs 2 and 3). But they are behind paywalls. If you know of a good available account I’d very much like to see it.

I do not know of a relevant paper. However, one of Lapworth’s equations is that
Wave period =wavelength/(group velocity + current velocity).

Using numbers derived from
ftp://ftp.wmo.int/Documents/PublicW...CUMENTS_JCOMM/005_Presentations/01_Warren.ppt It should be possible to calculate the change in wavelength for typical wind waves for different values of current velocity. Obviously, you would have to remember the limitation imposed by the height/ wavelength before waves break. You could then do the same for swell waves of the same height and a range of wavelengths meeting a current.

Material for a nice YM article?

 

[guernseyman]

Deep water is normally taken to be more than twice the wavelength.

Sorry I got that upside down; it should be half the wavelength.

 

[Hydrozoan]

From an interesting article by Eric J Heller in Cruising World, February 2006, p88:

“On the first crossing, the tide was flooding 1 or 2 knots to the north, running in the same direction as the wind. On the second, the tide was ebbing, opposing the wind at only a knot or so. The difference in the seas, however, was truly exponential. This difference can’t be explained simply by adding the water speed to the wind speed to get the wind-over-water speed, although that does make matters worse.”

In the first case, the seas were described as “eerily calm”; in the second, there was “a 4- to 6- foot chop” such that “Working on the foredeck, I was treated to green walls of water above my head as the bow crashed through the seas”.

The author invokes wave refraction in a wave vs. current situation in a channel to explain the difference described above. Something that has not, I think, been much discussed in this thread.

https://books.google.co.uk/books?id...=onepage&q=waves bigger on flood tide&f=false

 

[guernseyman]

From an interesting article by Eric J Heller in Cruising World, February 2006, p88:

“On the first crossing, the tide was flooding 1 or 2 knots to the north, running in the same direction as the wind. On the second, the tide was ebbing, opposing the wind at only a knot or so. The difference in the seas, however, was truly exponential. This difference can’t be explained simply by adding the water speed to the wind speed to get the wind-over-water speed, although that does make matters worse.”

In the first case, the seas were described as “eerily calm”; in the second, there was “a 4- to 6- foot chop” such that “Working on the foredeck, I was treated to green walls of water above my head as the bow crashed through the seas”.

The author invokes wave refraction in a wave vs. current situation in a channel to explain the difference described above. Something that has not, I think, been much discussed in this thread.

https://books.google.co.uk/books?id...=onepage&q=waves bigger on flood tide&f=false

Anecdotes like the above are worrying: not for what they say, but for what they don't.

First we are not told the origin of the waves: are the raw waves the same in both instances.
Secondly, what of the depth of water: was the tide high in the first instance, and low on second?

 

[Dockhead]

Re: wave length and wave height

I had to dig out my engineering fluid mechanics notes...

Two factors seem to be at play.

Firstly, the wavelength shortens. Consider a wave travelling at a certain speed through the water. As it enters a region where a current flows against it it will still have the same speed through the water, so the waves will bunch up. Hence a shorter wavelength. Except my notes seem to have used far more equations to say this...

Secondly, the wave height increases.

This really does get mathsy (equation with 8 varibles, and all sorts of operators: sinh, sqrt, ratios etc). Essentially because the wave is moving from a region of no current to one of current there will be a change in the rate of energy transfer of the wave. The effect of this is to increase the wave height. The graph that comes out of the horrendous equation looks like the one posted above. Essentially its a curve. If the current is going with the wave (the RH side of the graph) we get a slight reduction in wave height. If the current is going against the wave (the LH side of the graph) we get a rapidly growing increase in wave height.

Hope this helps. Sorry it is so complicated. I could post a picture of the equations if you like, but they didn't really help me understand it much and I was apparently in the lectures writing the notes at the time.

Regards,

Robin

That's the best plain English explanation I've heard. :thumb:

It's not "friction". The two knots more or less make little different to the amount of friction with the water. As others have said -- that's just the difference in wind speed in relation to the water surface. So if it were friction, the waves should be the same in the same wind speed relative to the water surface, but as we all know from experience they are not.

It's like a Doppler effect --as Magdalena wrote, the waves "bunch up" when they hit an area with current flowing relative to the water where they were formed, shortening the frequency and making them higher and steeper. That's all there is to it.

 

[Hydrozoan]

Anecdotes like the above are worrying: not for what they say, but for what they don't. ...

Yes, I agree that it's anecdotal and more information would have been helpful - and he did say the wind 'piped up' during the first crossing, but was already up when leaving for the second. But I think the proposed mechanism is interesting, nonetheless.

 

[lw395]

Sorry I got that upside down; it should be half the wavelength.

In that case, the water qualifies as deep, the channel is about 10 to 12m deep around HW, the wavelength is much less than twice that, more like 4m in places? That's very very wet in a ~4m long racing dinghy.

 

[lw395]

From an interesting article by Eric J Heller in Cruising World, February 2006, p88:

“On the first crossing, the tide was flooding 1 or 2 knots to the north, running in the same direction as the wind. On the second, the tide was ebbing, opposing the wind at only a knot or so. The difference in the seas, however, was truly exponential. This difference can’t be explained simply by adding the water speed to the wind speed to get the wind-over-water speed, although that does make matters worse.”

In the first case, the seas were described as “eerily calm”; in the second, there was “a 4- to 6- foot chop” such that “Working on the foredeck, I was treated to green walls of water above my head as the bow crashed through the seas”.

The author invokes wave refraction in a wave vs. current situation in a channel to explain the difference described above. Something that has not, I think, been much discussed in this thread.

https://books.google.co.uk/books?id...=onepage&q=waves bigger on flood tide&f=false

Refraction is the change in direction of a wave due to crossing a boundary between two media at an angle. Does not seem too relevant here?
But it can have interesting effects, for instance where waves cross a channel or bar obliquely.

 

[lw395]

Re: wave length and wave height

.....It's like a Doppler effect --as Magdalena wrote, the waves "bunch up" when they hit an area with current flowing relative to the water where they were formed, shortening the frequency and making them higher and steeper. That's all there is to it.

I think that's a significant effect well explained, but not 'all there is too it'.

This thread has digressed from its title to cover a whole range of interesting (and wet!) effects that can happen with wind and waves.

The Heller quote about 'eerily calm' brought to mind a different effect, waves from no wind at all, just due to current and a change in depth. I've seen this a few times at St Aldhelms, glassy smooth water, then quite vigorous as you cross the step change in depth.

I think there are a whole range of effects, mostly observed in combinations.

 

[Hydrozoan]

Refraction is the change in direction of a wave due to crossing a boundary between two media at an angle. Does not seem too relevant here? ...

Or the change in direction as waves move into faster or slower moving water (which is different media in a sense). The relevance is that current variation across a channel can focus waves at its centre in the swell against ebb situation (or the converse). As I understand his proposed mechanism, anyway.

 

[Hydrozoan]

I do not know of a relevant paper. However …
Material for a nice YM article?

Thanks (but no thanks, I’m in too deep water to contemplate writing anything!) I think I see what you’re suggesting, though from a quick bit of reading around I believe one would need a different version of the forecasting nomograph for shallow water generation.

 

[NormanS]

It's discussions like this that help to show how ridiculous it is that weather forecasts pretend to give a forecast of "Sea State".

 

[franksingleton]

It's discussions like this that help to show how ridiculous it is that weather forecasts pretend to give a forecast of "Sea State".

The French, sensibly, predict swell which can be done quite well by the NWP We humans cannot predict swell very well. After that, they give what seems to be the wind wave effect using the “probable” wave height according to the Beaufort scale equivalents.

The UK gives a composite value based on swell forecasts but adding the “probable” wind wave height based on the Beaufort scale. This is probably not too bad over the open sea but not near the coast where tidal currents can be strong. It is up to us, as users, to know when we will have current with or against swell or wind waves. Although these could, no doubt be predicted objectively, there is no way in which the information could be presented in an understandable form, The sea state terms are as in the Douglas scale.