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Boo2
25-03-10, 08:40
Hi,

I keep reading that conditions often become rough when the wind is against the tide and I'm curious to know the physical reasons why this is so ? Obviously you need to add the windspeed to the tide speed to get an idea of how much of a sea will result, but if the wind is say 30 kt and the tide is 4 kt, does the resulting effect amount to worse conditions that a wind of 34 kt would have given ? From what I've read the answer is "yes", but I'm interested to know what is going on for that to be true ? Is it an interaction with the sea bed or what ?

Thanks,

Boo

fergie_mac66
25-03-10, 08:52
In simplified terms the tide changes the shape of the wave. It slows the deep part of the wave in relation to the top of the wave, sort of trips the wave up.The waves become higher and unstable and start to break

FullCircle
25-03-10, 08:54
Basically, the wind gets under the waves and makes them stand up, so they will be steep and choppy, hindering your forward progress.
Excellent thread here on Cruisers Forum....

http://www.cruisersforum.com/forums/f131/challenge-explain-the-physics-of-wind-over-tide-32570.html

Channel Ribs
25-03-10, 09:04
We had an interesting one yesterday while drifting in a tide of 2kn, a ferry went past (at 3 cables) and when the wake eventually arrived it was moving diagonally across the tide. You could plainly see the waves grow in height and then fall over themselves into breakers.

I hope that description helps?

LONG_KEELER
25-03-10, 09:05
Hi,

I keep reading that conditions often become rough when the wind is against the tide and I'm curious to know the physical reasons why this is so ? Obviously you need to add the windspeed to the tide speed to get an idea of how much of a sea will result, but if the wind is say 30 kt and the tide is 4 kt, does the resulting effect amount to worse conditions that a wind of 34 kt would have given ? From what I've read the answer is "yes", but I'm interested to know what is going on for that to be true ? Is it an interaction with the sea bed or what ?

Thanks,

Boo

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.

dt4134
25-03-10, 09:17
It shortens the wavelength of the waves, making them steeper.

The wind is not necessary for the effect, albeit it does make things worse, as a train of waves from the open sea encountering an area of strong tidal flow will steepen and tend to break. It can get pretty bad even on a calm day in some specific areas where a large swell encounters a strong ebb.

If the tidal flow is in the opposite direction, i.e. wind with tide, the wavelength will increase and the sea will feel smoother and calmer.

Skysail
25-03-10, 09:27
A previous thread:

http://www.ybw.com/forums/showthread.php?t=31281&highlight=Here+in+Cook+Strait

Roberto
25-03-10, 09:28
It does not explain anything about physics, but this graph (I think it comes from the Scripps Institution of Oceanography) gives some insight into the relationships between wave characteristics (length/height, curves L and H) and current speed (related to wave period/celerity, horizontal axis); it may also give an idea why in a tide against wind situation some waves break and some do not.


http://i174.photobucket.com/albums/w111/brancaleone_2007/correntecontro.jpg[/URL]



the full description can be found here

http://sybrancaleone.blogspot.com/2009/04/eng-waves-and-tidal-streams.html

Ricd
25-03-10, 09:37
It does not explain anything about physics, but this graph (I think it comes from the Scripps Institution of Oceanography) gives some insight into the relationships between wave characteristics (length/height, curves L and H) and current speed (related to wave period/celerity, horizontal axis); it may also give an idea why in a tide against wind situation some waves break and some do not.


http://i174.photobucket.com/albums/w111/brancaleone_2007/correntecontro.jpg[/URL]



the full description can be found here

http://sybrancaleone.blogspot.com/2009/04/eng-waves-and-tidal-streams.html

Very simple and clear description..useful thanks

savageseadog
25-03-10, 09:46
Sea bed interaction nearer the shore is extremely important. The other effect, not mentioned is that the relative wind speed is increased by the current

AliM
25-03-10, 10:10
Well, i am a physicist and I am still quite puzzled - but this is my intuitive explanation:

In my experience, the effect is much worse in shallow water. The wavelength of the waves, and therefore the steepness of them, has a strong dependence on depth (shallow water leads to short waves which are therefore steeper).

No wind:
The molecules of water actually move in a circular trajectory within the wave, with a bit of extra motion with the tide - so end up in a spiral, with their fastest motion forward at the top of the wave.

Add the wind:
The wind hits the front of the wave (in the wind against tide case) and blows the water molecules back, steepening the wave.
If the wind is in the same direction as the tide, it hits the back of the wave blowing the molecules forward, flattening it. The difference is only a few knots, but the difference is very marked - as we all have experienced!

bazobeleza
25-03-10, 10:18
'Sea bed interaction nearer the shore is extremely important. The other effect, not mentioned is that the relative wind speed is increased by the current'

Don't forget to add in overfalls, wonderful things that can turn a mere wind over tide situation in a really hair raising experience when added to the mix.

In a F7 on a spring tide the overfalls off carmel head wales once brought Beleza to a shuddering halt and she's a 20 ton boat and motorsailing.

Salty John
25-03-10, 10:36
Wind-over-tide works on an oceanic scale too, so seabed interaction is a separate issue. For instance, the Gulf Stream runs north between Florida and the Bahamas at around 3k. Wind over 15k from the north creates very uncomfortable conditions whereas wind from a southern quarter leaves the sea state quite benign. Water depth is around 1800 ft.

dt4134
25-03-10, 11:15
Well, i am a physicist and I am still quite puzzled - but this is my intuitive explanation:



Hi AliM,

I'm an ex-physicist. I've just tried dusting the cobwebs off an old textbook on waves to see if there were any diagrams I could still understand, but there was none that was appropriate to this.

I suspect the main effect is pretty much the same as the Doppler Effect. The waves are generated at a certain frequency based upon wind speed (and of course fetch etc.) and the velocity of propogation is determined by that (could look up all the numbers for that I guess but haven't yet).

When the waves enter an area of moving water the wave is slowed down by the same amount as that water is moving (I'm assuming for simplicity that the moving water is moving in completely the opposite direction to the waves) yet the frequency remains the same. Therefore the wavelength will shorten.

The above assumes constant depth of water. When you combine that with the effect of the shallowing of the water (which is often the case where you get waves encountering a strong ebb) thats when that bit of sea becomes famous and gets its picture in the pilot books.

Of course there's also the effect of the wind on the steeper seas, the increase in the apparent wind speed (from F7 to F8 in the OP's example) and any disturbance caused by the topology of the sea bed, particularly abrubt changes in depth.

AliM
25-03-10, 11:38
JCP,

Yes, but when you look at it as a simple Doppler effect, then I don't think it can be the full explanation. In a 20kt wind in a river with a 1kt current, say, the sea state with the wind against tide is a lot nastier than in a 21kt wind with slack tide. Similarly, with the 20kt wind with the 1kt tide, it feels a lot nicer than a 19kt wide against 1kt tide. It must be very non-linear.

The Doppler effect gives the shortening of the wavelength (linearly), which is more obvious when the water is shallow, and this tells you about the gradient of the wavefronts (again linearly) along with the fetch which gives the amplitude.

Perhaps I'll look up the scientific papers, when I've done some of the stuff I'm supposed to be working on!

Ali

Channel Ribs
25-03-10, 11:48
Are you sure your not over complicating it? The water under the surface is doing one speed and the air above the water is going in the opposite direction, therefore you end up with a turbulent area where the two interact. Given that water forms waves whenever it's flow is not the subject of a single force, you get waves that increase dramatically as soon as the forces increase.

Quandary
25-03-10, 11:57
The Malthouse answer (without molecules) seems to make most sense to me.

dt4134
25-03-10, 12:04
JCP,

Yes, but when you look at it as a simple Doppler effect, then I don't think it can be the full explanation. In a 20kt wind in a river with a 1kt current, say, the sea state with the wind against tide is a lot nastier than in a 21kt wind with slack tide. Similarly, with the 20kt wind with the 1kt tide, it feels a lot nicer than a 19kt wide against 1kt tide. It must be very non-linear.

The Doppler effect gives the shortening of the wavelength (linearly), which is more obvious when the water is shallow, and this tells you about the gradient of the wavefronts (again linearly) along with the fetch which gives the amplitude.

Perhaps I'll look up the scientific papers, when I've done some of the stuff I'm supposed to be working on!

Ali

Hi Ali,

I was thinking more of the comparison between the wave speed in stationary water and the tidal flow. That I think is more significant than the wind speed itself & the rate of tidal flow.

I remember seeing a table of the relationship wave height, wavelength, wave speed, fetch and wind speed but can't find it now.

Anyway, I'll have to drop out of the debate now as I'm off to do some applied chemistry (applying expensive chemicals from a tin of International Mega-gunk to the bottom of the hull).

Regards,

JCP.

TQA
25-03-10, 13:03
Don't care about the physics just know that I do not want to do it again.

20 to 30 knots from the North while out in the Gulf Stream south of Cape Hatteras [ mini depression that was not forecast ] gave sea conditions that had large amounts of green water crashing aboard from all sides and the only time I ever worried about the stability of my tough 38 foot steel ketch.

That was a nasty day!

Sans Bateau
25-03-10, 13:37
Both wind and water provide friction. Putting your hand out of the car window you will 'feel' the wind. Likewise, put your hand in the water from on a moving boat, you will 'feel' the pressure of the water against your hand.

Next, get a glass of water and blow across the top of it, you will produce little wavelets. If the wind is blowing across water that is not moving then also wavelets will be created. That is, the same as the glass of water, the friction of the wind dragging the water. If the water is moving (tide) and the wind is in the same direction the effect of the wind on the water is reduced. For example if the wind is blowing at 20 knts and say the water is moving at 5kts in the same direction the effect on the water will be as if the wind was only doing 15knts. The wind will also smooth the water and push it in the direction it is already flowing.

The reverse is the case when the wind and tide are in opposing directions (wind against tide). Say the wind is blowing at 20knts and the water is moving at 5knts in the opposite direction, not only is the effect on the wind on the water equal to 25 knts, more importantly the friction of the wind will be trying to push the water back in the direction it is flowing, this will cause the surface of water to build up into steeper wavelets, these in turn, depending on the strength of the wind are providing a rougher surface for the wind to push against, so the wavelets continue to build until the force of the flow of water allows it to flow in the direction it is trying to go. Depending on conditions, sometimes the waves can be very steep. And you dont have to go off shore to experience this, try running down the Solent past Cowes on spring ebb with a strong SW blowing.

The stronger the wind the steeper the waves. On a boat, its a wave that is more likely to cause you a problem than the wind.

PeterGibbs
25-03-10, 14:06
Hi,

I keep reading that conditions often become rough when the wind is against the tide and I'm curious to know the physical reasons why this is so ? Obviously you need to add the windspeed to the tide speed to get an idea of how much of a sea will result, but if the wind is say 30 kt and the tide is 4 kt, does the resulting effect amount to worse conditions that a wind of 34 kt would have given ? From what I've read the answer is "yes", but I'm interested to know what is going on for that to be true ? Is it an interaction with the sea bed or what ?

Thanks,

Boo

Why it is dangerous, is because the frequency of waves, as well as their height, increases and, as you can imagine, in extreme circumstances a boat cannot make headway and could be overwhelmed by breaking seas.

PWG

Channel Ribs
25-03-10, 14:12
Why it is dangerous, is because the frequency of waves, as well as their height, increases...

Also the conditions can be localised and variable, so they can trip up the unwary or the unlucky.

BlowingOldBoots
25-03-10, 16:23
Going Against The Tide
Having sailed for 48 hours arrived at the Mull of Kintyre (West to East) with wind against tide. This is quite a big overfall. Spent 3 hours surfing up to 13 kts with 1 kt ground speed. I was tired. I didn't want to go inshore because that would have been too dangerous in this case, and going offshore would have cost me the 3 hours anyway. Stupid decision ended up causing me 4 hours of hard concentration. The reason I was in the over fall was because I got the tides totally wrong by 6 hours.

Going With The Tide
Rounding Garroch Head (Southish to Northish) which has a small overfall. Blowing force 6 form the North, rounded the corner in the overfall, started luffing up and stuffed her bow into a bigish standing wave. Not quite like hitting the breaks, but green seas on the bow and crew tumbling about the cockpit. I have done this twice rounding the headland, one time I was on the bow and head butted the anchor.

Both events were very physical.

tel1
25-03-10, 18:46
Hi,

I keep reading that conditions often become rough when the wind is against the tide and I'm curious to know the physical reasons why this is so ? Obviously you need to add the windspeed to the tide speed to get an idea of how much of a sea will result, but if the wind is say 30 kt and the tide is 4 kt, does the resulting effect amount to worse conditions that a wind of 34 kt would have given ? From what I've read the answer is "yes", but I'm interested to know what is going on for that to be true ? Is it an interaction with the sea bed or what ?

Thanks,

Boo

simple answear, Its one of the most uncomfortable rides in sailing. slam, slam, slam on every wave. I done an xmas trip in my 38 footer, 25 knots on the nose and tide against the wind around goodwin sands, i was glad of my 78hp engine as i manged to plow along at 7kn and get out of the horrible sea!

jack_tar
25-03-10, 22:01
quite a good example of wind over tide outside my window at the moment. well not exactly but near enough.

i am in a town on the side of the river rhone in france and my hotel room overlooks the river. As you will be aware the river flows its way southwards down to the med. and being a sizable river it travells along at a fair lick, no idea how fast but i would suspect about 4 to 5 knts.

this last few days there has been a stiffish breeze from the south straight upstream breeze would be a good 10 15 knts I would think. Amazing waves on the river not spectacular but interesting seeeing what they are like and comparing the wind speeds when i venture out.

little off subject perhaps but i thought it may be of interest

Jack

magdalena
27-03-10, 12:11
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

duncanmack
27-03-10, 19:50
Years ago I saw a brilliant demo of the effect of shortening the wavelength of a sinusoidal curve.

Was done on an oscilloscope.

As you shorten the wavelength the amplitude increases. Dramatically!

Perhaps some others will remember it.

Watergeus
15-08-11, 11:40
To reawaken this thread...

Whilst the Scripps graphs are interesting (see thread http://www.ybw.com/forums/showthread.php?t=232362) they don't really address the issue because they refer neither to the wind strength or the water depth, both of which are important.

If there's no wind, nothing happens. If you're in deep water little happens.

I got caught off Dungeness recently in a force 5 and a flat bottom boat: it was quite uncomfortable and impossible to turn in towards Rye until the tide changed. (I expected a 3). The length to height ratio was about 10 (30m to 3m) and nearer the shore the waves were breaking.

Chris Moss

electrosys
15-08-11, 12:14
Nothing could appear more benign than the River Welland running from The Wash up to Fosdyke Bridge, as it's well sheltered on both sides - but last autumn I experienced a really bad 'wind against tide' when coming in on the flood, with the wind howling directly down the channel.
At times white-topped rollers, several feet high, came marching down the river - I couldn't believe my eyes, and the little open boat I was in became airborne several times. I don't know why it happens, and to be honest I don't care much - I just know it's something to be very leery of.

GinjaNinja
15-08-11, 12:20
This may shed some light :-)

http://magicseaweed.com/Wave-Fundementals-Article/323/

fisherZ
15-08-11, 12:32
The wind holds the wave back, allowing the following waves to catch up and therefore add to the height of the wave.

dt4134
15-12-12, 15:48
When this thread was in full flow I meant to go away and try to find the science behind wind against tide, but I never really found the time.

Anyway, thanks to EStarzinger who posted a link to this website a couple of weeks or so ago, I've been browsing a few modules and I think I can throw in a bit more science on the subject.

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

by the way, it is a mine of information of Meteorology and can really help to while away the winter months. You do need to create an account to log in but other than that the courses seem freely available.

Anyway, waves have a period T and a wavelength L and a waveheight H. Generation depends upon the wind strength, time it blows and fetch (fetch is a lot more complex than simply the distance to the nearest land upwind, but that is covered in the modules)

As swell propogates, wave period increases and heigh decreases (which is what we all know intuitively anyway)

Anyway, a few basic formulae:

c = L/T, where c is wave celerity (why they use celerity instead of velocity I don't yet know)

c also closely approximates SQRT(((g * L)/(2 * Pi)) * tanh ((2 * Pi * h) / L))

where g is the gravitational constant and h is the water depth.

The formulae can be simplified for deep water which is defined as water deeper that L/2 to give

c = SQRT((g * L)/(2 * Pi))

This can be used to give a wave celerity of 1.56 * T for deep water. that's in m/s. In knots it is approximately 3 * T. Wavelength is 1.56 * T^2.

Similarly for shallow water, which is defined as water shallower than L/20 it can be simplified as

c = SQRT( g* h), i.e. is water depth dependent.

Between L/2 and L/20 the full formulae is needed. The module also goes into a lot of detail on the effect of waves entering shallow water and breaking heights but I'll skip over that for this post.

Also worth noting that waves tend to break when H > 1/7th of L.

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.

So 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.

What is really interesting is the effect of waves or swell entering an area where an adverse current or tide is flowing. The period T remains unaffected, but the wave is slowed by the tide, so wave length reduces using the formula

L = T (c-u) where u is the speed (velocity) of the tide.

The H of the wave is increased in proportion to the reduction in wave length

I'll use the Greenwich Light Vessel as an example. At the moment it is reporting a wave period of 7 seconds and a wave height of 6.2 ft (I'll use 2 metres)

These waves will have a wave length of 76.4m in slack water and will travel at 10.9m/s (or 21 knots). Now imagine an adverse 3 knot tide (I'll use 1.5m/s)

The wavelength reduces to 7 * (10.9 -1.5) which is 65.8m and height correspondingly increases to 2.3m

If the tide is flowing in the same direction as the waves at the same speed the corresponding figures are 86.8m and 1.76m height.

Converting to imperial units, for the same waves, the effect of a 3 knot reversing is to make a change from a wavelength 285 ft and a height of 5'9" to a wavelength of 216 ft and a height of 7' 6"

that doesn't seem too bad, but the effect on shorter period waves (which are moving slower and hence will suffer a greater proportional effect from the tide) will be a more significant change in the steepness.

By the way, wave energy E = 1/8 * (Ro * g * H^2) where Ro is the water density and H is the wave height.

johnalison
15-12-12, 16:05
I hope you will excuse this non-physicist not following the whole of that argument. What puzzles us laymen is why with one body of water going one way at, say, 2 knots, with, say, 18 knots of wind, and another going the other way at 2 knots with 22 knots wind should in practice show such a considerable difference in wave character compared to still water with the same difference of wind speed. It appears as if the wind "knows" that the water is moving.

This is something we all understand in practice because we have experienced it. What is hard to grasp is how just changing the frame of reference changes the wavelength and thus the wave character. I know the answer is in post 32 but I'm not sure I'll live long enough to digest it.

dt4134
15-12-12, 16:09
I hope you will excuse this non-physicist not following the whole of that argument. What puzzles us laymen is why with one body of water going one way at, say, 2 knots, with, say, 18 knots of wind, and another going the other way at 2 knots with 22 knots wind should in practice show such a considerable difference in wave character compared to still water with the same difference of wind speed. It appears as if the wind "knows" that the water is moving.

This is something we all understand in practice because we have experienced it. What is hard to grasp is how just changing the frame of reference changes the wavelength and thus the wave character. I know the answer is in post 32 but I'm not sure I'll live long enough to digest it.

Give me an hour or two and I'll go back to the module and do an example calculation on the waves generated to show the difference.

lw395
15-12-12, 16:13
......

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.

So 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.

......

My first hand observation would suggest that's bollux.
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.

dt4134
15-12-12, 16:32
Actually, it didn't take me as long as I though to find the right nomogramme.

So 18 knots of wind blowing for six hours from a flat calm will produce waves with a height of about 3'8" and period of about 4.5 seconds.

So the wavelength will be 31.6m or 103'. In reality there a spectrum of waves of course. The waves will be travelling at 13.5 knots (7m/s).

22 knots of wind blowing for six hours will produce waves of about 5' with a period of about 6 seconds, so wavelength will be about 56.2m or 184'. The waves will travel at 18 knots.

Now imagine the tide reverses (I'm simplyfying this a lot). The 3'8" waves will now find themselves travelling against the current (4 knot change from the current they were generated in).

Period remains unaffected at 4.5 seconds but using L=T(c-u) so the wavelength reduces to 22.5m or 74' (that reduction gives a proportionate increase in height, so that gives a wave height of approx 5'). - same height as the waves generated against the tide but a much shorter wavelength, therefore steeper waves.

It is interesting that the main effect seems to be caused by the tide changing from wind with tide to wind against tide rather than the waves being generated as the wind blows against the tide.

dt4134
15-12-12, 16:48
My first hand observation would suggest that's bollux.


I guess that's why I'm beginning to give up on this forum. You are perfectly entitled to go through the modules yourself. I've posted the link. There is a lot of it that covers waves entering shallow water, which I have deliberately omitted to keep things simple. That's why I chose an example in mid-channel.

If after going through the modules you still disagree with the science I'm sure they'll be perfectly happy to hear from you. However, your scientific argument might have to be a tad more developed than that of a semi-literate adolescent.

lw395
15-12-12, 16:56
Well I thought your
""
The effect of the tide simply increases or decreases the wind speed used for determining the waves generated.""
was condescending and clearly blatantly wrong, as will be attested by anyone who has actual experience of sailing in wind over tide conditions.

It's called a reality check.

dt4134
15-12-12, 17:07
Well I thought your
""
The effect of the tide simply increases or decreases the wind speed used for determining the waves generated.""
was condescending and clearly blatantly wrong, as will be attested by anyone who has actual experience of sailing in wind over tide conditions.

It's called a reality check.

It's not blatantly wrong, I use in an example calculation for JohnAlison above, which I also simplified by doing the calculations starting from a flat calm. Wind blowing over the tide is one effect, waves entering a current (or a changed current) is another, waves entering shallow water is another. Before mentioning the effect of wind on existing waves or the combination of different seas to give the observed effect.

You've simply dismissed it out of hand without any effort to research it yourself, enquire about other effects or even doing some example calculations of your own.

machurley22
15-12-12, 17:29
...The waves will travel at 18 knots.
Are you quite sure of your calculations?

johnalison
15-12-12, 17:29
Actually, it didn't take me as long as I though to find the right nomogramme..

Many thanks. I have to go out now but I've saved it for later.

lw395
15-12-12, 17:32
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.

Dockhead
15-12-12, 17:49
I guess that's why I'm beginning to give up on this forum. You are perfectly entitled to go through the modules yourself. I've posted the link. There is a lot of it that covers waves entering shallow water, which I have deliberately omitted to keep things simple. That's why I chose an example in mid-channel.

If after going through the modules you still disagree with the science I'm sure they'll be perfectly happy to hear from you. However, your scientific argument might have to be a tad more developed than that of a semi-literate adolescent.

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
15-12-12, 18:14
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.




+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
15-12-12, 18:30
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.



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
15-12-12, 18:38
scroll down half page or look "ocean currents", the text makes it very clear that opposing or same-direction currents act upon *wave steepness*.



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
15-12-12, 18:41
Are you quite sure of your calculations?

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
15-12-12, 20:54
Waves with a period of 6 seconds should travel at 18 knots

But some of them are very naughty!;)

Boathook
15-12-12, 21:22
Hi,

I keep reading that conditions often become rough when the wind is against the tide and I'm curious to know the physical reasons why this is so ? Obviously you need to add the windspeed to the tide speed to get an idea of how much of a sea will result, but if the wind is say 30 kt and the tide is 4 kt, does the resulting effect amount to worse conditions that a wind of 34 kt would have given ? From what I've read the answer is "yes", but I'm interested to know what is going on for that to be true ? Is it an interaction with the sea bed or what ?

Thanks,

Boo

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
15-12-12, 21:24
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

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
15-12-12, 21:39
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.

guernseyman
15-12-12, 22:00
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.

Dockhead
15-12-12, 22:07
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.

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
15-12-12, 22:13
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

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
15-12-12, 23:05
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 :)

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.

alahol2
16-12-12, 01:55
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."

macd
16-12-12, 06:39
...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.

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
16-12-12, 07:45
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.

This particular noddy thinks you have explained the phenomena quite well.:D

Uricanejack
16-12-12, 07:54
Hi AliM,

I'm an ex-physicist. I've just tried dusting the cobwebs off an old textbook on waves to see if there were any diagrams I could still understand, but there was none that was appropriate to this.

I suspect the main effect is pretty much the same as the Doppler Effect. The waves are generated at a certain frequency based upon wind speed (and of course fetch etc.) and the velocity of propogation is determined by that (could look up all the numbers for that I guess but haven't yet).

When the waves enter an area of moving water the wave is slowed down by the same amount as that water is moving (I'm assuming for simplicity that the moving water is moving in completely the opposite direction to the waves) yet the frequency remains the same. Therefore the wavelength will shorten.

The above assumes constant depth of water. When you combine that with the effect of the shallowing of the water (which is often the case where you get waves encountering a strong ebb) thats when that bit of sea becomes famous and gets its picture in the pilot books.

Of course there's also the effect of the wind on the steeper seas, the increase in the apparent wind speed (from F7 to F8 in the OP's example) and any disturbance caused by the topology of the sea bed, particularly abrubt changes in depth.

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:D

CAPTAIN FANTASTIC
16-12-12, 08:25
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.

Dockhead
16-12-12, 08:35
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
16-12-12, 08:36
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
16-12-12, 10:01
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
16-12-12, 10:13
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
16-12-12, 10:15
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
16-12-12, 11:07
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
16-12-12, 11:16
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
16-12-12, 11:44
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
16-12-12, 11:47
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
16-12-12, 19:42
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
16-12-12, 20:02
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
16-12-12, 20:05
....

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
16-12-12, 20:11
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
16-12-12, 20:24
....

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
16-12-12, 22:53
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
16-12-12, 23:52
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
17-12-12, 00:26
[QUOTE=lw395;3919819]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
17-12-12, 08:11
[QUOTE=lw395;3919819]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
17-12-12, 08:24
[QUOTE=dt4134;3919842]

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
17-12-12, 09:40
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.

guernseyman
17-12-12, 10:10
[QUOTE=lw395;3919971]

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.



Looking in Reeds Almanac around Rathlin Island (assuming that is the Rathlin of Rathlin Sound) it looks a nightmare with tidal streams of up to 6kn. I wonder if there are overfalls complicating the wave systems?

Reeds says, of Ballycastle, "Outside of the harbour is a fair weather anchorage clear of strong tidal streams, but liable to sudden swell...".

I don't know what sudden swell is. But it seems possible that the phenomenon is not pure 'wind over tide' or, as I like to think of it, 'wind-generated waves reacting with a tidal current'.

Elsewhere I see a mention of standing waves being set up in the locality. Clearly this is not a place where simple 'wind over tide' occurs.

duncan99210
17-12-12, 10:20
'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.

Yeah, but at the Bar it only ever builds up when the wind is blowing over the tide??? Otherwise, fast flowing stream, just like a river but no waves.

Slycat
17-12-12, 10:29
[QUOTE=dt4134;3919990]


Reeds says, of Ballycastle, "Outside of the harbour is a fair weather anchorage clear of strong tidal streams, but liable to sudden swell...".

I don't know what sudden swell is. But it seems possible that the phenomenon is not pure 'wind over tide' or, as I like to think of it, 'wind-generated waves reacting with a tidal current'.

Elsewhere I see a mention of standing waves being set up in the locality. Clearly this is not a place where simple 'wind over tide' occurs.

We do get simple wind over tide, the complications occur because the tide moves in circles at some points! The standing waves are more usually generated when we've a big sea swell as well as tide/wind.

In my experience the sudden swell probably refers to the possibly of a big ground swell arriving quite quickly. I've been out surfing in the area with 2-3 foot waves and there been the very sudden arrival of a much larger (9-10ft) swell. With such a large fetch (the entire Atlantic) these waves can arrive very quickly.

See here for the peculiar tides around Rathlin:

http://i40.tinypic.com/330gpcw.jpg

lw395
17-12-12, 10:37
Yeah, but at the Bar it only ever builds up when the wind is blowing over the tide??? Otherwise, fast flowing stream, just like a river but no waves.

I think there are other effects with a bar, which when combined with wind over tide, need to be treated with respect.
There are lots of other interesting effects to do with wave propagation, the material linked to by dt4134 is quite interesting.

lw395
17-12-12, 10:39
[QUOTE=dt4134;3919990]

.....

Could you edit your post please? it looks like you are quoting me, and I ihave never sailed the Rathlin Sound.

Cheers,

dt4134
17-12-12, 18:48
Looking in Reeds Almanac around Rathlin Island (assuming that is the Rathlin of Rathlin Sound) it looks a nightmare with tidal streams of up to 6kn. I wonder if there are overfalls complicating the wave systems?

...

Elsewhere I see a mention of standing waves being set up in the locality. Clearly this is not a place where simple 'wind over tide' occurs.

I've never pushed my luck in Rathlin Sound, either aborting or slowing down if the timing isn't going to work. It is a difficult area with ocean swells coming in form the Atlantic to add their bit. There are some historians that believe Bhreachan met his end there rather than in the whirlpool that bears his name.

And yes there are effects from overfalls, but I don't believe there were any in the instance I used as an example.

It was about an hour into the flood where a F5 blew up suddenly as I was passing between fair Hd & the island. There were calm seas and no swell beforehand, so it was the best example that sprung to mind of waves being generated in wind over tide conditions from a flat calm. Other cases I remembered all had added complications.

And doing my tourist board bit ... well worth a visit if you're passing, especially when the sun is out so you can view the scenery.

dt4134
17-12-12, 19:06
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. :-)


Interesting observation. Perhaps it occurs because the waves have had longer to grow before entering the tidal areas, so even though the tidal flow has started to ease it has bigger waves to work on.

Anyway, if you're going through the modules you'll have found the nomogramme for wave generation so can give it a go at trying to calculate the waves yourself. I guess you've found the bit on group celerity so you should be able to calculate how far the wave group has come and therefore make a stab at how long they have had to develop.



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.

Don't I know it. The mast of the main boat I race on was strengthened after coming down four times in a couple of years under the previous ownership so she will really pitch in the wrong size of chop in a beat. We might as well go home if we can'y avoid it. With bigger waves, she's fine upwind and brilliant downwind.

SmileyGiley
19-12-12, 09:00
FWIW - Short steep seas is a devil of a toungue twister - just been discussing WoT offshore Borneo.

I'd rather be home for Xmas

franksingleton
19-12-12, 16:03
Hi,

I keep reading that conditions often become rough when the wind is against the tide and I'm curious to know the physical reasons why this is so ? Obviously you need to add the windspeed to the tide speed to get an idea of how much of a sea will result, but if the wind is say 30 kt and the tide is 4 kt, does the resulting effect amount to worse conditions that a wind of 34 kt would have given ? From what I've read the answer is "yes", but I'm interested to know what is going on for that to be true ? Is it an interaction with the sea bed or what ?

Thanks,

Boo

Not my field. But, there is an interesting paper by Alan Lapworth in the R Met Society Journal, "Weather", in April 2011. It might be worthwhile somebody getting the paper and liaising with the author to produce an article for one of our magazines. The paper is copyright - Wiley.

Contact details: Alan Lapworth, Met Office, Field Site, Cardington Airfield, Shortstown, Bedfordshire, MK42 0SY, UK ajlp@waitrose.com


He points out that, swell waves will move at a speed given by
Period of the waves = wavelength divided by speed.

If a current develops opposing the swell then, as the period will be unchanged but the speed will be lessened by the speed of the current. Therefore, the wavelength must be shorter and the steepness (ie the ratio of height to wavelength) will increase.

If the current is with the swell, the wave speed increases and the wavelength must increase in proportion. The steepness will decrease.

As an example, if a wave of wavelength 15 m and speed 10kn encounters an opposing stream of strength 2 kn, its wavelength will be reduced by 50% while its height is increased by 75%, giving a dramatic increase in steepness.

As somebody else has already said, it is really a case of waves (swell, really) against current.

lw395
19-12-12, 18:33
Not my field. But, there is an interesting paper by Alan Lapworth in the R Met Society Journal, "Weather", in April 2011. It might be worthwhile somebody getting the paper and liaising with the author to produce an article for one of our magazines. The paper is copyright - Wiley.

Contact details: Alan Lapworth, Met Office, Field Site, Cardington Airfield, Shortstown, Bedfordshire, MK42 0SY, UK ajlp@waitrose.com


He points out that, swell waves will move at a speed given by
Period of the waves = wavelength divided by speed.

If a current develops opposing the swell then, as the period will be unchanged but the speed will be lessened by the speed of the current. Therefore, the wavelength must be shorter and the steepness (ie the ratio of height to wavelength) will increase.

If the current is with the swell, the wave speed increases and the wavelength must increase in proportion. The steepness will decrease.

As an example, if a wave of wavelength 15 m and speed 10kn encounters an opposing stream of strength 2 kn, its wavelength will be reduced by 50% while its height is increased by 75%, giving a dramatic increase in steepness.

As somebody else has already said, it is really a case of waves (swell, really) against current.

Frank, in your example, where has the energy come from to make the wave 75% higher?


I don't doubt or deny that swell hitting a current in shallow water can have an effect.
But there is also a pure 'wind over tide' effect, not dependent on any swell.

I suspect in reality, both effects often occur together. But not always.
I think true 'wind against tide' is largely associated with the presence of the seabed affecting the motion of the wave. I suspect this causes a steeper front to the wave, which then has a higher wind resistance, hence more wave energy is extracted from the wind.
If there is any swell to kick-start the process it will be exaggerated.

But if the process is entirely dependent on swell, you would not get 4ft+ waves at the bottom end of a Northerly force 5 in Chichester harbour, when the waves have only come a couple of miles down the harbour.

I accept that often any messy stuff will be loosely called 'wind against tide' when other things are happening.

guernseyman
19-12-12, 19:06
But if the process is entirely dependent on swell, you would not get 4ft+ waves at the bottom end of a Northerly force 5 in Chichester harbour, when the waves have only come a couple of miles down the harbour.



I'm sure Frank will have an opinion but, for the time being, I think you need a fetch of about 15 miles to get 4ft waves. Possibly the waves you have seen are those that have worked their way up from outside. I've seen 3ft waves travelling up the River Trieux just 400yds past the sharp 'S' bend above Lézardrieux. Very clever of them, because I don't think they were generated in 400yds.

lw395
19-12-12, 19:35
I'm sure Frank will have an opinion but, for the time being, I think you need a fetch of about 15 miles to get 4ft waves. Possibly the waves you have seen are those that have worked their way up from outside. I've seen 3ft waves travelling up the River Trieux just 400yds past the sharp 'S' bend above Lézardrieux. Very clever of them, because I don't think they were generated in 400yds.
No, they are generated by the wind against tide.
They are traveling south, the wind direction.
Outside the harbour, away from the bar channel it is typically very flat, if the wind is off the land.
Classic example would be the Thursday of Fed Week last year.

guernseyman
19-12-12, 21:10
No, they are generated by the wind against tide.
They are traveling south, the wind direction.
Outside the harbour, away from the bar channel it is typically very flat, if the wind is off the land.
Classic example would be the Thursday of Fed Week last year.

Well if the waves are travelling against the tide, that would help explain why they are building in height. The thing is what is causing the waves in the first place? Wash? Waves that have come up and been reflected?

franksingleton
19-12-12, 23:20
Frank, in your example, where has the energy come from to make the wave 75% higher?


............
.

The example is taken from the Lapworth paper. As I see it, when there is no current the energy in the waves goes down wind – really, I suppose down swell. When the current is against the swell, the energy “piles up” causing the increase in wave height. When the current is with the swell, the energy is taken away more quickly. I cannot quantify that effect but I can see that is the reason.

alahol2
19-12-12, 23:52
The example is taken from the Lapworth paper. As I see it, when there is no current the energy in the waves goes down wind – really, I suppose down swell. When the current is against the swell, the energy “piles up” causing the increase in wave height. When the current is with the swell, the energy is taken away more quickly. I cannot quantify that effect but I can see that is the reason.

I would think it's more likely a change from kinetic energy to potential energy. ie as the wave slows (meeting the opposing current) it loses kinetic energy. That energy has to go somewhere so it goes into potential energy by lifting the water higher.
Maybe

franksingleton
20-12-12, 09:32
I would think it's more likely a change from kinetic energy to potential energy. ie as the wave slows (meeting the opposing current) it loses kinetic energy. That energy has to go somewhere so it goes into potential energy by lifting the water higher.
Maybe

Lapworth is a little coy on the energy aspect. I have to assume that the eneergy goes into increased wave height. that coupled with the increased steepness is what makes such a difference with a change of tidal current from i knot in one direction to 1 knot in the other. I agree, the energy gets concentrated with a contrary current and removed when current is with swell.

The official way of getting the paper is to go to http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1477-8696 and search for wind and tide.

lw395
20-12-12, 10:17
Well if the waves are travelling against the tide, that would help explain why they are building in height. The thing is what is causing the waves in the first place? Wash? Waves that have come up and been reflected?

The thing that is causing the waves is the wind.
Why do people not want to see this?

In my simple scenario there are only three significant things:
The seabed, at a depth of say 10m
The incoming tide, probably reducing from 3 knots to about 1.
The wind, blowing against the tide. Typically F4.


I suppose most on here don't see that simple case with no significant outside influences very often. If you are sailing along the coast, there will be waves coming downwind.
When I sail at Lymington, the system is a lot more complex, with swell coming in from Christchurch Bay, changes of depth, changes of stream etc.
Cowes is more complex again.

franksingleton
20-12-12, 22:51
Probably the most definite comment that can be made about sea state is that it can certainly be complex. The variables are swell, wind-sea, bottom topography, coastal topography, current, reflection and refraction. Have I missed any? Near the coast, all can come into play. Which one or ones will be dominant will vary from time to time and place to place. Another definite statement – as ever, with anything connected to weather, there no hard and fast rules. There is no single answer.

NormanS
20-12-12, 23:02
Probably the most definite comment that can be made about sea state is that it can certainly be complex. The variables are swell, wind-sea, bottom topography, coastal topography, current, reflection and refraction. Have I missed any? Near the coast, all can come into play. Which one or ones will be dominant will vary from time to time and place to place. Another definite statement – as ever, with anything connected to weather, there no hard and fast rules. There is no single answer.

Thats why I wonder why the Inshore Forecast, forecasts "Sea State". In practice, it doesn't mean anything.:confused:

guernseyman
21-12-12, 08:58
Probably the most definite comment that can be made about sea state is that it can certainly be complex. The variables are swell, wind-sea, bottom topography, coastal topography, current, reflection and refraction. Have I missed any? Near the coast, all can come into play. Which one or ones will be dominant will vary from time to time and place to place. Another definite statement – as ever, with anything connected to weather, there no hard and fast rules. There is no single answer.

Not to forget diffraction - unless you include it in coastal topography.

franksingleton
21-12-12, 09:14
Thats why I wonder why the Inshore Forecast, forecasts "Sea State". In practice, it doesn't mean anything.:confused:




I agree with you. In my opinion, swell forecasts are far more useful. NWP models can predict swell and we sailors cannot. Sea-state is dependent on so many factors including tidal current that it is nigh impossible to give a meaningful value. However, it is a requirement defined as required under the GMDSS.

CROSS inshore forecasts give a sea-state but also swell. It is a shortcoming of UK forecasts that they do not. Is that because swell is more important as a determining factor governing entry to French (and Spanish) ports than too many British ones?

lw395
21-12-12, 10:12
Probably the most definite comment that can be made about sea state is that it can certainly be complex. The variables are swell, wind-sea, bottom topography, coastal topography, current, reflection and refraction. Have I missed any? Near the coast, all can come into play. Which one or ones will be dominant will vary from time to time and place to place. Another definite statement – as ever, with anything connected to weather, there no hard and fast rules. There is no single answer.

I agree.
Because there are usually so many factors applying at once, I feel it's useful to look at the cases where some of these factors are absent, like my Chichester Harbour example, where there is wind over tide but no incoming swell, or the way swell propagates around the Solent in no wind. Or to drift over St Aldhelm's ledge when the sea is a mirror one side but simmering the other.

I'm not sure there are 'no hard and fast rules' though. The laws of physics remain, it's just quite hard to apply them clearly.

franksingleton
21-12-12, 16:20
..................

I'm not sure there are 'no hard and fast rules' though. The laws of physics remain, it's just quite hard to apply them clearly.

The same is true about weather. How many hard and fast rules are there?

lw395
21-12-12, 18:29
The same is true about weather. How many hard and fast rules are there?

Conservation of energy
Conservation of angular momentum
If you can see Wales from Clevedon it is going to rain.

franksingleton
21-12-12, 18:53
Conservation of energy
Conservation of angular momentum
If you can see Wales from Clevedon it is going to rain.

Yes. Obviously. What I was trying to say, clearly inadequately, that sailors have no rules that they can apply. In principle, you can apply all these rules and get a forecast or a prediction of sea state. In practice that is not the case. NWP makes a pretty good fist of it but has to use many approximations, estimates and simplifications.

bitbaltic
21-12-12, 19:05
If a current develops opposing the swell then, as the period will be unchanged but the speed will be lessened by the speed of the current. Therefore, the wavelength must be shorter and the steepness (ie the ratio of height to wavelength) will increase.

If the current is with the swell, the wave speed increases and the wavelength must increase in proportion. The steepness will decrease.



I've just read this whole thread (goodness knows why). That's the only completely correct passage anywhere in it.

There are a few considerations core to the concept here which haven't been mentioned in any post.

Surface friction (between water and air, and between water and the sea bed, between water of different densities)

Compressibility of water in comparison to air

The fact that most estuaries and all oceans are stratified.

Just my two penny worth.

lw395
21-12-12, 21:01
I've just read this whole thread (goodness knows why). That's the only completely correct passage anywhere in it.

There are a few considerations core to the concept here which haven't been mentioned in any post.

Surface friction (between water and air, and between water and the sea bed, between water of different densities)

Compressibility of water in comparison to air

The fact that most estuaries and all oceans are stratified.

Just my two penny worth.

The thread wandered off from discussing why wind over tide is so horrible (tech term) to people denying that wind over tide in itself exists.
In my view, the seabed causes a velocity profile in the water, which changes the shape of the waves, causing more friction between air and water, hence more energy is transfered.
As the roughness of the sea increases, the air also becomes more turbulent, so the faster moving air a few metres above the water is mixed with the slow moving air at the boundary, hence there is faster air driving the waves.


Are you suggesting that strata in the water make it behave as shallow water, i.e. the waves only operate in the top layer? I'm not disagreeing, haven't considered that before.

franksingleton
22-12-12, 09:28
I may be wrong but I get the impression that some on this thread are forgetting that, in a wave, the water goes up and down and not horizontally. It is the energy which is propagated in the form of wave action. The longer the wavelength, the further down will the wave effect extend. As the depth gets less, at some stage it becomes too shallow for the wavelength. The critical depth is about half the wavelength.

When the depth is less than the critical value the wave energy has to be realised somehow and that is by shortening the wavelength. This results in the steepness – the ratio of height to wavelength increases. When the steepness exceeds a critical value then waves break.

webcraft
22-12-12, 09:36
Anyone who doubts the effects of wind over tide should try sailing down (or up) the Sound of Jura in a steady SW F5 over a change of tide. It's chalk and cheese.

- W

franksingleton
22-12-12, 10:14
Anyone who doubts the effects of wind over tide should try sailing down (or up) the Sound of Jura in a steady SW F5 over a change of tide. It's chalk and cheese.

- W

For the reasons at #89.

pagoda
22-12-12, 20:26
For the reasons at #89.

Quite.. A lot of anguish .. and some useful analysis mind you.

In general, if tide or current / meets wind. Simple. generally nasty short waves ( but perhaps deep) meet boat. NOT comfortable on Boat. Much worse on close reach.
Avoid by looking at tidal atlas and weather?
Try going west against tide a Westerly in the North Channel?? madness - hide in Sanda or Rathlin. Use your head... your crew will thank you for it :D
Application of common sense solves it.

franksingleton
22-12-12, 21:57
What we have never understood is why, when beating into a swell, one tack is often much more easy than the other. we have found this even when both tacks have a similar angle to wind or swell.

Any ideas? or should this be a new thread?

Of course, gentlemen do not beat but i have never presumed to be a gentlemen.

LeoLind
25-10-15, 01:13
I don't think it's as complicated as you make it sound. The effect happens just as much in deep water and sometimes without any wind, so many previous explanations fail. As a mariner I have observed the effect often but in 1992 my eureka moment came one day while heading east on a motor yacht through the Current Rock passage in the Virgin Islands where I had a birds-eye view of whole effect clearly displayed in front of me. I could see that the current causes the waves in the center to travel a little slower than on the outsides, and that this bends the wave train, and now the waves on the left and right were converging towards the center where they added their energy and height. I have illustrations here http://www.3dym.com/waves/waves.htm

Peroo
25-10-15, 11:45
Having read the whole thread (I had nothing else to do) I think I sort of get it - my question is what is the effect of surface pressure on wave formation and indeed wind over tide conditions. Logic and intuition may suggest that low pressure may result in even higher waves and high pressure less - but do the models include this at all? Is there another force at work here we sailors should also be aware of?

I think so, but wondered if those far more technically nous may be able to educate me?

franksingleton
26-10-15, 17:39
Having read the whole thread (I had nothing else to do) I think I sort of get it - my question is what is the effect of surface pressure on wave formation and indeed wind over tide conditions. Logic and intuition may suggest that low pressure may result in even higher waves and high pressure less - but do the models include this at all? Is there another force at work here we sailors should also be aware of?

I think so, but wondered if those far more technically nous may be able to educate me?

Pressure affects the height of the sea. An increase/decrease in pressure of 1 hPa results in lower. higher sea level heights of 1 cm. Changes in air pressure cannot affect wave height simply because the whole sea level will be lower/higher. Of course, with tidal surges, lower pressure will result in waves being higher but it is the sea as a whole and not individual wages.

jac
26-10-15, 18:15
THis is a 5 year old thread - started again by a new person after a gap of 3 years. Can we not stop these somehow?

NormanS
26-10-15, 18:53
THis is a 5 year old thread - started again by a new person after a gap of 3 years. Can we not stop these somehow?

Why? That seems a very elitist attitude on your part.

macd
26-10-15, 19:07
THis is a 5 year old thread - started again by a new person after a gap of 3 years. Can we not stop these somehow?

They can be comical when 'advising' someone with, say, an engine problem that was probably rectified a decade ago. But, although LeoLind may now be wishing he'd paid more attention, is there really any time issue on general matters such as wind over tide? Unless, of course, you're suggesting it's no longer a phenomenon (being in the Med, I wouldn't know: plain old wind over water has been bad enough lately) ;)

Ardenfour
26-10-15, 21:22
It's not the wind against the tide, but the waves against the tide that creates the effect...

nemodreams
27-10-15, 00:24
All I know is whem I'm halfway to Scarweather - on a spring ebb into a force 5 SW - I ain't thinking about physics :ambivalence:

Boo2
27-10-15, 00:50
THis is a 5 year old thread - started again by a new person after a gap of 3 years. Can we not stop these somehow?

What for ?

Boo2

ghostlymoron
27-10-15, 08:50
I think it's unlikely that anything new will be said on this topic after 13 pages of discussion spanning 5 years. I would suggest that old threads coulb be made read only although I don't feel strongly about it.
I've sometimes contributed to old threads, and felt rather silly when someone - usually Vic or Nigel - have pointed it out.

jac
27-10-15, 19:29
What for ?

Boo2

As others have said, is anything new being said? It brings a dead topic back to life, loads of people respond to what they think is a live issue and the OP has long gone. (Although not in this case) so those thinking they are being helpful are rather wasting there time.

I think all it needs is for those who review posts by new members to also look at what they are replying to. If it adds value, or is a genuine reactivation of the thread then fine.

It doesn't stop oldies reactivating long dead threads but it tends not to be the old hands that do it.

LittleSister
27-10-15, 23:50
I think I read this thread in its prime. (Either that, or one uncannily like it.) But it's still interesting to read it again now, and perhaps get a bit more out of it and see the new posts.

What does it matter if some of the posts are old? Wind against tide hasn't gone away.

It's not as if it's crowding out more vital and up to date threads. If you're not interested, then don't read it!

MagicalArmchair
28-10-15, 08:57
For my part, its the first time I've seen the thread, so quite interesting reading going back to the first page, so thanks for 'necroing' the thread (I believe that is what the 'hip' dudes call it...:cool:)

franksingleton
28-10-15, 11:17
What we really need is a good search engine and then use it before posting questions. There have been several/many threads starting with the same weather query. I am always happy to try to answer them but sometimes go back and copy a post from a previous thread to do so.

There is chapter in Reeds Weather Handbook on the tidal stream/ wave issue.

macd
28-10-15, 11:42
As others have said, is anything new being said? It brings a dead topic back to life, loads of people respond to what they think is a live issue.

Do feel free to provide evidence that wind over tide is not a live issue.

For the life of me I can't see the problem with born-again threads. Opening them isn't compulsory.

Gladys
28-10-15, 12:35
Coming out of Vlissingen with a SW on the ebb, I defy anyone to say it wasn't dangerous, Gladys was being thrown around like never before, I couldn't hold on to the crash bar whilst steering. Stand? Sit? Very dangerous to move around.

MagicalArmchair
28-10-15, 12:42
"first holiday" with the girlfriend (she loved it, so now she's the wife!), wind over tide on the Wallet on the East Coast. The conditions were pretty moderate, gusting praps 28-30 knots apparent. That short steep and wet chop shows the great fun that wind over tide heralds, you can see how that would ramp up pretty quickly given either a stronger tidal flow or the wind building.


https://youtu.be/diM46wXiN4Q

LittleSister
28-10-15, 21:22
What we really need is a good search engine and then use it before posting questions. There have been several/many threads starting with the same weather query. I am always happy to try to answer them but sometimes go back and copy a post from a previous thread to do so.

There is chapter in Reeds Weather Handbook on the tidal stream/ wave issue.

That's all very well , but this is a discussion forum. If we only wanted to know 'the facts', there are probably better sources than YBW forum that Google and other search engines could find for anyone sufficiently keen enough to follow up.

I think the mix of views, observations, perspectives, solid expert information and insights from people such as yourself, plus a generous dose of blind prejudice and wilful provocation from hither and thither, is what makes the forum entertaining and informative. That, and the way that subjects come up (or resurface!) that one probably wouldn't have set out to research, but is pleased to have drawn to one's attention, engage with, and perhaps gain some insights from.

Long live old threads!

Floyd Raser
28-10-15, 22:35
That's all very well , but this is a discussion forum. If we only wanted to know 'the facts', there are probably better sources than YBW forum that Google and other search engines could find for anyone sufficiently keen enough to follow up.

I think the mix of views, observations, perspectives, solid expert information and insights from people such as yourself, plus a generous dose of blind prejudice and wilful provocation from hither and thither, is what makes the forum entertaining and informative. That, and the way that subjects come up (or resurface!) that one probably wouldn't have set out to research, but is pleased to have drawn to one's attention, engage with, and perhaps gain some insights from.

Long live old threads!

Absolutely, but I see where Franksingleton is coming from; there have been two threads about painting engines within the last couple of days. Yes this is a discussion forum but that makes it an excellent source of facts. I have used the search engine here to great effect, finding out how to get my tiller pilot working for instance.

Maybe the moderators could use the code system to illustrate that a particular thread is over a certain age?
The search engine would be more effective if some people put a bit more care into the titles of new threads; "What's happening here?" may prevent useful info being found easily.
And it is interesting how threads usually get lively after around nine pages, which is why I looked in here! :cool:

macd
29-10-15, 04:35
Maybe the moderators could use the code system to illustrate that a particular thread is over a certain age?

Such a system has been in place for some considerable time. It's in the format DD-MM-YY and accompanies the name of the OP, just below the title of each thread. Granted, the reader would have to do some fairly tricky arithmatic for this elaborate code to fulfill the function you describe. Those not up to the task could perhaps get their carer to help.

Floyd Raser
29-10-15, 11:48
Such a system has been in place for some considerable time. It's in the format DD-MM-YY and accompanies the name of the OP, just below the title of each thread. Granted, the reader would have to do some fairly tricky arithmatic for this elaborate code to fulfill the function you describe. Those not up to the task could perhaps get their carer to help.

+1 :encouragement:

Buck Turgidson
29-10-15, 12:42
Such a system has been in place for some considerable time. It's in the format DD-MM-YY and accompanies the name of the OP, just below the title of each thread. Granted, the reader would have to do some fairly tricky arithmatic for this elaborate code to fulfill the function you describe. Those not up to the task could perhaps get their carer to help.

It's almost a brilliant post.
Except that this thread is still active as indicated by the name and date of the LAST post ;-)

macd
29-10-15, 13:30
It's almost a brilliant post.
Except that this thread is still active as indicated by the name and date of the LAST post ;-)

Shucks....you are almost too kind.
You're also right, of course. But I daresay that there's not a single active thread on the main forums that was begun even one year ago unless it's had once of these frightful resuscitations.

franksingleton
29-10-15, 17:32
It is difficult to know whether it is better to start a new thread or carry on with an old one. Using the search engine for a specific topic may take you to a thread with many pages. It is not always easy to scan through several pages and sort out the wheat from the chaff particularly when there is some banter. It can be s easier to start a new thread with your question and risk being told that the answer is at ……..

In any case the world moves on. In my specialism, over the past year NOAA has improved its model and is now able to provide GRIB data on a 0.25 degree grid and Météo France now makes its high resolution GRIB data freely available. Somebody wanting some information about GRIBs and going to an old thread rather than starting a new one might not find those facts. Nevertheless, it can be a little frustrating to answer a question, maybe at some length, only to see the question asked again a few weeks later, perhaps in another forum. As it is, I think that we just have to accept being frustrated at times. It is always best to ask a question rather being deterred from doing so.

It is easy to miss a discussion on some topic of interest until it I too late to make a, hopefully useful, comment or ask a follow-up question. I find it difficult to monitor forums for new and potentially interesting threads. It is not helped by the search engines not always working properly although they seem to have improved lately. Particularly when away cruising, what is the best way to look out for new threads on particular topics?

Apologies for rambling on but, like others, I see this from the point of view both as a provider of information and a searcher for help.

Hydrozoan
30-10-15, 12:51
I’m adding to the resuscitation rather than starting afresh. It’s a long and rather techie post, and in the end it won’t tell you how big the waves will be. So don’t say you’ve not been warned!

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.

Lapworth’s paper (http://onlinelibrary.wiley.com/doi/10.1002/wea.606/pdf) cited by franksingleton at #89 covered A - and said of its subject: “... the phrase ‘wind against tide’ … should really be ‘swell against tide’ …”).

My digging into B has been constrained by the fact that much of the detailed material is behind paywalls and seen by me only in partial ‘previews’ - and by my old and limited physical and mathematical oceanography. But with those two caveats I’ll offer what I’ve found.

The introduction to a 2000 paper by Suh et al. emphasizes the distinction between A and B (full text of paper at http://www.researchgate.net/profile/Kyung-Duck_Suh/publication/245292866). It cites papers on A, in terms of the effect of current on wave spectra – papers much more mathematical than Lapworth’s I imagine, so I didn’t pursue them further.

Regarding B, Suh et al. cite two earlier studies and say that both of them ‘… found that when the wind is opposing the flow, the waves tend to be higher than when it follows the flow’. But Suh et al. also argue (and later show), contrariwise, that high frequency waves in the equilibrium range of the wave spectrum may be enhanced with a following (rather than opposing) current. However, I believe that the ‘equilibrium range’ means microscale (cm to m wavelength) waves (Ref 1 below).

A book chapter by D H Peregrine in 1976 (Ref 2 below) cites amongst others the same two sources as Suh et al. It notes that a paper (of 1975 by Vincent, which I failed to find) rationalized their results and pointed out that the effective fetch was increased by an opposing current because the wave energy had to travel more slowly, and that large amplitude waves could arise from very short fetches with an opposing current equal to the group velocity of the waves (IIRC). A book by I Lavrenov (Ref 3 below) also refers to studies of wave generation in a current, to the effective fetch in the presence of a current and to the problems for ship handling of ‘wind against current’ waves.

Those sources all support lw395’s repeated point about the importance of B, as well as A (with ‘current’ as inflowing tidal current in his case). Regrettably, with what I’ve been able to see and do, I cannot use them to put numbers to the amplitudes likely with various winds, currents and fetches.

Perhaps someone with full access to the sources and the expertise will be able to do that - or at least check and correct any omissions or misunderstandings in this post, as I’m conscious of being outside my technical comfort zone. But I hope - with that warning - it may be helpful. And as the late John Ebdon used to say: ‘Anyway, if you have been, thanks for listening’.

(1) In introduction of paper by Dai et al. ‘The equilibrium range of wind wave spectra: an explanation based on white noise’, Journal of Ocean University of China, 2007, Vol6, 345-348.
(2) ‘Interaction of Water Waves and Currents’ in Advances in Applied Mechanics, Vol. 16, Academic Press, 1976.
(3) ‘Wind Waves in Oceans. Dynamics and Numerical Solutions’, Springer, 2003.

LittleSister
30-10-15, 21:48
I’m adding to the resuscitation rather than starting afresh. It’s a long and rather techie post, and in the end it won’t tell you how big the waves will be. So don’t say you’ve not been warned!
. . . .
But I hope - with that warning - it may be helpful. And as the late John Ebdon used to say: ‘Anyway, if you have been, thanks for listening’.

Excellent stuff! It's great that you've highlighted that there are two different issues, given that a lot of the thread seemed to me to be people talking past each other because they had missed the distinction.

guernseyman
31-10-15, 08:50
[QUOTE=Hydrozoan;5496532]snip...

Lapworth’s paper (http://onlinelibrary.wiley.com/doi/10.1002/wea.606/pdf) cited by franksingleton at #89 covered A - and said of its subject: “... the phrase ‘wind against tide’ … should really be ‘swell against tide’ …”).

...snip

Personally, I prefer 'wave train against current' as it is more general than 'swell against tide' and correctly summarises the physical effect.

Lapworth's paper is so clear that I keep a copy on board.

Hydrozoan
31-10-15, 10:57
Excellent stuff! It's great that you've highlighted that there are two different issues, given that a lot of the thread seemed to me to be people talking past each other because they had missed the distinction.

Thank you. Some people did see it and lw395 made a good point at #102, that it was useful to try to look at real situations in which one mechanism is operating largely in isolation (if one can find them, of course!).


… Personally, I prefer 'wave train against current' as it is more general than 'swell against tide' and correctly summarises the physical effect. Lapworth's paper is so clear that I keep a copy on board.

Yes, I agree that is a better description; I was quoting Lapworth himself to emphasize that the wave generation side was not covered, in what is indeed an admirably clear paper.

I hoped to find a similarly clear exposition of the ‘wind generation in current’ case, but I could not get the primary sources cited by Suh et al. and Refs 2 and 3 (nor a full view of the latter texts themselves). I wonder if general estimation of enhanced wave generation by estuary/coastal currents might have been addressed (e.g. by a body like the US Corps of Engineers) in guidance for the preliminary assessment - i.e. before any detailed modelling - of proposed port or other developments. Perhaps someone with access to that literature can turn up something.

franksingleton
01-11-15, 17:12
…………
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
01-11-15, 18:58
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
01-11-15, 20:21
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
01-11-15, 20:45
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
01-11-15, 20:51
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
01-11-15, 20:59
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
01-11-15, 21:54
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
01-11-15, 21:57
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/PublicWeb/amp/mmop/documents/JCOMM-TR/J-TR-21-WWSS-CAR/DOCUMENTS_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
01-11-15, 22:54
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
02-11-15, 01:20
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=pj5tdTd_u_kC&lpg=PA88&ots=m4N_dpl6PC&dq=waves%20bigger%20on%20flood%20tide&pg=PA88#v=onepage&q=waves%20bigger%20on%20flood%20tide&f=false

guernseyman
02-11-15, 09:20
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=pj5tdTd_u_kC&lpg=PA88&ots=m4N_dpl6PC&dq=waves%20bigger%20on%20flood%20tide&pg=PA88#v=onepage&q=waves%20bigger%20on%20flood%20tide&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
02-11-15, 09:20
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
02-11-15, 10:24
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
02-11-15, 10:28
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
02-11-15, 10:40
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=pj5tdTd_u_kC&lpg=PA88&ots=m4N_dpl6PC&dq=waves%20bigger%20on%20flood%20tide&pg=PA88#v=onepage&q=waves%20bigger%20on%20flood%20tide&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
02-11-15, 10:50
.....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
02-11-15, 11:06
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
02-11-15, 11:26
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
02-11-15, 12:30
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
02-11-15, 22:44
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.

LeoLind
03-11-15, 03:31
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.

Yes, but regarding waves, refraction refers to the bending of the wave travel direction towards shallower water, such as approaching a beach at an angle, or passing around a shallow headland. I think it usually reduces the wave size as there is divergence, i.e. spreading of the wave energy.