johnalison
Well-Known Member
Many thanks. I have to go out now but I've saved it for later.
Wind against tide - physical reasons why this is dangerous ?
- Thread starter Boo2
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Many thanks. I have to go out now but I've saved it for later.
lw395
Well-Known Member
If you consider the simple case of a tidal channel running north-south, close to the shore which runs east-west. The main channel, running north in Langstone harbour. South coast UK.
A Northerly wind, so the incoming tide is wind over tide.
There are no waves coming from any significant distance, the wind is blowing off the land.
The waves in the channel are big and steep on the flood, far beyond what you see with the same wind relative to the water blowing the other way, and far beyond what you would get if the wind was stronger at slack water.
The tide is probably 1.5knots an hour before HWS if that.
The middle of the channel is probably 10m deep at HW.
It is quite clear that in shallow water, waves generated locally are not simply related to the relative speed of the wind and water at the surface.
Obviously when there are pre-exisiting waves entering shallower water, the situation is more complex, but your model breaks down against observation in the simplest case.
When I have time I will read your references, thank you for providing them.
A Northerly wind, so the incoming tide is wind over tide.
There are no waves coming from any significant distance, the wind is blowing off the land.
The waves in the channel are big and steep on the flood, far beyond what you see with the same wind relative to the water blowing the other way, and far beyond what you would get if the wind was stronger at slack water.
The tide is probably 1.5knots an hour before HWS if that.
The middle of the channel is probably 10m deep at HW.
It is quite clear that in shallow water, waves generated locally are not simply related to the relative speed of the wind and water at the surface.
Obviously when there are pre-exisiting waves entering shallower water, the situation is more complex, but your model breaks down against observation in the simplest case.
When I have time I will read your references, thank you for providing them.
Dockhead
Well-Known Member
Sorry, but I have to agree with his observation, however crudely it may have been expressed. In Post 32 you said that the effect of wind against tide results from the increase in relative speed produced by velocities of wind and water being added. If that were true, then a 16 knot wind blowing against a 2 knot tide would produce exactly the same effect as 18 knots of wind at slack tide. Resisting the temptation to refer to parts of the male anatomy, I must say that this, let us just say, starkly contradicts decades of first hand observations, therefore, I also don't buy it.
You back and fill in a later post, referring to the change of flow of tidal streams. But this is also boll . . . ummm, contrary to the experience of anyone who has sailed in the Gulf Stream, which does not change direction, especially to that of anyone who like me has had the misfortune to be out in the Gulf Stream in a Northerly, where a little F7 blow can produce a sea state which one might be more inclined to associate with a tropical rotating storm.
So what causes wind-over-tide? Damned if I know. It's a total mystery to me and doesn't actually make any sense at all. All the beer you can drink in one evening in the Fountain Inn in Cowes to anyone who can actually shed some light on the question, which has bothered me for decades.
Roberto
Well-Known Member
+1
I do not know the modules he is quoting, but for example this one:
http://www.meted.ucar.edu/marine/mod2_wlc_gen/print.htm
scroll down half page or look "ocean currents", the text makes it very clear that opposing or same-direction currents act upon *wave steepness*.
Which is what is currently observed in real life (steeper, sometimes breaking waves), what is indicated by the Scripp's institution graph I posted earlier, and oh, what is indicated by the attached reference from Van Dorn, "Oceanography and Seamanship" which basically leads to the same conclusion: wave steepness changes...
dt4134
Well-Known Member
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
dt4134
Well-Known Member
Exactly. It is not the wind against tide that is the biggest factor, it is the increase in the steepness of previously-generated waves when they encounter opposing tide. Even the example I posted in response to JohnAlison's post shows that (albeit it shows a simple case starting from flat calm, in deep water, with a very much simplified model for the tide).
I got it from the same set of modules as you. I've posted the link in two separate posts above.
dt4134
Well-Known Member
Probably not, but I've checked that one. Waves with a period of 6 seconds should travel at 18 knots, using c=3T as an approximation when working in knots for waves where the depth of water exceeds L/2.
LittleSister
Well-Known Member
Boathook
Well-Known Member
As others have said sea bed and water depth makes a difference, but in the Solent 30 knots wind and tide in same direction gives flat sea. Tide turns and the waves become fairly tall and very steep. I'm normally in harbour / shelter at this stage in the Solent.
Dockhead
Well-Known Member
You have offered nothing but incoherent and contradictory explanations, and suggestions to take a complete weather course in multiple modules.
Let's get back to the thesis of the OP, which I thought was well expressed, and which has not yet been answered by anyone:
Every sailor who sails a boat more than his armchair, knows that waves stand up and become ugly when wind and current are opposed. What is the cause of this well documented and widely observed phenomenon? A person who understands it will be able to explain it, and will need to resort to references and formulae only for proof. Proof is not needed here, so far, since we have not even heard a coherent thesis for someone to challenge.
So perhaps there are multiple and complex mechanisms at work -- let's hear about them. I am somewhat skeptical about how complex these mechanisms could be since they occur between a very clear and simple cause, and a very clear and simple effect. In any case, this should be difficult to explain, only to someone who does not understand it.
Since we observe this phenomenon just the same in deep water as well as shallow, it cannot be a matter of seabed effects or wave reflections. Since we observe this phenomenon just the same in a constant ocean current like the Gulfstream, it cannot be a matter of echoes of of the opposing stream. So WTF is it?
I would love to be proven wrong (and there's a river of beer in the Fountain riding on it
), but I have a pretty good idea that no one who has participated in this thread so far has the vaguest idea. Some are just a bit more honest about it 
SHUG
Well-Known Member
I'm afraid I can't read all this stuff but I would contribute that when there is a wind shift which puts the wind and wave direction in opposition then thats the recipie for big surfing waves. Here the wind quite clearly slows down the exposed part of the wave and gives the steep front beloved by surfers.
From my own extensive sailing experience I think this is a far more significant effect than the more general wind against tide.
From my own extensive sailing experience I think this is a far more significant effect than the more general wind against tide.
guernseyman
Well-Known Member
After a quick reading of this thread, I think the explanation has been given even if it was obscured by a lot of details.
AFAIK when waves encounter a parallel or anti-parallel current the wavelength is altered, becoming larger in the first case, and smaller in the second. As the steepness depends on the wavelength, the observed effect is explained.
The usual measure of steepness is wave height divided by wavelength and does not give the intuitive steepness as might be estimated by a sailor, but it's as good a measure as any.
Notice that there is no mention of wind; its relevance is only as the generator of the waves.
AFAIK when waves encounter a parallel or anti-parallel current the wavelength is altered, becoming larger in the first case, and smaller in the second. As the steepness depends on the wavelength, the observed effect is explained.
The usual measure of steepness is wave height divided by wavelength and does not give the intuitive steepness as might be estimated by a sailor, but it's as good a measure as any.
Notice that there is no mention of wind; its relevance is only as the generator of the waves.
Dockhead
Well-Known Member
I think we can all agree about that -- the wavelength is altered and with it steepness.
But why is the wavelength altered? Nothing here explains why 20 knots blowing against a 2 knot current is so much different from 22 knots at slack tide. But it is very different, as we all know.
Here's another discussion on the subject: http://www.cruisersforum.com/forums/f131/challenge-explain-the-physics-of-wind-over-tide-32570.html
guernseyman
Well-Known Member
Forget the wind: think about a wave train encountering a current. If the current is moving is the same direction as the waves, they will be stretched out in that direction; and compressed if the current is head on.
dt4134
Well-Known Member
You have offered nothing but incoherent and contradictory explanations, and suggestions to take a complete weather course in multiple modules.
Let's get back to the thesis of the OP, which I thought was well expressed, and which has not yet been answered by anyone:
Every sailor who sails a boat more than his armchair, knows that waves stand up and become ugly when wind and current are opposed. What is the cause of this well documented and widely observed phenomenon? A person who understands it will be able to explain it, and will need to resort to references and formulae only for proof. Proof is not needed here, so far, since we have not even heard a coherent thesis for someone to challenge.
So perhaps there are multiple and complex mechanisms at work -- let's hear about them. I am somewhat skeptical about how complex these mechanisms could be since they occur between a very clear and simple cause, and a very clear and simple effect. In any case, this should be difficult to explain, only to someone who does not understand it.
Since we observe this phenomenon just the same in deep water as well as shallow, it cannot be a matter of seabed effects or wave reflections. Since we observe this phenomenon just the same in a constant ocean current like the Gulfstream, it cannot be a matter of echoes of of the opposing stream. So WTF is it?
I would love to be proven wrong (and there's a river of beer in the Fountain riding on it
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.
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alahol2
Well-Known Member
Just had a read of K Adlard Coles' Heavy Weather Sailing, Appendix 1.
His explanation is the same as that given by dt4134. ie the wave is slowed down and the wavelength decreases increasing the steepness of the wave front. Also because the energy of the wave remains the same it's height increases. This is a 2-fold effect (and you will notice the wind does not play any part in either effect).
He also adds "the shorter the incoming waves (and stronger the currents) the more pronounced the effects".
He quotes a study by the Scripps Institute... "waves, entering an area of opposing currents, can quite easily have their heights raised by 50 to 100 per cent in currents as low as 2 to 3 knots. Thus the breaking of waves may be a frequent occurrence even without much local wind."
His explanation is the same as that given by dt4134. ie the wave is slowed down and the wavelength decreases increasing the steepness of the wave front. Also because the energy of the wave remains the same it's height increases. This is a 2-fold effect (and you will notice the wind does not play any part in either effect).
He also adds "the shorter the incoming waves (and stronger the currents) the more pronounced the effects".
He quotes a study by the Scripps Institute... "waves, entering an area of opposing currents, can quite easily have their heights raised by 50 to 100 per cent in currents as low as 2 to 3 knots. Thus the breaking of waves may be a frequent occurrence even without much local wind."
macd
Well-Known Member
Good, practical, ancillary point, yet rarely mentioned in pilot books. The phenomenon's quite common along the north coast of Spain where Atlantic waves from the west are often met by a local easterly. The result is steep and often confused local seas which can be unpleasant.
Uricanejack
Well-Known Member
Uricanejack
Well-Known Member
I got a "C" in O Grade Physics 35 years ago. The effect of doppler shift is the apparent change in wave length due to the velocity of of the object the waves are emanating from as it passes the observer or the apparent change in wave length due to the velocity of an observer through the medium in which the waves are traveling. The actual wavelength is unaffected. unlike the wind against tide senario
CAPTAIN FANTASTIC
Well-Known Member
The effect of wind and waves against tide is evident in many places but nothing like the Bristol channel where the tide force vector collides against the opposite wind and wave vector, resulting in a very steep and confused sea.
