Wind against tide - physical reasons why this is dangerous ?

[guernseyman]

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]

'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]

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]

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]

.....

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

Cheers,

 

[dt4134]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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.

 

[guernseyman]

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.