Wind against tide - physical reasons why this is dangerous ?

[Dockhead]

Your post is astounding in its arrogance and ignorance. You haven't bothered to make an effort to understand what I'm saying and are too lazy to look at the source material I've quoted. I've spent seven or eight hours over the last couple of weeks going through modules on the web and have simplified it down to a what is still quite a lengthy post, but I've condensed an awful lot of detail.

The whole point is to try to understand it mathematically. They're not difficult formulae. I've also provided worked examples to help illustrate the points, but you don't seem to have looked at them in any detail.

I'm not sure I can even simplify it to your terms. You're looking for a one dimensional answer to something that is not one dimensional. I guess you read abridged versions of the classics.

So anyway, here goes in noddy terms.

1) waves are generated by the wind. The period and height depend upon the wind speed, fetch, which is the distance over which the waves are affected by the wind not the distance from the nearest land, and time. Sometimes they develop from scratch and sometimes they develop from existing waves already created by a gentler wind.

2) After generation is finished waves propagate as swell (I'm not going into that as it's a whole area in itself). Swell is generally less steep because energy transfer and dispersion causes swell period (hence wavelength) to increase and height to decrease.

3) Wave speed in deep water is proportional to period, in shallow water it is proportional to water depth.

4) The period of a wave does not change when it enters shallow water so its wavelength decreases as it slows down. That again is a whole area in itself and I'll skip that to keep it simple as my earlier posts confined themselves to deep water.

5) I must've already posted this three or four times, but here goes again. WIND OVER TIDE IS NOT IN ITSELF THE MAJOR CAUSE OF THE STEEP WAVES YOU ENCOUNTER IN 'WIND OVER TIDE' CONDITIONS. In other words, although the wind opposing the tide causes bigger waves to be generated, it is not the major factor.

6) When an existing wave enters an area of water that is flowing in the opposite direction its period remains the same. It slows down and the wavelength decreases. It's height increases by a proportionate amount. In other words it becomes much steeper. THAT IS THE MAJOR CAUSE.

7) The wave energy is a mixture of kinetic and potential energy. Although energy is lost to the sea bed in shallow water, we're not considering that case, so as the wave slows down kinetic energy is transferred into potential energy. The potential energy is the gravational potential energy of the wave crests.

Now we're getting somewhere. At least 90% of this verbiage is irrelevant to the question (Waves are generated by wind? You don't say!) but at long last (!) a clear thesis emerges, in Point 6 - so it is, according to you, the transition between areas of water moving in relation to one another, which causes the foreshortening of the wavelength. So for the Gulfstream, you have swell built up over a long fetch which is then caught up short against the moving mass of the Stream.

I think all of us "noddies" know that the water in waves moves up and down (or in circles), contrary to appearances. So I think we can imagine that if you impart a real horizontal moment to this whole system, you might have something like a Doppler shift in the wavelength. Hmm.

 

[duncan99210]

Then there are places like Caernarfon bar where you get standing waves (i.e. the waves are present but do not move relative to the underlying surface) in wind over tide conditions. How does that happen?

 

[Quandary]

Decades since I did physics, but my simple explanation to myself has always been based on forces; water is much heavier than air, so water moving at two knots exerts more force than air moving at sixteen. When the two are in concert there is little energy at the interface so minimum disturbance, when they are opposed there is considerable energy which has to be used in some form or other, hence the agitation.
Obviously this theory is much too simplistic but it still makes more sense to me than anything else I have read so far.

 

[dt4134]

Now we're getting somewhere. ... a clear thesis emerges, in Point 6 - so it is, according to you, the transition between areas of water moving in relation to one another, which causes the foreshortening of the wavelength.

It's not my thesis at all, I've simply gone off to find out what the established theory is and report back. The original thread was getting nowhere because of a lack of a scientific theory that could be used mathematically. That already existed, just none of the posters knew it. I simply found out what it was.

We were there in my first post of yesterday, as I stated what I later repeated as item 5) and did a worked example to illustrate item 6).

In my second post of yesterday I used the formulae I obtained (along with a nomogramme that is available at the link) to show that generating waves with wind with tide then reversing the tidal flow (as tidal flows in coastal waters are wont to do) gave steeper waves than waves generated wind against tide against the same tidal flow.

Anyway, glad the message is starting to get across.

 

[NormanS]

It amazes me that weather forecasters can give "Sea State" predictions, presumably based on wind strength, when we all know that it depends on so many other factors, - fetch, depth, and tide etc. My favourite is when they tell me that it will be "Smooth in Shelter". Doh!

 

[lw395]

Y......

5) I must've already posted this three or four times, but here goes again. WIND OVER TIDE IS NOT IN ITSELF THE MAJOR CAUSE OF THE STEEP WAVES YOU ENCOUNTER IN 'WIND OVER TIDE' CONDITIONS. In other words, although the wind opposing the tide causes bigger waves to be generated, it is not the major factor.

.....

You can post it as many times as you like, but those of us who actually observe what happens will know that wind against tide is in itself a significant cause of rough water.
There are of course other causes of rough water, but when you remove those, wind over tide remains to be seen.

Of course the other effects you describe often occur in conjunction.
The very worst places like St Aldhelms Ledge, you get a wave generated by the change in depth with no incident waves and no wind.
Add in either wind over tide or big moderately incoming waves and the effect is spectacular.
It is places like that where wind-over-tide becomes dangerous overfall conditions.

It's a nice day, I'm going sailing now.

 

[guernseyman]

Wind at moderate speed exerts a second-order effect on waves when they impinge on a current. This becomes obvious if you consider the special case of a swell encountering a current. This can occur in the absence of wind, a complete calm. And yet if the current is against the swell it will be observed to steepen. That underlines the irrelevance of wind despite the usual term of 'wind against tide', which is used because it is perhaps more usual to observe the steepening of waves generated by a local wind.
That is not to forget that extreme winds are a different matter as in the Fastnet disaster.

 

[dt4134]

You can post it as many times as you like, but those of us who actually observe what happens will know that wind against tide is in itself a significant cause of rough water.

I made the mistake once of posting on a non-sailing thread on this forum relating to my day job. I gave an opinion that I would normally charge for, backed up with a lot of data. Two posters told me I was wrong and there were about thirty subsequent posts that were complete rubbish. I didn't bother coming back as nobody showed enough knowledge to form the basis on any meaningful discussion.

Not surprising then that when I've just based a few post simply on looking into the science of waves for a few days that there's going to be a few that want to remain happy in their ignorance.

I'm satisfied that I've learnt something useful from my time studying the MetEd modules. There's plenty more I've yet to look at. I hope posting a summary of the wave modules was helpful for some but given the response I doubt it is worth my while posting anything I learn from the others.

Have a nice sail.

 

[charles_reed]

It does seem to get a bad press.

But the other direction, down wind, is a vote grabber !

In a F6 & F7 it's as good as sailing gets. Finding a good enough reason for going in an opposite direction than you intended is often the problem though. Normally to do with going back to work.

Work is, indeed, the curse of the sailing classes.

 

[lw395]

Then there are places like Caernarfon bar where you get standing waves (i.e. the waves are present but do not move relative to the underlying surface) in wind over tide conditions. How does that happen?

'Standing waves' has two meanings as far as I can tell.
I think what you are referring to is a wave which is stationary relative to the land, due to a big change in depth or width of the channel. Imagine a rock in the middle of a stream with a bow-wave, or the wave pattern you observe in a river, downstream of a weir when the river is up. Canoeists call these 'stoppers', they stop you from going down the river, as you have to go up the wave to progress.
These things do not need wind to cause them, but when added to wind-over-tide can become chaotic and wild. Even swell coming in with no wind can be lumpy.

The meaning of 'standing wave' in electronics is completely different, it is where you have waves reflecting back from something, which can cause the amplitude of a wave to vary with distance away from whatever is causing the reflection. You can see this happen in water waves, but it is different in that the wave is still going up and down at any point in space, but with a varying amplitude.
If you have two reflecting ends to your bit of water, you can get a resonance with very big standing waves building up. The most spectacular demonstration of this on water was when they used to do power boat racing on Bristol Docks.

 

[lw395]

FFS, it's like trying to tell someone they'll not fall off the edge of the world.

I was refering only to the component of the waves that are generated in the period when the wind is blowing against the tide. You, like lw395, are trying to compare it to a real life observation of many other combined effects. I'm happy for a sensible discussion of the other effects, but you'll have to bring yourself up to speed first.

I've posted the link. It is publically available, just work your way through the modules.

https://www.meted.ucar.edu/training_detail.php?page=1&topic=3&language=1&orderBy=publishDateDesc

Actually, I'm trying to point out that my real life observation of the simplest case of the wind blowing against the tide with no other effects to combine, contradicts your personal interpretation of the ucar modules. I think you are taking something out of context.

 

[lw395]

....

Have a nice sail.

Thank you it was very pleasant, nice to finish our season on a high note. A little more wind would have been welcome, but we are pleased with our second place.

 

[lw395]

I made the mistake once of posting on a non-sailing thread on this forum relating to my day job. I gave an opinion that I would normally charge for, backed up with a lot of data. Two posters told me I was wrong and there were about thirty subsequent posts that were complete rubbish. I didn't bother coming back as nobody showed enough knowledge to form the basis on any meaningful discussion.

.....

Such are opinions. My work partner and I have just paid for two 'professional opinions' which largely contradict each other. Between the two of them we are a bit closer to understanding the issues. It's often a case of applying perfectly good logic to different interpretations of the starting point.

 

[lw395]

....

Not surprising then that when I've just based a few post simply on looking into the science of waves for a few days that there's going to be a few that want to remain happy in their ignorance.

......

Unfortunately I think it is you being ignorant of a great deal of accumulated 'real world' knowledge.
I apologise for using the word 'bollux' yesterday, I will try to think of something polite but equally apt to assess your assertion:
"Anyway, the basic wind over tide calculation is simple. The effect of the tide simply increases or decreases the wind speed used for determining the waves generated."
That is a statement that anyone sailing on tidal water would do well to be ignorant of.

If anyone can shed any light on a proper mathematical treatment of wind over tide, I would be interested.

 

[dt4134]

Unfortunately I think it is you being ignorant of a great deal of accumulated 'real world' knowledge.
I apologise for using the word 'bollux' yesterday, I will try to think of something polite but equally apt to assess your assertion:
"Anyway, the basic wind over tide calculation is simple. The effect of the tide simply increases or decreases the wind speed used for determining the waves generated."
That is a statement that anyone sailing on tidal water would do well to be ignorant of.

If anyone can shed any light on a proper mathematical treatment of wind over tide, I would be interested.

You persist in taking that one line out of context. It makes me think you haven't even read the rest.

Absolutely my post was based on my interpretation of what I read. Yours aren't even based on that. Your I've-been-sailing-and-you-haven't attitude in order to defend your position is just bollux (there 1-1 ). It's the sort of thing I'd expect from Seajet.

I take it that your 'real world' is the Solent? You claim you've observed seas that have been generated entirely by wind over tide, but you've failed to account for the variations in tidal flows across quite small areas (bit disappointing for a racer that ) so that the effects you see even in The Solent will consist of even recently generated waves entering areas of adverse flow. Yet you have simply rejected that effect, the predominant effect, without properly considering it.

Accepting that the mathematics I gave does not account for the spectrum of waves generated, the combination of sea from different wave trains, or tides that are not directly opposing the wind, the model does tie in reasonably usefully with my real world observations. You just haven't made yours carefully enough.

I deliberately posted the link so that anyone who was interested can go to the same sources as I did. The modules are really not difficult at all. So if you want to open your mind and study them, feel free.

 

[lw395]

You persist in taking that one line out of context. ....

So, do you retract that one line?
A lot of what you say makes sense, but that line is the simplest part and it is wrong.
I'm not sure I am taking it out of context.
The context is the thread title, 'wind against tide'.

A lot of what you discuss is ocean swells reaching shelving water, which is a separate subject, equally significant in itself.
The real world (and even the Solent) often has all these effects added together.

If you do not accept that wind vs tide can create big waves where no waves were before the tide turned, you may be in for a shock at some point. I suggest reading Bethwaite and Morris's views on the subject.

 

[dt4134]

So, do you retract that one line?
QUOTE]

Absolutely not. I think you're just misinterpreting that one line and focusing on it to the detrement of the rest of what I've said.

I'll give it another go.

The wind blowing over the surface of the moving water has an effect but it is simply a result of the increase in relative speeds. It will produce bigger waves but it is not the major component of what is generally called 'wind over tide' even though it is literally the 'wind over tide'. But please wait for the rest...

Waves already produced elsewhere (and they don't have to be ocean swells they can be very recently generated waves produced nearby) will be shortened, will grow in height and will steepen because of the combination of the two changes, when they move into an area of adverse tidal flow (or an increase in adverse tidal flow). That effect is the biggest component. The effects combine.

Note that even though the generation of seas has started elsewhere it will continue even when opposed by the tide if the wind continues to blow and the seas are not fully developed. Even if fully developed before encountering the adverse tide, the increase in the relative wind speed will lead to further growth of the already steepened seas.

Think of what happens when you're reading the wind on the water. You look at the smallest ripples to read the up-to-date effect. The waves themselves are the result of what has happened previously. They tell you about the past not the present. That applies to this as well.

 

[lw395]

So, do you retract that one line?
QUOTE]

Absolutely not. I think you're just misinterpreting that one line and focusing on it to the detrement of the rest of what I've said.

I'll give it another go.

The wind blowing over the surface of the moving water has an effect but it is simply a result of the increase in relative speeds. It will produce bigger waves but it is not the major component of what is generally called 'wind over tide' even though it is literally the 'wind over tide'. But please wait for the rest...

Waves already produced elsewhere (and they don't have to be ocean swells they can be very recently generated waves produced nearby) will be shortened, will grow in height and will steepen because of the combination of the two changes, when they move into an area of adverse tidal flow (or an increase in adverse tidal flow). That effect is the biggest component. The effects combine.

Note that even though the generation of seas has started elsewhere ......

Very interesting in the case of waves coming from elsewhere, but have you never experienced 'wind over tide' in a situation where the waves never originated elsewhere?

Many of us have, the waves are very much bigger than would be produced by the wind speed simply being increased by the water speed. That is 'wind against tide' in its purest form.

 

[dt4134]

Very interesting in the case of waves coming from elsewhere, but have you never experienced 'wind over tide' in a situation where the waves never originated elsewhere?

Many of us have, the waves are very much bigger than would be produced by the wind speed simply being increased by the water speed. That is 'wind against tide' in its purest form.

I'm not sure that really ever happens. To have the waves generated purely by wind over tide you'd need a body of water moving at a constant speed from the time generation starts from a flat calm. Even with a change from a tidal flow of 1 knot at HW+1 to 2 knots at HW+2 the second mechanism comes into the play.

Trying to remember my best example from memory of waves being generated from a flat calm in tidal waters, was coming through Rathlin Sound when a F5 sprung up. The various patches of tidal flow where clearly delineated by the sea surface, but there were still wavelets moving from one patch to the other and so changing shape.

Remember also that smaller waves in the first hour or two of generation will still travel quite slowly, so the impact of tidal flows will be much greater causing a much great degree of steepening. I can use the theory to do a calculation of that to show the effect (just not yet as I've to go off to work). maybe I'll find time later in the week in between the Christmas parties.

 

[lw395]

I'm not sure that really ever happens. To have the waves generated purely by wind over tide you'd need a body of water moving at a constant speed from the time generation starts from a flat calm. Even with a change from a tidal flow of 1 knot at HW+1 to 2 knots at HW+2 the second mechanism comes into the play.

....

What we tend to observe is the waves continuing to build after the tide has passed its peak, by which time the effect you describe would be in reverse.

Often the wind is light early in the day and the waves start to build as the wind increases through the morning (as the tide is decreasing), making our lunchtime HW race a very wet affair in a N or NE breeze off the land.
I think it's fair to say we have studied this closely. As in 'waves in the face' close.

In those races there is a balance between sailing upwind in maximum favourable tide, and sailing in less waves which don't stop the boat so much.