Tides

[qweetcher]

The Bristol Channel has the second highest tidal range in the world, only exceeded by the Bay of Fundy in Canada. I'm always astonished when I think about the Bristol Channel having the second highest tidal range in the world. Are there any other parts of the UK which have amazing tides?

 

[johnalison]

Not really in the same league, but I remember refuelling at the old station in St Helier years ago at LW and being amazed at the length of fuel hose being lowered, with about 14 1/2 metres tide.

And I believe there is somewhere in the middle of the North Sea with no change in height at all. I used to know the long Latin or Greek name for it.

 

[Woodlouse]

I believe Jersey comes third in the tidal height stakes with over 12 metres on the big springs. I have to say it's amazing the range of tides we have around our coasts, all the way from over 15 metres in some parts to practically nothing in others.

 

[Quandary]

At Gigha, the max. spring range is about 3', half of that at neaps, makes it very easy to get in and out of your dinghy at the jetty.

 

[Allan]

I believe Jersey comes third in the tidal height stakes with over 12 metres on the big springs. I have to say it's amazing the range of tides we have around our coasts, all the way from over 15 metres in some parts to practically nothing in others.

I recently read a report by Dave T Pugh of the Natural Environment Research Council in Swindon. In the report he states that the Bristol channel, my present sailing area and Granville in France, my previous sailing area, are the second and third highest tides in the world. I imagine Jersey comes into the Granville area as they are only about 25 miles apart. Having sailed in these two areas, I am fasinated by tides. Some people have said that I must be a gluton for punishment! I think it makes life easier, I often get the SOG into double figures. Passage plans are easy from Cardiff, if the tide is up, turn right, if it's down turn left!
Allan

 

[Captn D]

I have often thought that the subject of tides and its fundamental principals, plus the various factors that make certain areas of world to have greater or lesser tidal ranges etc., interesting enough to be the subject (plus suitable photos of races, bores and other tidal phenomena) of a good book. However a post a few months back here appeared to deem the subject too boring to be of interest. Perhaps today, where there are very boring books on nearly everything, the time is right for a properly researched and interesting book on this very significant subject.

 

[Allan]

I have often thought that the subject of tides and its fundamental principals, plus the various factors that make certain areas of world to have greater or lesser tidal ranges etc., interesting enough to be the subject (plus suitable photos of races, bores and other tidal phenomena) of a good book. However a post a few months back here appeared to deem the subject too boring to be of interest. Perhaps today, where there are very boring books on nearly everything, the time is right for a properly researched and interesting book on this very significant subject.

I agree, I have read a couple of papers on the subject and find it a very interesting subject.
Allan

 

[reginaldon]

In contrast with the Bristol Channel, I lauched my Heron at Lulworth Cove and when I asked about the tides, I was told it was only a foot or two and could be ignored - I never tried to verify it - I suppose I could check Wiley or Reeds.

 

[Allan]

In contrast with the Bristol Channel, I lauched my Heron at Lulworth Cove and when I asked about the tides, I was told it was only a foot or two and could be ignored - I never tried to verify it - I suppose I could check Wiley or Reeds.

Reeds give a Mean level for Lulworth Cove of 1.2m. I know the tides in the Bristol channel are due to the geography but I find it interesting that tides can vary within a relatively short distance.
Allan

 

[macd]

At Gigha, the max. spring range is about 3', half of that at neaps, makes it very easy to get in and out of your dinghy at the jetty.

The sea area S and W of Gigha, and perhaps the one referred to in the N sea, is know as an amphidrome and has an even lower range. (You can check it out on Wikepedia although this particular one is not mentioned there.) Useful to note that the absence of tidal range is not the same as an absence of tidal streams.

 

[Marsupial]

The sea area S and W of Gigha, and perhaps the one referred to in the N sea, is know as an amphidrome and has an even lower range. (You can check it out on Wikepedia although this particular one is not mentioned there.) Useful to note that the absence of tidal range is not the same as an absence of tidal streams.

at last! a bit of sense about tides and tital streams. Thank you for helping me remember that. YM Instructors _ do your worst!

 

[maxi77]

Reeds give a Mean level for Lulworth Cove of 1.2m. I know the tides in the Bristol channel are due to the geography but I find it interesting that tides can vary within a relatively short distance.
Allan

And that is very much down to resonance or the lack of it as I was taught by my oceanography lecturere many many moons ago.

 

[OGITD]

River Wyre

Already posted but relevant to this thread as I have seen the Bore down the river Wyre, and looking at the tide range of ten meters recently I am not surprised, as it is quite a narrow river.

http://www.wyrebc.gov.uk/Page.aspx?PgeID=56175

 

[qweetcher]

I have often thought that the subject of tides and its fundamental principals, plus the various factors that make certain areas of world to have greater or lesser tidal ranges etc., interesting enough to be the subject (plus suitable photos of races, bores and other tidal phenomena) of a good book. However a post a few months back here appeared to deem the subject too boring to be of interest. Perhaps today, where there are very boring books on nearly everything, the time is right for a properly researched and interesting book on this very significant subject.

It's definitely not a boring subject. A good book on the subject woud be great. Are you going to write one?

 

[Allan]

I'm off sailing now but will try to remember to post details of some interesting papers about Tides. I do know that one is by David T Pugh and in another, which I found by google his name and details, by another writer who seems to prove that the gravitational pull of the sun is greater than that of the moon although the tides are more down to moon than the sun. I got too confused at that point.
Allan

 

[wirralbells]

Good sources of information

The book published in 2004 by David Pugh is called "Changing Sea Levels" (by Cambridge University Press). It's a good book - not too heavy a read. His earlier book which is very hard to get hold of now is called "Tides, Surges and Mean Sea Level" published in 1987. This is much more technical. Another very technical book and even older is the "Admiralty Manual of Tides".

There's obviously a lot of free information on the web if you google 'Tides' but just be careful. I've read information that is fundamentally wrong when describing some of the physics.

Regarding the highest tides in the world, here's a list I compiled a couple of years ago. The problem of course is deciding when to ignore a location because it is so near somewhere else. The first number is the extreme range and the second is the mean spring range.

Minas Basin (East end of Bay of Fundy) 16.0m 13.1m
Avonmouth (Bristol Channel) 14.8m 12.2m
St. Malo, France 13.5m 10.7m
Punta Loyola, Argentina 13.2m 10.4m
St. Helier, Channel Isles 12.2m 9.6m
Bhavnagar, India 11.7m 8.8m
Liverpool, England 10.7m 8.4m
Inch'On (Chemulpho), South Korea 10.6m 8.1m
Swansea, Wales 10.4m 8.5m
Barrow (Ramsden Dock), England 10.3m 8.2m
St. Peter Port, Channel Isles 10.3m 7.9m
Dieppe, France 10.1m 8.5m


Lastly, Allan mentioned about the strength of the gravitational pull of the moon and the sun. The gravitational pull of the sun is indeed much greater than that of the moon - this is because gravitational pull is inversely proportional to the SQUARE of the distance between the objects. However the tide-producing force is actually inversely proportional to the CUBE of the distance between them. This means the nearness of the moon suddenly becomes more significant than when just considering the gravitational pull.

Interesting thread.

Colin Bell
National Oceanography Centre.

 

[AntarcticPilot]

Lastly, Allan mentioned about the strength of the gravitational pull of the moon and the sun. The gravitational pull of the sun is indeed much greater than that of the moon - this is because gravitational pull is inversely proportional to the SQUARE of the distance between the objects. However the tide-producing force is actually inversely proportional to the CUBE of the distance between them. This means the nearness of the moon suddenly becomes more significant than when just considering the gravitational pull.

Interesting thread.

Colin Bell
National Oceanography Centre.

Just a little more explanation - tides are caused by the gravity gradient across the earth, that is, the difference in the gravitational attraction of the sun or moon from one side of the earth to the other. As the Sun is much further away than the moon, this gradient is smaller for the sun than for the moon. As Colin points out, this is because gravity obeys an inverse square law, so the gravity gradient is proportional to the differential of 1/(distance squared), i.e. 1/(distance cubed)

I guess the first correct explanation of tides is probably in Newton's Principia! However, as it is in Latin, it is not very accessible these days

 

[Porthandbuoy]

And I believe there is somewhere in the middle of the North Sea with no change in height at all. I used to know the long Latin or Greek name for it.

Does the term "Degenerate Amphidrome" ring a bell? Where cotidal lines of zero change intersect.

 

[Ubergeekian]

The sea area S and W of Gigha, and perhaps the one referred to in the N sea, is know as an amphidrome and has an even lower range. (You can check it out on Wikepedia although this particular one is not mentioned there.) Useful to note that the absence of tidal range is not the same as an absence of tidal streams.

All sorts of funny things happen there. As I recall, for example, spring tides (or maximum ranges) at Machrihanish correspond with neap tides (or minimum ranges) at Port Ellen, just the other side of the sound.

I once saw an honest-to-god waterspout linking cloud and sea just off Machrihanish. Nothing to do with tides, but it was very impressive.

 

[alan_d]

All sorts of funny things happen there. As I recall, for example, spring tides (or maximum ranges) at Machrihanish correspond with neap tides (or minimum ranges) at Port Ellen, just the other side of the sound.

Not in my copy of Reed's - the standard port for both is Oban, although it does say the mean range for Macrihanish is 0.5 metres. Difficult to see how it could work, as springs occur around full moon and new moon, which occur at much the same date everywhere on Earth. I know that high water in many places does not coincide with the time of the Moon's meridian passage but lags by a period of hours (because of friction etc.), but a week to get from Port Ellen to Macrihanish does strain credulity.

As the tidal ranges are so small, what you are describing could happen from time to time due to the effect of prevailing winds etc. but I do not think it could be a consistent harmonic.