Tidal Range

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SteveA

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The mention by Lakesailor of the spring tide today got me thinking - what are the physics behind there being such a different tidal range between places fairly near together.
An example being Heysham HW today is 10.5m whereas Douglas only about 50 miles away over open water is 7.7m. at about the same time How does this difference of nearly 3m occur?
 
The differences are mainly down to the configuration of the land, for example in the Bristol Channel which has the 2nd highest tidal range in the world, the range increases as you go upstream due to the funnelling effect. Then as you get nearer to the tidal limit of the Severn they drop off again.

For a similar reason you get the variation of tide times along the South coast as the water sloshes up and down the channel rather than just going in and out. The double tides in the Solent also come from the topography as the tide comes in from the Needles and Bembridge at different times.
 
Yes I can understand variations due to different land configurations will make these differences but in this case I'm meaning two shorelines facing each other with nothing but 50 mikes of sea in between.
 
Well, they happen to be on the shores of the Irish Sea, which has a large channel into it from the south and a narrower one to the north, that the water has to come in and out of. A configuration not unlike the English Channel
 
Most of the tidal effects we see are caused by the land: the tidal hump, or dome, is only around a metre high in the open ocean - but when a land mass ploughs into it all hell breaks loose. The general flow of tidal water in the Celtic/Irish seas is SW -> NE, so Heysham gets it full on whilst Douglas is in the "lee" of the island.
 
I would venture the opinion it is effectively a reflective wave of tide off the opposite shore. Therefore the answer before that its the topography in meaning the coastline shape is correct.
 
Almost correct but..

....Poole is not in the Solent (thank heaven and contrary to the thoughts of many) yet has double tides and more marked than in the Solent. I believe the correct cause is not the IOW but the topography of the Cherbourg Peninsular sticking out in Gallic Phallic style into the Channel so that a large slug of water gets hung up in the bay by St Malo/Mont St Michel before it is swept up into the east end of the Channel, thereby arriving a few hours after the first bit of water that went the direct route. The Solent tides I believe rise in sequence in ports from West to East not both ends towards the middle so to speak notwithstanding that was wot they tort us in geography GCE!
 
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Most of the tidal effects we see are caused by the land: the tidal hump, or dome, is only around a metre high in the open ocean - but when a land mass ploughs into it all hell breaks loose. The general flow of tidal water in the Celtic/Irish seas is SW -> NE, so Heysham gets it full on whilst Douglas is in the "lee" of the island.

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OK - let's make it interesting - how do you explain tidal ranges of around 10-15 cm in the Caribbean? Surely on islands surrounded by lots of water one would expect the oceanic range but they get far less.
 
Many tyhanks for the replies - i think I understand it a lot better now - I realise the big difference between the waters of Morecambe Bay and the IOM is the actual depth of water. Generally MB is shallow (alright i know there's Lune deep) whereas IOM is deep.
 
Re: Almost correct but..

But even more peculiar is the Sound of Jura.

Great tidal steams at each end (Sound of Luing and Correyvrekan at one end and Mull of Kintyre at t'other) - yet the tidal rise/fall is normally less than 1m.

The topography is of a funnelling effect also.

Donald
 
Many years ago I heard a Yachtmaster Theory Instructor state that recent information from satillites indicated that tides were caused by the sun and moon attracting the earths plates. Each plate rotated about some fault lines and small movements caused large movements in the sea.

I found this a plausable argument as the plates float on a liquid magnum base and they could gently rock, they are also more dense so would have a greater attraction of the masses than water. Hence edges of the plates would have the greatest tides and that is why the med nor islands in the middle of oceans have little tides.

My understanding is that movement of a few inches at the edge would cause a sea movement of many feet and that it also explained why the tides go the opposite way to what you would expect if a standing wave of water attracted by the sun went round the earth!

I have mentioned this a few times to find out more but as no one has ever heard of it and I concluded that its probably not true!
 
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OK - let's make it interesting - how do you explain tidal ranges of around 10-15 cm in the Caribbean? Surely on islands surrounded by lots of water one would expect the oceanic range but they get far less.

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If you think of the Atlantic as a big bathtub, with the water sloshing back and forth across the Equator, then near the Equator you would expect to get a fairly small rise and fall of tide. Work by Sterneck (back round about 1920) suggested an amphidromic point (one where there was no tide) somewhere near the Virgin Islands.

If you want to see what happens near amphidromic points, try doing secondary port calculations for Port Ellen, Islay.
 
Unfortunately we don't have the second highest range any more - there is another site in N.Quebec (Leaf Bay I think) which has the same range as the Bay of Fundy.
BTW Part of the reason for big tides is due to the shape and size of a basin - a bit like the sloshing of bath water coming over the rim if you get the frequency right.

Flood warning just issued for Porlock to Avonmouth (1630) for tomorrow morning tide - we are expecting 14.5m here at Portishead
 
The space in which the volume of water can fill. Sadly as usual it comes down to physics and Maths. Isn't it boring but great!. Most people who sunbathe on a beach have no knowledge of why the tide comes in and out neve rmind the difference between neaps and springs!
 
Tidal range is decided by the shape of the basin. I suggest that you do web search on amphidromic centres and co-tidal lines. These usually a small diagram in lavers Tide tables for Liverpool
 
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