How can the Continental Shelf cause larger waves?

[jerrytug]

Hello we would be grateful if someone could settle this argument,which is:
So... "everybody knows", and it has achieved myth/legend status, that when the seabed shoals from (very round figures) 1000` up to 100` coming back in to Biscay from sea,the waves grow due to the shallowing, as in a surf beach for example.
My side of the argument is that the edge of the continental shelf is too deep to affect the waves down at 50 m depth,70 fathoms whatever,or whatever,and it`s all psychological and cultural.
If sailors didn`t know it was there from looking at soundings on their chart,or heard or read of its reputation,they would not know about the edge of the continental shelf.

It`s clearly an area with some naughty weather (on the surface) but putting that aside,how could the seabed so far down affect the surface conditions? Technically it can`t surely?
Submarines only have to drop down to 10 or 20 metres to avoid a storm,if I understand it right.
So the continental shelf definitely does not make the waves higher because it couldn`t according to the laws of physics.
Would be grateful for some scientific back-up on this one because I know I am right,please any oceanographers or physicists out there on this forum would you be kind enough to put the myth out of its misery so we can all crash out,mucho obligado,Jerry et al.

 

[Deleted member 36384]

Ocean currents are directed upwards on a continental shelf. The upward current produces a water velocity and direction which may counter the surface water velocity and direction. The energy behind the ocean current is sufficient that it keeps powering the water in the direction it has adopted more or less independently of any surface direction caused by a storm.

 

[AntarcticPilot]

Basically, the surface wave extends below the surface by about the same depth as it's wavelength. For ocean swells, that is hundreds of metres, so even deep water by our standards can exert a drag on oceanic swell waves. The effect of the shallowing water is to slow the wave down, so the waves don't actually get higher - but they do get much steeper, as the crests get closer together. You also get interference effects with shorter waves, so the chances of a big crest or a deep trough are much increased.

The formal definition of the edge of the continental shelf is usually either the 500m or 1000m contour - there isn't a lot of difference in the location; the continental slope is very steep. So, you're going from water that is kilometres deep to water that is 100s of metres deep in a short distance.

 

[jimi]

My thoughts are that the surface wave produces a wave in the water below it but containing a bit less energy ad infinitum until the bottom is reached. When shallower water is reached the energy compresses back up into the available depth and therefor more energy is imparted to the surface wave. However that is my theory and I'd be glad to hear someone who actually knows what happens to explain it ;-)

 

[Roberto]

During an approach we once passed here, there are just a few miles miles between the 1000m and 50m contours with a bit of SW swell (not much, say 2-2.5m) and it was definitely, definitely rougher than a few miles before or after , we had the feeling that the boat was moving up and down while remaining horizontal, like if pushed by a vertical piston on the keel, rather unnatural movement.
Once well inside the 50m contour all back to normal.
I guess the depth variation is among the causes, somehow

 

[maxi77]

A long time ago the submarine I was in got caught in a rather nasty winter North Atlantic gale. We went down to around 100 metres to conserve power and wait for the storm to blow itself out. While we were there the boat was oscilating up and down some 50 feet. So yes you do get wave effect quite a long way down. Mind you that was an awful lot better than being near the surface

 

[RocnaONE]

there are also deep sub-surface currents which can trot along quite happily on the ocean floor, until they hit a slope, then up pops the flow, and causes an upwelling near the surface, against which the superficial currents and winds can cause havoc,


Similar to orographic clouds, in a way.

 

[binch]

Waves.

Antarctic Pilot is exactly right.
"Pieces" of water trace a vertically circular locus under the crest of the wave.
When the bits underneath touch the sea-bottom the wave is held back and starts to topple, the leading side gets steeper, in extreme cases the wave breaks .
Our submariner friend is right too. Most submarines go down below the surface disturbance. Their only snag is coming up to the surface again. The boat will have no stability for a moment or two as she breaks surface.

 

[G12]

Hey, this is interesting! So much better than the usual anchor/how big is your boat/Tom Cunliffe doesn't use enough fenders threads of late!
I would be interested to see some sort of animated graphic of this phenomenon.....

 

[Baggy]

Any comments on what makes Biscay particularly bad..
Is the confused sea also due to reflected waves from the surrounding coast.. ?

 

[jerrytug]

Thank you for the interesting and educational replies folks,what an eye opener. One of the great things about seafaring is there`s always something new I`m learning. Nice one good watch Jerry.

 

[cawarra]

Hey, this is interesting! So much better than the usual anchor/how big is your boat/Tom Cunliffe doesn't use enough fenders threads of late!
I would be interested to see some sort of animated graphic of this phenomenon.....

Here here....

 

[maxi77]

Any comments on what makes Biscay particularly bad..
Is the confused sea also due to reflected waves from the surrounding coast.. ?

I think that is part of it but also the bad areas are close to main shipping routes too so a lot of ships are affected. I have experienced some pretty bad weather west of Scotland and also west of Norway but these places were not on main shipping routes so do not have the same reputation.

 

[Nostrodamus]

Look, take no notice of any of the rubbish they are spouting on about or the diagrams. I am going to let you into a seret but you must keep it to yourself.

Sometime ago our Victorian ancestors in their infinate wisdom developed a machine that could really churn up the sea. Several of these have been strategically placed around France to keep them in. A bit like an eletric cattle fence to gently remind them to stay where they are.

What they don't know is that just beyond the machines the Biscay is a millpond but we have to keep telling them stories of how rough it is.

Just in case you were wondering the Americans still believe our stories that the world is flat an ends 20 miles off their coastline..

Just keep it to yourself.

 

[KellysEye]

>Any comments on what makes Biscay particularly bad.

If you look at the contours posted by Roberto you will see they go from 1,000 metres to 50 metres in a very short distance. That really kicks the seas up. We've crossed Biscay twice and kept outside the 1,000 metre contour. On the first crossing we got 50 knots and big seas. I dread to think what it would have been like inside 50 metres.

 

[Leighb]

Here here....

Where? Where?

 

[Dromedary]

http://en.wikipedia.org/wiki/File:Deep_water_wave.gif

This illustrates particle motion in a deep water wave (possibly using Stokes fifth order wave equation)