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Pooh-Bah
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Pooh-Bah
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Dear faldage, High Tides along the south coast of the UK:This is all strictly non-technical and whether it will assist you in universal understanding I hesitate to predict! I don't think it helped me much - knowledge is one thing, understanding another. What I am told and what I read do not seem to agree. An amateur yachtsman has assured me, I think mistakenly, that the high tide along the south coast moves from west to east. The Atlantic high tide he tells me swells back around both sides of Ireland and around the north of Scotland, moving up the channel and down the North Sea (would this form a sort of mælstrom where they meet, I wonder?). When I look at tide tables for south coast ports, however, my interpretation is that the tide moves east to west along the Channel coast. I think I prefer to trust the tide tables and to avoid cruising on my friend’s yacht! It seems from the tables that there is a high tide approximately every 12 hours 20 minutes. The double tide effect at Poole Harbour causes a secondary peak that occurs about 4 hours after the main high tide. I don't know why it occurs, perhaps Bean can explain; it is generally lower than the main peak, but there appears to be variation in this, I guess depending on where you are in the spring/neap tide cycle. A similar effect is seen at Southampton due I believe to the Isle of Wight and the two arms of the Solent, but there the two peaks are mostly about the same height with a drop of only a few inches between them, so this has the effect of lengthening the duration of high water – obviously very desirable for a port. Again this effect seems more or less marked depending, I guess, on the spring/neap cycle. Where I have given conclusions rather than facts above they are based on my deductions arrived at from looking at the tide tables so could well be completely erroneous! Two interesting links: http://www.ukho.gov.uk/easytide.htmlhttp://www.alia.ie/sailing/tides.htmldxb
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old hand
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old hand
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This has been very interesting indeed! From a glance at the dimensions of Poole Harbour, I would at first guess that the situation is much, much more complicated than first appears. I Googled "double tides" and got this great link for a place in Scotland, which at least states that it's due to the what we call the bottom topography (which is what I suspected all along): http://www.rampantscotland.com/know/blknow31.htmI think to get a good, definitive answer to your question may take some hunting in research papers. Perhaps someone's done a model of the water level and currents in Poole Harbour for a thesis or something. Indeed, from the bit I've read in my book (an introductory oceanography book with merely a single chapter on tides) it would be quite complicated! And from looking at the map, with two sets of narrows and Brownsea island in the middle - I'd say it must be quite complicated - no simple rectangular box for this model! Edit: Oh! Oh! I've found something great, and not technical! Try http://www.weston.org.uk/tides.htm. (I am reading it right now, haven't finished it yet. Just wanted to post in case you were still on Board!)
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Joined: Jan 2001
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old hand
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old hand
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OK, dxb, here you go. Online, I found a reference to a book:
Author = Officer, Charles B.
Title = Physical Oceanography of Estuaries (and Associated Coastal Waters)
Publisher = John Wiley & Sons
Year = 1976
Pages = 465
ISBN = 0-471-65278-4
Apparently, it has a chapter which has a section dealing with tides at Solent, etc. So there you go! In a free moment maybe you or I will pick it up from a university library and have a gander at it!
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Carpal Tunnel
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Carpal Tunnel
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Posts: 2,636 |
Hi y'all. I have moved these posts to the Spendrift thread. I'd hate to see such interesting material lost in cyberspace.
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Carpal Tunnel
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Carpal Tunnel
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crossthreading.
Bean: tides at Solent.
I think UK usage would be tides in the Solent.
Bingley
Bingley
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Pooh-Bah
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Pooh-Bah
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Bean, thanks for the links and the book ref. In my bookshop browsing I shall keep an eye open for it. Meanwhile there are all the links that have come out of this discussion to explore.
dxb
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Carpal Tunnel
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Carpal Tunnel
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I had an unpleasant experience with a "sinusoidal" tide. My brother forgot to mention he had used up all the shear pins that protected outboard motor when propeller got tangled in seaweed. My father and I were half a mile down the channel from the mooring place when some weed caused pin to shear, and we found there was no spare pin. I elected to swim back up the channel to the skiff at our mooring place. When I started, the tide was high, but halfway to my goal, it started to turn, and picked up speed quite rapidly, until I was going back down the channel. I had to swim to channel bank, and walk through horrible ooze filled with sharp broken sheel that cut my bare feet. If I had plotted my progress during the swim it would have made a sort of sinusoidal curve, I now recognize.
One of the most spectacular tides in the world is the one in the Bay of Fundy, which in spring can reach forty feet. Alas the few times I was able to visit the Tidal Bore it was comparatively tame. There is a rather long list of terms used to describe tides, too long, and too easy to forget to be worth posting.
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old hand
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old hand
Joined: Jan 2001
Posts: 1,156 |
Yes, I know this thread is getting long, but I have one last thing for dxb on this subject. I was in the library today anyway and went to look at that book. It is basically a review of the scientific work in estuaries up to that date (already more than 25 years ago). I read the bit on the Solent, etc., and found it wasn't very helpful. I asked my supervisor what he thought and we looked at a map and worked it out. The Kelvin waves in the NH, as I said, travel with the coast at their right. If the English Channel is about 100 m deep, the the velocity of the Kelvin waves is about 100 km/h. If you look at a map and picture the tide propagating, first along the north coast of France from west to east. Then since the Channel near Dover is so narrow, the Kelvin wave "sees it" as closed altogether, and instead continues along the south coast of England, in an east-west direction. The distance between the closest point near the Isle of Wight and Cherbourg, France, if you measure along the coastline, is such that it takes about three hours for the tide to work its way around there (i. e. ~300 km). So if, say, it is high tide at Poole, the tide is on its way out at Cherbourg, and though they are some distance apart it is not so far that the tide going out at Cherbourg is "unnoticed" by the water at Poole. There are also probably strong topographical effects as well, and what you get is the tide "meeting itself" in the region between the Solent and Poole Harbour. Whew! What an odyssey. If you have any more questions, do PM me (and don't forget to turn your PMs on in that case). (I think others may not be as interested in this discussion as we are!) 
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Pooh-Bah
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Pooh-Bah
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Thanks Bean, I shall sit down quietly with a map and a glass (scotch!) and think on that. As you say, any more by PM.
dxb
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