Waves below the surface this time!

[saltwater_gypsy]

For years I have tried to understand sea-wave formation. and the one that still intrigues me is the situation in the Bay of Biscay.
The common understanding is that Atlantic waves roll into Biscay and about 100miles from the coast they reach the Continental Shelf where the depth changes abruptly from about 3,000metres to about 120metres this then causes the waves to rear up to great heights.
Hang on though, the waves are a surface effect (Tsunami excluded) and the effect of a surface wave is not going to extend much more below the surface than its height above. so how can there be an interaction with the seabed until the wave reaches the shallows which are typically only five miles offshore?
I know that the situation in Biscay can be horrendous as the start of the 2006 Velux race demonstrated.
So my basic question is: “How far below the surface are surface waves effective?”
Is there a submariner out there who can report on stormy conditions at 100m below the surface?
I suspect that a lot of the problems in Biscay stem from low pressure systems moving through rapidly so that the change in wind direction causes large confused seas similar to the 1979 Fasnet storm.

 

[NickiCrutchfield]

Nothing informative to offer I'm afraid, but it is a pleasure to have such an interesting post to read.

 

[colingr]

From my diving experience the waves only have an effect proportional to their height. As such I would be suprised if the waves in the Bay of Biscay are caused by the normal wave effect. It might be due to the mass of water circulating around the Atlantic suddenly meeting a sharp reduction in depth causing an upward bulge?

Sort of a vertical Severn Bore if you see what I mean.

I'm sure someone will be along shortly to pick a fight though, sadly.

 

[Phoenix of Hamble]

Can only echo the comments of above posters. What a pleasant change to have an interesting question to discuss.

I'm afraid I have no light to shine on the question, either, but will wait the theories/knowledgeable answers with enthusiasm!

 

[colingr]

Perhaps we can make this a bicker free thread? /forums/images/graemlins/ooo.gif

If we all hold hands and sing a happy song /forums/images/graemlins/grin.gif

Mines a /forums/images/graemlins/smirk.gif

 

[john_morris_uk]

I am not a submariner by trade, but I have been in a few submarines dived so I will give a quick two pence from my limited exterience.

In rough weather you need to go fairly deep to avoid the effects of the waves on the surface. Perhaps deep enought to hit the continental shelf. If I remember rightly, at 50 or 60 metres you can sometimes still feel the motion of the storm on the surface.

No doubt someone will be along with a more accurate memory than me in a few minutes.

Does that help?

 

[johnalison]

Although what you say is broadly true, a wave is associated with a circular movement under the water (as seen from the side) and the drawings I've seen suggest that this extends somewhat deeper than the wave height and I would think was related to wavelength rather than height, though still a long way short of the kind of depths you're talking about. I too look forward to an answer from someone who actually knows.

 

[Twister_Ken]

My guess - there are transoceanic currents which work throughout the water column. When the moving water suddenly bashes into by the continental shelf, it has nowhere to go except upwards. The rising water will interact with the surface water, in ways which may not be entirely pleasant for the passing yottie, especially if there's a hatful of wind about to stir things up a bit.

Hence continental shelf (or any other sudden change of depth) will create worse sea condits than otherwise.

 

[AliM]

Well, I don't know either, but the wavelength of water waves depends on the depth in shallow water - presumably not in the deep ocean, but certainly when you get to the continental shelf - that's the effect you see when waves break on the beach, or when the tsunami came into shallow water. So we have long wavelength waves suddenly having to become shorter wavelength when they hit the continental shelf, so they must pile up. This can, presumably, be done calmly when the wind is moderate, because Biscay has its calm periods, but when there is any strong wind to complicate the whole thing, or the original waves (which came from the other side of the Atlantic) are particularly big or confused, then all hell breaks loose.

All speculation, but no one else seems to know, so I might as well speculate! I'll defer to anyone who has a clue, though.

 

[sarabande]

here's a fine wee website on the subject, with a lot of clear graphs to show the relationship between wave "depth" and speed, and frequency.

It also explains about wave height, which was a recent question on board the forum.

http://www.seafriends.org.nz/oceano/waves.htm

 

[peterb]

There is a mathematical answer to this one; I have the textbook and I'll look it up. But in the meanwhile I'll quote from the Met Office's book "Meteorology for Mariners".

"The disturbance set up by wave motion must necessarily extend for some distance below the surface; but its magnitude decreases very rapidly in accordance with a definite law, the trochoids becoming flatter and flatter as the depth increases and the water particles revolving in ever-decreasing circles. At a depth of one wave's length the disturbance is less than a 500th part of what it is at the surface, so that the water at that depth may be considered undisturbed."

The problem is to know what the wavelength is. Roughly speaking, in deep water the wavelength (metres) is 1.56 times the square of the period (seconds). So a wave which takes 5 seconds to rise and 5 seconds to fall will have a wavelength of about 150 metres. If the water gets much shallower than that, then the front face of the wave will start to steepen towards breaking.

The problem at the continental shelf is that some of the oceanic swells have very long periods; 20 to 30 seconds, giving wavelengths of a kilometre or more. To such waves the shelf is quite shallow; the wave slows and steepens considerably. A breaking wave of this type has a lot of energy and can do very nasty things to a boat.

 

[mobeydick]

The depth of the water does not have much effect on waves as long as it is > L/2 (where L = wavelength), and has most effect when < L/20. The wave does not 'feel' the bottom when depth > L/2.

However, ocean wavelengths range from a few meters to more than 1400km (for tides, which are just long-period waves). I believe the it is the interaction between the currents caused by the long period waves 'feeling the bottom' and the short period storm waves that actually cause the confused seas. (That plus wave refraction from the the coast of the bay)

 

[alahol2]

Apparently there are deep water waves...
Invisible waves

 

[AntarcticPilot]

Another possible effect is that surface waves are not the only waves in the sea. You also get internal waves between different thermal layers in the sea, and I THINK that these have very long wavelengths and would be affected by shallowing onto the continental shelf. And, of course, the energy in these waves also has to go somewhere - potentially into increased surface waves.

 

[snowleopard]

[ QUOTE ]
ocean wavelengths range from a few meters to more than 1400km (for tides, which are just long-period waves). I believe the it is the interaction between the currents caused by the long period waves 'feeling the bottom' and the short period storm waves that actually cause the confused seas. (That plus wave refraction from the the coast of the bay)

[/ QUOTE ]

Here is an article covering that point plus others.

Apologies for Fred Rift, but while googling I came across this statement in an article on the Bay of Biscay:

"The British Yachtswoman Ellen MacArthur finished her first trip around the world here, and there is a famous song based on this."

Anyone shed light on that?

 

[saltwater_gypsy]

Thanks Sarabande I think the website gives a good part of the answer!! In the excellent article it shows that the depth for interaction depends more on the distance between waves (the wavelength) than the height.
It seems that the depth of interaction is about half of the wavelength
An extreme example quoted is that from a 60knot wind the waves may have a height of 6m and have a wavelength of 450m. The depth of interaction is therefore of 225m which is getting close to the Continental Shelf depths.
Thank /forums/images/graemlins/smile.gifyou gentlemen for an interesting and courteous thread.I think some others indicated that this was the solution but also raised some interesting ideas about deep ocean waves. -thanks.

 

[maxi77]

Unlike the poster above I was a submariner and I can confirm that wave motion extends quite far down, and although the vawe effect is diminished as you get deeper it is still quite noticable below 300 feet.

In general the ocean waves extend to a depth similar to their wave length and this whole mass ofd water is moving constantly. As you move into shallowere water the deep part of the wave is slowed causing the wave to 'trip' which makes it rear up and break.

You can watch the same effect on a beach when there is surf and breakers, hence the problem in the bay where you have a rapidly shelving continental shelf fed by a substabtial North Altantic swell and in storms big waves.

 

[stuey_two]

This week the wave period at Biarritz peaked at 15 seconds. Using the excellent website identified by Sarabande, this gives a wavelength out in the deep sea of 360m which means that waves were being affected by the seabed at any depth less than 180m - ie the edge of the continental shelf.
Fascinating!

 

[l'escargot]

I don't know about 100m but from my diving days I recall feeling the effects of waves 30m below the surface when we used to dive after stormy weather and there was a sea running.