RYA Tidal Calculations

[marisca]

If there is anyone out there who religiously follows "the method" to end up with an answer to the nearest centimetre here is today's Millport tide graph.

[mm5aho]

I wonder why there's such a difference? Is that all due to wind and atmospheric pressure?
Half a metre is a significant error.

[Bodach na mara]

I wonder why there's such a difference? Is that all due to wind and atmospheric pressure?
Half a metre is a significant error.

Not really a great variation from predicted values. These assume equal pressures over the water surface locally and out at sea. For every 1mB difference in pressure we expect 10mm difference in level. Now that is not all rise, as it is split between the Atlantic (drop) and our local rise, but in January 1994 (I think) there was a depression of 950mB over Scotland with high pressure of 1020 about 400 miles west of Malin head. The predicted level difference would be 700mm and coupled with a SW gale, the tide rose in Largs to the extent that I saw people in canoes as far up the main street as the square at the station.

Now Marisca, I would really like to know how you get the data for the actual tidal level curve?

[NorthUp2]

Pick your pier-
http://www.ntslf.org/data/uk-network-real-time

[marisca]

Pick your pier-
http://www.ntslf.org/data/uk-network-real-time

Spoilsport! I was going to spin a yarn about paying school kids to go paddling with rulers.

[pagoda]

Agreed on the spoilsport aspect, but interesting stuff nevertheless. Surge prediction in particular..

[Nick]

Half a metre is a significant error.

Why 'significant' ?

Atmospheric conditions cause variations up to a metre from the predicted heights several times a year.

[NorthUp2]

I'll b u g g e r off back to TOP then!

[mm5aho]

Significant in that if you failed to take the other factors into account, there's a chance to have not enough margin.
In doing a similar calculations recently, I failed to take account of the other factors. Half a metre would have had me significantly aground!

[Bodach na mara]

Pick your pier-
http://www.ntslf.org/data/uk-network-real-time

Spoilsport! I was going to spin a yarn about paying school kids to go paddling with rulers.

The sad thing is, I would have believed it!

Thanks for the link, a useful site.

[Bodach na mara]

Significant in that if you failed to take the other factors into account, there's a chance to have not enough margin.
In doing a similar calculations recently, I failed to take account of the other factors. Half a metre would have had me significantly aground!

Maybe that could explain a well-known TV presenter running aground on the reef across the exit from Arisaig which was filmed for "Landward" and can be found on youtube.

[Silkie]

Thanks for the pointer.

[youtube]http://uk.youtube.com/watch?v=983Yz0O4R-A[/youtube]

[marisca]

There but for the grace of god ..........

Here is a picture of Euan perched on the arse of another craft learning his navigation skills at the hands of a master.

[Storyline]

Quite proud that all these tide gauges feed data to Bidston Observatory on the Wirral - this rather grand gothic building is one of the few remainders from a time when Liverpool was not only mega rich but also a centre of all things nautical.

Although I was aware of these data feeds as we sometimes see the one in Portpatrick we also rely too much on the tidal predictions being accurate. Our modus operandi is for swmbo to do the tidal calcs on the approach to an anchorage (hence I am very rusty when it comes to these) and we sometimes rely on them being accurate to half a metre of so. Have not been caught out yet but came perilously close in Plockton this summer. The harbour authority have kindly left the best part of the anchorage free of moorings but this central area is quite flat bottomed (with excellent holding in course gritty sand). From the calcs we knew we were on a falling tide when we arrived with another half metre or so to go down and by the time we dropped back we had just 0.8 under us. We became gale bound and failed to remember that we were moving to springs so by the third night swmbo had me watching for the depth sounder to start to rise at 02:30 with about 3 inches under the keel ! She was not convinced when I told her that it would be good to give the bottom of the keel a free scrub. Fortunately it was a calm night with flat water so we did not touch.

So thanks for this wake up call, we will allow more depth in future when the pressure is high - or should that be low ? Low pressure allows the tide to rise higher than predicted but does it follow that high pressure pushes the water down further ? (any answers to this question will prove that someone at least made it through to the end of my very long winded post )

[cpedw]

Low pressure allows the tide to rise higher than predicted but does it follow that high pressure pushes the water down further ? (any answers to this question will prove that someone at least made it through to the end of my very long winded post )

Yes and Wight Dawn's rule (10mm tide level down/up per mb atmospheric pressure up/down) still applies. But as his observation about Largs shows, wind can be a much more significant factor. AFAIK there's no simple rule relating wind to level change. That's not surprising since wind strength and direction, duration of wind and local topography will all have an influence. There are probably other factors too.

So if there's anything funny happening to the weather, allow extra anchoring depth and even more scope to compensate for the unpredictable...

Derek