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Tides
Oct 19, 2012 12:26:12 GMT 1
Post by jonjel on Oct 19, 2012 12:26:12 GMT 1
We have recently had the highest tides of the year.
I know why we have spring tides and neap tides, and why some of those have a larger rise and fall.
But, why are there some areas of the world where tides are far greater than in other parts? For example, the Bristol Channel, the Bay of Fundy and a few other hot spots.
I could Google it, but thought I would seek your collective wisdom.
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Tides
Oct 19, 2012 13:55:19 GMT 1
Post by alancalverd on Oct 19, 2012 13:55:19 GMT 1
Mixture of resonance and restricted-flow phenomena.
For all that the alternative energy brigade would wish otherwise, it seems that only the Severn, Bay of Fundy and Rance estuary are remotely economic for large scale tidal electricity production. The fact that the Rance barrage has been working for years and the others haven't progressed much beyond proof of concept just demonstrates how marginal it all is.
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Tides
Oct 19, 2012 14:39:54 GMT 1
Post by Progenitor A on Oct 19, 2012 14:39:54 GMT 1
Mixture of resonance and restricted-flow phenomena. You took the words right out of my mouth! I am on page 22 of the book 'How to Sound Like A Convincing Politician'. You must be chapters ahead as you write like a a pastmater of the genre!
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Tides
Oct 19, 2012 18:57:22 GMT 1
Post by alancalverd on Oct 19, 2012 18:57:22 GMT 1
Not sure I understand your compliment, if it is such.
The difficulty one has in answering your question in any more detail is that the world's coastlines are enormously variable so whatever equation one might propose for Southend wouldn't work for Southampton, let alone Sydney.
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Tides
Oct 19, 2012 19:34:58 GMT 1
Post by principled on Oct 19, 2012 19:34:58 GMT 1
As I understand it, the tidal range is a function of the gravitational pull of sun and moon, the atmospheric pressure, volume of water, funnel effects of the topography and Earth's rotation. However, for a given latitude a number of the above must be common, so I would assume that eventually range comes down to the middle 3: pressure, water volume and topography. But as the nearest I've got to being a mariner is when I was a sea scout, I could be talking utter bottox! P
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Tides
Oct 19, 2012 19:50:42 GMT 1
Post by Progenitor A on Oct 19, 2012 19:50:42 GMT 1
As I understand it, the tidal range is a function of the gravitational pull of sun and moon, the atmospheric pressure, volume of water, funnel effects of the topography and Earth's rotation. However, for a given latitude a number of the above must be common, so I would assume that eventually range comes down to the middle 3: pressure, water volume and topography. But as the nearest I've got to being a mariner is when I was a sea scout, I could be talking utter bottox! P ;D What I dont understand is that the Mediterranean does not have tides. Does that mean that at Gibraltae and Port Said we have a cliff-edge of water 6' high at what should be high tide?
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Tides
Oct 19, 2012 20:23:16 GMT 1
Post by alancalverd on Oct 19, 2012 20:23:16 GMT 1
The "funnel effects of topography" are by far the largest variable.
The gravitational effect is much more one of dragging the oceans around the world than of heaping them up. Think about pendulum clocks: the effect of daily variation in the gross g vector is for all practical purposes negligible. But when that tiny variation affects the liquid that covers most of the planet, it's not surprising that over the last umpteen zillion years it has developed something of a circulating wave.
The depth of the mid-ocean doesn't vary nearly as much as the height of the sea where it hits the land. It's not only the 2-D "map" topography that affects tidal range at a given point, but the nature of the continental shelf too.
The Mediterranean is only weakly coupled to the major ocean system, through a very narrow gap that itself is not subject to a large tidal range (topography!). The tidal current through the straits of Gibraltar is very strong but the volume that can flow through in 6 hours is tiny compared with the volume of the Med, so not much range inside the basin.
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Tides
Oct 20, 2012 15:13:04 GMT 1
Post by mrsonde on Oct 20, 2012 15:13:04 GMT 1
Mixture of resonance and restricted-flow phenomena. You took the words right out of my mouth! I am on page 22 of the book 'How to Sound Like A Convincing Politician'. You must be chapters ahead as you write like a a pastmater of the genre! 22 pages! Admirable perseverance Nay; even if 20 of them are pictures on how to move your hands like Tony Blair.
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Tides
Oct 20, 2012 15:19:07 GMT 1
Post by mrsonde on Oct 20, 2012 15:19:07 GMT 1
Not sure I understand your compliment, if it is such. A blithe assumption of benevolent allowance is the best policy around here Alan, good move. Hard to prove whether it means your wits are dim or sharp. I always hope. Australians are not renowned for their mathematical competence, granted, but that seems a mite unfair.
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Tides
Oct 20, 2012 22:00:54 GMT 1
Post by rsmith7 on Oct 20, 2012 22:00:54 GMT 1
"Mixture of resonance and restricted-flow phenomena." Yeah, understand that but have never got why Orkney has about 4m range, Shetland about 0.5m, Bristol about 13m yet Stavanger has less than 0.5m. You'd have thought Stavanger would have had more considering the topography isn't that different to Bristol. Remember diving in Scapa Flow and coming across a weird opaque curtain like tide boundary. Swam into it and could feel the flow across the part of my body that was in it. Wasn't strong enough to spin you round but it was enough to change your direction of travel. Very strange...
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Tides
Oct 22, 2012 15:12:46 GMT 1
Post by jonjel on Oct 22, 2012 15:12:46 GMT 1
I thought there was a very small tide in the Med, around 8" from memory. And someone once told me there were measurable tides on some of the great lakes. How you measure 'em is another matter.
As for diving, I remember on many occasions diving with a snorkel and around 10' down the water is suddenly very much colder. almost like a boundary layer.
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