Idle tidal question

A

alan_d

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I know that for any particular place the times of high water springs and high water neaps are easily predictable. For example, HW springs at Oban is always about 6am and 6pm UT, while HW neaps is around midday & midnight. What I do not know is how stable this is over time. Will it be the same in 10 years, 50 years, 200 years, or does it gradually drift? Why (or why not, as the case may be)?

Alan

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I don't think so, the times of the tides you are talking about depend on the moon's position relative to the sun and local, large scale underwater topography, so as the moon gradually slows down over the millenia it may have an effect, but it is tectonic activity will cause the changes as Ireland changes position relative to Scotland and Scotland itself changes shape. I don't expect to notice any difference in my lifetime, though.

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it's all in the physics....

high tide occurs when the moon is overhead or directly below. when that happens at noon the sun is also directly overhead, in other words in line (spring tides). when the moon is overhead at 6am/pm the sun is at right angles to the moon (neap tides).

that's the theory. in practice high tide occurs a bit after the moon passes overhead but the delay between the passage of the moon and high water is a constant for any given place.

so the answer is that the relationship between time of high water and height of tides is fixed.

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All in the physics is correct in one sense...

I work on survey ships at the moment (although I am not a hydrographic surveyor myself). Recently I made the foolish mistake of asking about tides in one area that we were working in. The surveyors eyes lit up - and mine glazed over as he started to transmit on the approx 240 (yes thats two hundred and forty) factors they take into account when doing tidal calculations. Thats sun moon, planets, cororiolis (sp?) etc etc...... (and each of these have several factors each...)

If you want I will ask whether the tides will change their times of HW springs etc over a period of decades or centuries.

Finally, if its any consolation, hope that you would be impressed by the care that is taken to ensure tidal data is accurate - and thus surveys are accurate as well. They don't use tide tables, Tides are measured with a tide gauge and then calculated from first principals.



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Looks like your supposition must be corect for as long as it matters to me!!
I think these look like spring tides..
OBAN
Times are GMT
16 Nov 2054 (Mon)
06:00 3.90
12:10 0.79
18:20 3:93

17 Nov 2054 (Tues)
00:30 0.77
06:40 3:93
12:50 0:81
18:50 3.87

Cheers Bob E..


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it does drift, and its complicated. Lunar precession, i believe - the same sort of reason why the stars aren't in the same place from the pyramids as the used to be in ancient times. Also, the earth is slowing down a teeny bit. Loads of webby stuff about this, and needs undersatnding of how there are several different ttype of lunar month, which buggers up the tides, hence yer need the tables, dang.

<A target="_blank" HREF=http://members.lycos.co.uk/marktovey/tides3.html>http://members.lycos.co.uk/marktovey/tides3.html</A>

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the question that really needs to be asked is where we can get hold of all these factors so we can plug them into tidal software and circumvent the UKHO's 'ownership' of the tides!

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Re: high water and height of tides is fixed

So what about wobbly orbits??? presume our wobbly orbit of the sun will affect angle and hence tides.. does the moon wobble too?

I know this is a bit nit picky but I'm just curious...

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Re: high water and height of tides is fixed

Yep, wobbly orbits were one of the factors as well. Precession of the earth being one of the larger minor elements if I remember the explanation.

I note that Total Tide which is accepted for commercial use in place of tide tables uses 60 constants.

If you go to the UKHO Totaltide webste:

http://www.hydro.gov.uk/ttflash/webfiles/exit.htm

there is an intro to tidal calculations. Click on "About Tidal Predictions"

Reference Snowleopard: Please don't get me going on why Charts and Tide info should be free. UKHO is a business. No income - no business...

And no my referring to the UKHO site is not a cheap plug for the Totaltide program. The site just happens to have a brief explanation of tides on it that goes beyond sun and moon. And you might be interested to know that when surveying the surveyors don't use Totaltide - they use a formula that takes every factor into account.

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Re: high water and height of tides is fixed

UKHO doesn't mention wobbly orbits.. or whether Total Tide takes these into account .. or indeed whether it is possible to factor in wobbly orbits..

Is it possible to calculate the expected wobble.. do we know when the next wobble occurs..? Otherwise wobbly orbits cannot be factored into tidal calcs..

I generally work on the basis that whatever the source of tidal data it is an imprecise science like the weather as the systems are as yet to complex/chaotic to fully model future tides/weather.

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I think everyone is missing the point really.
Yes I agree with issues of gravitational pull and lunar procession but the key factor missing in all responses is Oban.
The Oban issue must be factored in to all considerations - take for example the difficulty one can have just finding the exit from the supermarket when you are trying to get down to Loch Melfort. I systematically fail to find it ending up either in a Council Yard or going around the roundabout outside Nancy Blacks with Dear Heart murmering - "There, you've misssed it again you lunatic" Lunatic - Lunar - Moon - Tide - QED

<hr width=100% size=1><font color=purple>regards
Claymore<font color=purple>
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http://www.whoi.edu/services/facilities/CLAYMORE/
 
Re: One way tidal systems

My problems is that the one way tidal system model for Oban is regularly creates traffic overfalls and eddies.. races are not uncommon either and generally cannot be predicted.. I have found that rain increases the traffic overfalls tho..

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The theory of tides is covered well in the Admiralty Manual of Tides (NP 120), if it's still in print. It's by two men called Doodson and Warburg. Doodson (D.Sc., F.R.S) was Director of the Liverpool Observatory and Tidal Institute, and Warburg (MSc, Commander RN (retd)) was Superintendant of Tidal Work for the Admiralty Hydrographic Dept. NP120 was originally published in 1941, but was reprinted with minor additions in 1980.

Doodson calculated the harmonic components of the tide-generating forces in a paper to the Royal Society in 1921. Somewhere around I have a copy (though I can't find it at the moment), but if I remember correctly it quotes several hundred components.

On the usual explanation of tides one might expect that high water would always occur when the moon was overhead (or underfoot), or perhaps a little later if the high water "hump" were chasing the moon. That might be true on a totally water covered Earth, but the presence of land masses changes things completely.

The best indication of the way that HW time changes is given by Admiralty Chart 5058, which shows co-tidal and co-range curves for the British Isles and adjacent waters. The co-tidal lines are specified by what used to be called the "establishment of a port", i.e. the delay between meridian passage of the Moon and HW at the port. Chart 5058 doesn't just cover ports, though; it gives the co-tidal curves for the whole of the English Channel and the North Sea, and well out to the west as well. The co-range curves similarly join places having the same tidal range. (Anyone who uses the Admiralty Tidal Atlas for the Thames Estuary will find co-tidal and co-range curves for the Estuary in the back.)

Since it is the land masses that cause the behaviour of the tide, any change in the configuration of the land can cause changes in tide times and heights.

Probably the earliest recorded attempt at tidal prediction is given in the "Codex Cottonianus, Julius DVII", a manuscript held in the British Museum (or could now be the British Library). It contains information provided by one John Wallingford, who died as Abbot of St Albans in 1213. He quotes the time of HW at London Bridge, specifying it by the age of the Moon:-

New Moon: HW 0300
1 day old: HW 0348
2 days old: HW 0436 and so on.

Comparing this with modern tide tables, HW London Bridge is now about an hour earlier, mainly due to changes in the configuration of the river. And that's after nearly 800 years.



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Not just the shape of the river but its height and 'tidalness'. It has risen at least 13 ft in the last thousand years and the ford at Tilbury existed at least until the Norman Conquest. If you really want to see how much the Thames basin has changed look at the mardyke, which predates both London and its river and runs 'under' the Thames.
ps fantastic post, Peter
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Here's an interesting one for you.

Artic/Greenland ice sheets are melting fast. Tidal height in that area (and in most of nothern hemishphere) will decrease. Logic says it should go up. Why? I'll let people have a crack at this before I give the answer away. hint, tidal height in Antartica and southern hemisphere will increase

<hr width=100% size=1>Me transmitte sursum, caledoni
 
Because water is heavy and so will run down to the bottom of the earth.

No /forums/images/icons/smile.gif?

John

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But close you will have to concede?

But no more clues from me, I am off down to Antarctica with my graviton powered blowtorch to melt some ice to flood you lot up there /forums/images/icons/smile.gif.

In any event, supposedly the bit of NZ we live on here rises faster than the most optimistic forecasts for sea level rising - trouble is the ride up can be a little shaky here at times and may get wet before some of the shaky bits.

I think the same for SE England as well (without the shaky ride) maybe someone could comment?

Regards

John

<hr width=100% size=1>I am the cat but I am only 6.
 
Fairly simplistic article, but explains the concept well

http://www.newsandevents.utoronto.ca/bin1/010221b.asp



<hr width=100% size=1>Me transmitte sursum, caledoni
 
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