The Naked Scientists Forum

Author Topic: Why are the waves so much higher with 'wind against tide'?  (Read 15130 times)

lyner

• Guest
Why are the waves so much higher with 'wind against tide'?
« on: 08/08/2008 13:58:25 »
A tide of just 2kt can make a real difference to the height of waves in a 30kt wind. The relative windspeed is not very different so how does it work?

I suspect that it is something to do with the refraction of the wave underwater in a similar way that sounds seem louder when you are downwind of the source.

What do you think?
« Last Edit: 25/10/2015 09:33:15 by chris »

graham.d

• Neilep Level Member
• Posts: 2208
Why are the waves so much higher with 'wind against tide'?I
« Reply #1 on: 08/08/2008 15:14:22 »
The waves will be moving in the direction of the wind. The speed of the waves has little relation to the wind though. If the current is in the other direction, the wave will be moving opposite to the current. This creates a shear which steepens the leading edge of the wave. The wind strength has not much to do with it, except that the waves are usually bigger when the wind is strong, which is why you see the apparent paradox of the effect of a 2kt tide making a difference when there maybe a 30kt wind. The only effect of the wind is to give the waves direction, unless its strong enough to blow the tops off, but this is a different matter. Best not to go out in 30kt winds :-)

graham.d

• Neilep Level Member
• Posts: 2208
Why are the waves so much higher with 'wind against tide'?I
« Reply #2 on: 08/08/2008 21:55:14 »
I have thought about this a bit more and conclude that I may be wrong about the shear effect. This is more what happens to waves when they approach a gradual shelving beach. It may be more that the frequency of the wave remains constant but the moving water in an opposing direction causes the waelength to decrease. This produces steeper waves, and if steep enough can become unstable and break. Interestingly, I have just spent 20 minutes looking for this on Google but could not get any answers. Plenty confirming the phenomenon but no explanations.

lyner

• Guest
Why are the waves so much higher with 'wind against tide'?I
« Reply #3 on: 09/08/2008 09:53:56 »
Thanks for the answer; it's an interesting phenomenon.
Quote
the wave will be moving opposite to the current
In very deep water, the motion of  the waves relative to the water is (or should be) all that counts - not the actual flow of the water, as long as there is no 'shear' effect, which you refer to.
I suspect it is the shear effect (velocity gradient with depth) which must be responsible.
The effect is more noticeable in shallow water, I think, but tidal flow is sometimes more and sometimes less in shallow water - depending on the topography- so it's hard to be sure.
This is, clearly, a 'harder' question if  people just confirm rather than explain it.

The reason for breaking waves in shallow water is relatively easy to see - wave speed being dependent on depth.
The path of particles throughout the water is, basically, circular, as the wave passes through; the circles getting smaller and smaller as you approach the bottom. I guess the shear effect will alter  at a different rate (with depth) and have some effect.
Anyone else with any ideas?

LeeE

• Neilep Level Member
• Posts: 3382
Why are the waves so much higher with 'wind against tide'?I
« Reply #4 on: 09/08/2008 13:36:55 »
I think it's generally believed amongst surfers that an off-shore wind holds up (as in supporting, not retarding) incoming waves, making them higher and steeper, and therefore better to ride.  Dunno if it's actually true or not, or if it is, whether it would have a noticeable effect on small waves.

lyner

• Guest
Why are the waves so much higher with 'wind against tide'?I
« Reply #5 on: 10/08/2008 18:03:06 »
Oh I see: You mean that the waves have already been formed and the sind just 'holds them up a bit'.
That explanation can apply when the waves originate way out at sea and there happens to be an offshore wind, locally - not necessarily involving the tide at all. In general, though, a SW wind will cause waves from the SW and these waves are worse when the tidal current is towards the SW.

LeeE

• Neilep Level Member
• Posts: 3382
Why are the waves so much higher with 'wind against tide'?I
« Reply #6 on: 10/08/2008 18:42:53 »
Just a thought - have you considered the speed of the waves?  This will be a fair bit lower than the wind speed and an effective difference of four knots would be proportionally greater.  I think

graham.d

• Neilep Level Member
• Posts: 2208
Why are the waves so much higher with 'wind against tide'?I
« Reply #7 on: 10/08/2008 18:43:29 »
Logically it is not the strength of the wind that matters directly and, therefore, also not its direction (at least to a first order). As has been pointed out, a tidal stream of only 1 or 2 knots makes a huge difference to the state of the waves; ergo the windspeed relative to the tidal stream direction makes little difference. And (assuming reasonably deep water) the only factors seem to be the relative speed of the tide and the wind. The key factor played by the wind and its speed, combined with the fetch (the amount of water the wind has travelled over in order to create the waves), is that it simply determines the direction and amplitude of the waves. Over a great stretch of water I would expect that a constant tidal stream and a constant wind would therefore have no noticeable effect on the waveshape relative to any object in the water. If the tidal stream was opposite to the wind, the waves would be moving forward in the direction of the wind at about the same speed relative to the water, if slower relative to the sea bed. I am wondering if the phenomenon of the steep waves with wind over tide is actually only something that occurs inshore, in estuaries or anywhere where the fetch has been across deeper or wider stretches of sea where the tidal stream is low, but where the waves have progressed into an area where the stream is high. The stimulating frequency of the waves is determined in the deep water which creates a certain wavelength, but then this wavelength is shortened when a tidal flow in the opposite direction is encountered. In the opposite case, where the waves encounter a region where the tidal flow is in the same direction as the waves, the wavelength is stretched out and the sea much less steep. What do you think?

lyner

• Guest
Why are the waves so much higher with 'wind against tide'?I
« Reply #8 on: 10/08/2008 20:31:40 »
This is a complicated one.
If  there is a steady wind blowing for many hours, then the direction of the waves will be constant over several tidal cycles but the tidal flow may change by  180degs for the same depth of water  (half tide flood to half tide ebb). The waves have been produced by the wind for some miles or hours. Everything but water flow is the same over the same bit of seabed.
The general statement about wind and tide and waves would still apply, I am sure - I noticed it the other weekend whilst anchored for nearly 18 hours; very uneventful when wind was with tide  and very bumpy when the two were opposed - wind was around 30km/hr on my handheld jobby (That's not a euphemism) and from the same direction pretty well for all that time.
The water was quite shallow - 3m min to 7.5m max and, as the depth makes a lot of difference, the effect was possibly more than it would be in deep water.
I still feel it could be to do with the actual path taken by the moving water under the surface.  (That explains the difference in wave speed for different depths, for instance).In stationary water it will be circles (radius gets less as you go down and is zero on the bottom) but, when the water moves, the circles will be either stretched or compressed  to different degrees, depending on the wave motion and the linear motion (Which gets less as you go down and is zero on the bottom, also). If the two motions don't follow the same law of variation with depth, this could affect the shape / height of the waves on the surface by  artificially producing the effect of altering the effective depth of the water.
Where are all the marine specialists when you need them?

graham.d

• Neilep Level Member
• Posts: 2208
Why are the waves so much higher with 'wind against tide'?I
« Reply #9 on: 10/08/2008 21:25:35 »
I am sure that you are right about the depth having an effect, but it is my experience that the phenomenon occurs in relatively deep water too. There is correlation of course, because it is a change from deeper to shallower water that will increase tidal flows (but so does a narrowing of a deep channel with a similar effect. But this effect is also pronounced when the water is still deep enough to not affect the surface waves unduly (unlike the 3m to 7.5m water you were anchored in). I have sailed round St Albans head a few times and the overfalls, that produce the strong tides and the very steep waves when against the wind, are still 10s of metres deep. The depth variability does produce much turbulance though, and this does extend to the surface in some places.

I am trying to recall whether I have encountered wind-over-tide problems where it has been a narrowing of the channel (rather than shoaling) that causes the increased tidal flow. I think I must have but can't be specific.

paul.fr

• Guest
Why are the waves so much higher with 'wind against tide'?I
« Reply #10 on: 10/08/2008 22:58:41 »
Quote

Wave Height

Five factors affect the growth of wind waves.  First, the wind speed must be blowing faster than the transfer of energy from wave crest to wave crest.  The second factor is the amount of time the wind blows, or wind duration.  The third factor is the fetch, the uninterrupted distance over the sea for which the wind blows without a change in direction.  In the Solent, for instance, the fetch is limited by the surrounding coast of the mainland and the Isle of Wight and is rarely more than 10 miles. At sea it can be thousands of miles.

As waves enter shallow water their speed decreases, wavelength decreases, and height increases.  Waves therefore tend to break in shallow water, for example over a bar at the entrance to a harbour. If the tide direction is against the wind, this will also increase wave height and decrease wavelength. Shallow estuaries and harbours such as Salcombe, Chichester and Carteret will experience large waves in an strong onshore wind, particularly with an ebb tide, and must be avoided in such winds.
So in total wave height is affected by:

Wind speed
Wind duration
Fetch - distance of wind over water
Depth of water
Direction and speed of tide

http://www.btinternet.com/~keith.bater/waves.htm

paul.fr

• Guest
Why are the waves so much higher with 'wind against tide'?I
« Reply #11 on: 10/08/2008 23:06:28 »

lyner

• Guest
Why are the waves so much higher with 'wind against tide'?I
« Reply #12 on: 10/08/2008 23:27:51 »
Quote
There is correlation of course, because it is a change from deeper to shallower water that will increase tidal flows
But the converse can happen where the shape a the shoreline and a shallow area can reduce the tidal flow - used by racing types with 'local knowledge'.

It would be nice to have some serious theory  given to us because, as I said earlier, for extremely deep water, the only real effect must be due to the relative velocity of wind to water flow.

How's this for another idea? The wavelength of the waves arriving in an area will be governed by the wind speed relative to the water speed - imagine the water is stationary, to begin with. There is a gradual transfer of energy from KE of air to water wave energy - the waves build up in amplitude until a steady amplitude is reached. If the air starts traveling faster relative to the  waves (they are being carried against the wind by the tidal flow) then the waves can actually absorb more energy - getting higher. If the tide is carrying the  waves away from the wind, then energy is lost and the waves get less.

I am actually suggesting that there is an affect, rather akin to a kind of resonance  in which, for a given wind speed over the water, a particular wavelength of wave will be formed because the height and wavelength are related and both depend upon wind speed and fetch.. Then,a relatively small change in relative velocity can temporarily cause the waves to gain or lose energy . They already have quite a lot of energy  from miles out to sea and, even if the wind stops, they will keep going for hours, sometimes.
This 'resonance', in fact, is much sharper than you might expect and a small change in relative speeds can give a disproportionate effect.

I shall now look at the links, which appeared whilst I was ranting on. . . .

lyner

• Guest
Why are the waves so much higher with 'wind against tide'?I
« Reply #13 on: 10/08/2008 23:33:48 »
. . . . and the second link has a link "how waves are formed', which suggests that a small increase in relative air speed can quickly increase wave height - the 'exponential region' would take over from the 'saturation' region. For a drop in relative wind speed, the initial energy loss could be quite rapid, too.
It ain't linear, is my point.

2halfwitsdontmakewholewit

• First timers
• Posts: 1
Wind over tide wave face steepness - consider the source of the waves
« Reply #14 on: 09/12/2014 12:32:31 »
Stand on a high vantage point and watch the ebb stream from a fast flowing estuary flow out against an incoming set of waves. This will show the steepness effect in extremis and indeed, many such estuary entrances are flagged as navigation warnings on outgoing tides when the waves are onshore.

Could it therefore be that the wind over tide effect occurs when the waves concerned are previously established under (normal sqrt(1.4)*wl crest to crest conditions) and it is their subsequent introduction to the adverse tidal stream which causes the effect?

Consider a beach adjacent to the outflowing estuary mouth. Clearly the period for waves breaking on the beach will be the same as the period for waves fizzling out in the estuary mouth itself (if not, waves in the tidal zone would continue to bank up indefinitely!) However the waves now move more slowly so the only way to get the same number to break at the estuary mouth as on the beach is to have more waves, which is to shorten the wavelength and therefore increase the relative steepness (and for reasons of conservation of energy, possibly the actual height.)

Areas offshore with long fetches such as the English Channel where the tidal current flips through 180 degrees between tides also exhibit this steepening of existing waves when the wind is over the tide. However, under this contention, if a wind over tide were to "get up" from a calm state, in this case the steepness would not be present.

As to the physics of what happens when a circular wave motion at a certain velocity meets a head on current and the top part of the cirle appears to fall over the lower part producing the taller oval flow, the extent and nature of this and the conservation of various energies and angular momentum must be for theoreticians with more available time than me!

LeoLind

• First timers
• Posts: 5
Re: Why are the waves so much higher with 'wind against tide'?I
« Reply #15 on: 25/10/2015 01:02:36 »
I don't think it's as complicated as you make it sound, and it doesn't only happen in shallow water. 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 more explanations on my website 3D YachtManuals, tab trivia, bottom of page.

Colin2B

• Global Moderator
• Neilep Level Member
• Posts: 1770
• Thanked: 116 times
Re: Why are the waves so much higher with 'wind against tide'?I
« Reply #16 on: 25/10/2015 09:13:28 »
... 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...
You are right in your explanation of how this happens in a narrows. I've seen it in estuarys where current slows at the edge shallows due to friction. It is very dependant on bottom profile as a rise in the seabed will cause current to speed up in a Venturi like effect.

Looking back over the replies here, #2 by graham.d is the closest to what happens in deep water. The current opposes the wave direction reducing the wavelength causing steeper waves because they still have the same energy. The reason it is more noticeable with wind is that swell waves have a longer wavelength so the change due to current is smaller that that of wind waves, which have arisen due to a relatively shorter fetch and have a shorter wavelength and a proportionately greater effect.

I must say I find the behaviour of water waves a fascinating subject.

LeoLind

• First timers
• Posts: 5
Re: Why are the waves so much higher with 'wind against tide'?
« Reply #17 on: 25/10/2015 22:48:43 »
There are probably different explanations for different localities. I am referring to the effect caused solely by the current against a swell, in deep water and with no wind required. My opinion is that this effect I am talking about does not exist if the current is the same all the way across the body of water (imagine the whole ocean surface moving evenly in one direction at 2 knots but unbeknownst to the waves and wind: no effect). But it certainly exists locally way out in the Atlantic in the Gulf Stream, which is not a 'narrows'.  I am saying that the effect (increase in wave size where a current is against the swell) happens only because the current is not the same everywhere, causing bending of the wave-train. In the English Channel for example, the current varies considerably depending on distance from shore and local flows around islands or headlands. Where the current is locally different from the "upwind" waters, there will be bending of the wave train causing areas with larger seas and some with smaller seas, depending on whether the waves converge or diverge. Confused seas and rogue waves usually result from waves arriving at different angles from various sources. When 2 or 3 wave crests coincide, a short lasting big wave is created.

Colin2B

• Global Moderator
• Neilep Level Member
• Posts: 1770
• Thanked: 116 times
Re: Why are the waves so much higher with 'wind against tide'?
« Reply #18 on: 26/10/2015 17:29:58 »
... My opinion is that this effect I am talking about does not exist if the current is the same all the way across the body of water ........ In the English Channel for example, the current varies considerably depending on distance from shore and local flows around islands or headlands. Where the current is locally different from the "upwind" waters, there will be bending of the wave train causing areas with larger seas and some with smaller seas, depending on whether the waves converge or diverge. Confused seas and rogue waves usually result from waves arriving at different angles from various sources. When 2 or 3 wave crests coincide, a short lasting big wave is created.
It is interesting to hear your report of this occurring in deep water away from land. Do you recall any detail of the wave height and wavelength, depth etc?
I have seen the effects you mention in the English Channel and Scottish Islands around islands and headlands and also in the vicinity of submerged rises and hollows in the seabed. The refraction effects can, as you mention, be analysed by wave superposition.

The refraction effect you mention in deep water has the same cause as 'wind against tide' disturbance on a plane wavefront suggested by graham.d. Both are due to water waves maintaining a constant period when they enter the tidal stream, the current then causes a change of wavelength. If the wavelength and phase speed of the incoming waves are λ0 and c0 respectively, and the corresponding parameters are λ1 and c1 for a current velocity u then the period τ=λ0/c01/c1+u. We can see from this that for an opposing current ie u is negative, the wavelength will decrease. Energy conservation means that wave height will increase as will the steepness of the waves. As an example, a wave  of wavelength 15m and speed 10kn (5m/s) entering an opposing stream of strength 2kn (1m/s) its height will increase by 75%.
It is intersting to note that if the current is strong enough such that u>c0/4 then the phase speed becomes imaginary and the wave steepness infinite indicating a breaking wave.

If we consider the example you gave of a corridor of current entering an oncoming plane wavefront, we can see from the above that it will create a similar zone of higher, steeper waves within the corridor. However, the velocity of water waves varies with wavelength (unlike sound and light waves), longer wavelengths travelling faster than shorter. For deep water ie depth>λ/2, then c=√(gλ/2π) where g is gravitational constant. This means that the outer waves in your diagram will overtake those opposed by the current. This is exactly the situation which creates wave refraction and, as you show in your diagram, the outer wave trains will converge towards the centre adding their effect, by superposition, onto that of the current based effect. The result is a very confused area of sea.
As you say, in coastal areas differential currents, islands, sea bed variations etc will all add their effects.

It is worth noting that the same constancy of period and consequent variation of wavelength is also seen as deep water swell waves approach shallower water, causing them to become higher and steeper. This can be seen near many continental shelves eg Bay of Biscay, but also around most coastal waters.
« Last Edit: 26/10/2015 17:38:40 by Colin2B »

LeoLind

• First timers
• Posts: 5
Re: Why are the waves so much higher with 'wind against tide'?
« Reply #19 on: 02/11/2015 00:14:08 »
A deep water area where this is often seen is the in the Gulf Stream along the east coast of Florida, but also well offshore the Cape Hatteras area where the sea bottom should have no effect. I cannot give specific measurements of the conditions I have seen, but suffice it to say that the waves become noticeably larger and confused, just like you might describe them in the English Channel.

What makes you state that waves maintain a constant period when entering the counter current? I can see that your explanation "the current causes a change in wave lenght" would apply to the limited area of where the waves enter a counter current, but not to the whole length of the body of water. Isn't it a bit like running onto an airport conveyor belt walkway, where your body and legs catch up to the new increased air speed after a few strides?

I think my explanation only works when there is a difference in current speed left and right. Yours does not take that into account, but explains an increase in wave height near the frontal area of where the current initially encounters the wave-train, even if endlessly wide. But how far into the current does your effect work?

I feel that my explanation also deals quite well with the same effect on rivers. Current is usually stronger in the center of the river because the edges are shallower and have more drag. So the current causes the wave-train to bend. If against the wind, the bend is concave which causes convergence of waves from each half of the river width towards the center, where they combine their energy. Current with wind does the opposite and we get smaller waves.

« Last Edit: 02/11/2015 00:26:41 by LeoLind »

Colin2B

• Global Moderator
• Neilep Level Member
• Posts: 1770
• Thanked: 116 times
Re: Why are the waves so much higher with 'wind against tide'?
« Reply #20 on: 03/11/2015 00:20:00 »
What makes you state that waves maintain a constant period when entering the counter current?
Scientific observation and measurement from oceanographic survey vessels, weather ships and buoys.

You refer to this as my explanation, but I cannot claim it as mine, credit belongs to the oceanographers who study and teach this subject.
Up to the 1940s mariners believed that the effects of wind-waves and swell against current  were due to the combined velocity of current and waves, much like that of true and apparent wind effect. During WWII there was a concerted effort to try and predict sea state as part of weather forecasts to aid convoy and naval operations and wind against current was an important factor. The first person to document the now accepted explanation was Percy Unna in a paper for Nature in 1942. He realised that waves were too high to be explained by simple velocity addition. Since then the explanation has been studied and confirmed by NOAA, National Oceanography Centre, Met Office, and other oceanographic researchers.

I can see that your explanation "the current causes a change in wave lenght" would apply to the limited area of where the waves enter a counter current, but not to the whole length of the body of water. Isn't it a bit like running onto an airport conveyor belt walkway, where your body and legs catch up to the new increased air speed after a few strides?
No, the effect extends over the whole area, but how it works depends on what is providing the energy for the waves.

Take the case of swell in the absence of wind. You will be aware that such swell is created in distant storm centres where the waves have a wide variety of wavelengths. As the waves move away from the centre the longer wavelength overtake the shorter in what is known as radial dispersion. This leaves the longer more powerful waves which are no longer wind driven, they contain considerable inertia and will continue for long distances before losing significant energy. The current doesn't remove or add energy to the wave but the change in wavelength causes it to be redistributed, so the waves will continue with their new wavelength.

If the waves are wind driven then they will continue until the wind drops or changes direction. If the waves are generated over a large fetch then the longer wavelengths will behave like swell retaining their energy.

I think my explanation only works when there is a difference in current speed left and right. Yours does not take that into account,
As I explained in my previous post the central current causes a wavelength shift resulting in the central zone slowing down relative to the waves on the left and right. It is this difference which causes the waves left and right to swing inwards adding to the steeper waves caused by the shorter wavelength, so yes it does account for the effect.

Current with wind does the opposite and we get smaller waves.
This is as described by the formula I gave in my previous post, but instead of u being negative (current against waves) it is positive and results in longer, less steep waves.

The situation in rivers and estuaries is more complex with varying depth and differential salinity layering, however the principles are the same as I described.

If you consider the effects to be only due to the difference in current speed then wave height, steepness, speed and refraction angles will be underestimated. Under most conditions this is of little consequence as you will be able to see the sea state. However, Unna's work was used by Research Laboratory W as an input into calculating wave heights for the Normandy landings and an underestimate would have resulted in attempted landings during seas that would have been outside the operational limits of the landing craft.

If you are interested in the subject I could recommend you read:
Bascom, W: Waves & Beaches: The Dynamics of Ocean Survey - a classic, non maths book.
Coastal Engineering Research Centre: Shore Protection Manual - all about wave prediction, very practical, some maths.
Komen GK & others: Dynamics and Modelling of Ocean Waves - solid observation based work, but lots of maths

I will be happy to answer other questions or suggest other reading material, but I will have difficulty replying for a while as some harbours have limited Internet access.

LeoLind

• First timers
• Posts: 5
Re: Why are the waves so much higher with 'wind against tide'?
« Reply #21 on: 03/11/2015 04:30:14 »
Thanks for those detailed answers. I'll keep an eye out for those books.

Here is another scenario: imagine a long river with a current running north. Over only a part of this river there is a strong local northerly wind that creates waves running south. These waves never enter an area of stronger current since the current is the same along the whole length of the river. My guess is that the waves will still increase in height along the river center. The only current speed difference that could exist would be river center versus edges. Which phenomena would explain the size increase?

Regards.
« Last Edit: 03/11/2015 04:32:02 by LeoLind »

Colin2B

• Global Moderator
• Neilep Level Member
• Posts: 1770
• Thanked: 116 times
Re: Why are the waves so much higher with 'wind against tide'?
« Reply #22 on: 12/11/2015 14:19:06 »
Excellent, you would make a good researcher, putting up alternatives to be tested by observation. What you suggest is a good test of what happens and measurements confirm that the increase in wave height is predicted in the same way as for waves meeting an adverse current.
In my previous reply I used the example of preexisting waves meeting a current as it is easier to understand and closer to the question posed. In the situation where a constant current is already flowing and a wind develops, the current at the water surface opposes the formation of the waves and creates shorter waves than would have been created without the current - a sort of Doppler effect. The calculations used for wavelength change when waves meet an adverse current also accurately predict the wavelength when they are created over an adverse current. It is the wavelength which determines the speed of the waves, their steepness and the degree of refraction. Even though the wind and current might continue for a long time the waves do not settle to a length that might be expected for no current - think of your conveyor belt, if the person walks and the belt moves at a constant speed then the combined speed will continue until one or other changes.
As you pointed out the shelving of the river creates differing current speeds, but it has other effects on the waves. As the water shallows waves will be refracted towards the shore rather than the centre of the stream reducing the added effect of the converging wave groups.
Rivers are particularly complex and can easily create stopping lines where waves are unable to form due to faster currents over shoals, these can also be seen in tidal races such as The Bitches and Corryvreckan. One effect worth watching for is that under the right conditions waves entering a central current can be captured by total internal reflection and are unable to escape - as in a fibre optic - the result is a band of extremely chaotic water.
Another point worth remembering when dealing with rivers is that oceanographers often use a relative frame of reference related to a ship moving with the current, whereas river specialists will use an absolute reference frame related to the river bank. As with special relativity you just have to remember to convert between the 2 when necessary.

LeoLind

• First timers
• Posts: 5
Re: Why are the waves so much higher with 'wind against tide'?
« Reply #23 on: 15/11/2015 19:08:39 »
Hi Colin.

You say that the wind and waves are aware that there is a current flowing on the river, and take the current into account. This argument is sometimes given to explain the increase in wave height, but common sense tells me it should not because 'tis the apparent water versus wind speed that matters, not wind versus ground. After all the surface of the earth moves at a great speed but we do not factor that in. Unless you mean that in the case of a relatively shallow river there is an effect from a lower current speed below the surface speed.

So if I understand correctly, there are at least 2 different phenomenas (in deep water with no wind necessary) that often combine to increase wave size in a head current .
#1 A wave train encounters a head current which shortens the wavelength and increases the height (maintaining the same energy). This changed size will persist as long as the new current speed stays the same further ahead. If the head current dissappears, the wave train would return to normal size. This effect can be calculated knowing the wave and current speeds.
#2 A wave train encounters a head current of limited width, causing a bend and convergence of the waves from the left and right sides with increase in height but no change in wavelength. As the waves move from the side into the stronger central head current, effect #1 can also add to the wave height. The total effect would be hard to calculate, but could potentially be much stronger than only #1.

Or is effect #2 really the same as #1

Are there names for these effects?
#1 doppler effect?
#2 convergence, focusing?

Regards.

The Naked Scientists Forum

Re: Why are the waves so much higher with 'wind against tide'?
« Reply #23 on: 15/11/2015 19:08:39 »