Tides- sometimes ‘chust a mystery’ (as Para Handy put it)

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TimfromMersea

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Boat at West Mersea, Essex. Live in Wivenhoe, Esse
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Interesting situation last Thursday, the 30th September. Put my boat on the scrubbing posts at West Mersea - the main WMYC ones, not the shallower Dabchicks ones to the north. She only draws 1.4 metres so I didn’t anticipate any problems, even though the prediction was quite low, 3.6m at 06.30.

No issues getting on to the posts at 07.10. I normally leave it until the ebb has started, especially as on Thursday there was a fairly stiff SSW wind - I was single- handed so wanted there to be some ebb running to balance the wind trying to push the stern out. Not much water, maybe half a metre below the keel, but quite OK.

Duly scrubbed off when she dried out, with by then help from one of my friendly crew, who was joining me on the WMYC Autumn Trophy race, which was the reason for the scrub so late in the season. He offered to help me take her off the posts so with a prediction of 3.6m again at 19.10 we agreed to meet at 18.30.

At 18.30 that night the boat was still at least 2 feet dry on the posts! We waited until 19.15 but she was still not even afloat. The tide came nowhere near - I have never seen a ‘high’ that was quite so ‘low’.

I was seriously worried as the prediction for the next day, Friday morning, was lower, 3.5m at 07.50, and they didn’t start getting big until Saturday 2nd, when it was 4.10m, but at 22.10 by which time it would be pitch dark, and also 40 knots of wind and heavy rain was forecast.

I came down again on Friday morning with another friend and fortunately 30 minutes before tide time she was floating high, we got on board and motored her off with almost a metre under the keel. Sigh of relief!

Looking at Harwich Harbour web site, that compares the ‘actual’ with the ‘prediction’, on Thursday evening the ‘actual’ was 0.65m below the prediction, and on Friday morning the ‘actual’ was 0.45m above the ‘prediction’.

Can anyone more learned than me possibly please share any thoughts on why this should have been? In almost 60 years at Mersea I’ve certainly never seen a ‘high’ that came so ‘low’.
 
I never got involved in prediction of negative surges and know little about them. Intuitively, you might assume that a strong southerly might have such an effect although it is probably far more complex. There is a prediction service because of large tankers going through the Dover Strait where they can scrape the sea bed. This is, I believe, part of the storm tide warning service. See Flood Forecasting,
 
Remember all that when you get asked to "calculate" the tide height in a secondary port for your dazed kipper. Then ask yourself what use a calculation like that?

I have seen HW at Ramsgate drop by 1m over 24 hours. No tide prediction curve ever gives that sort of change.
 
franksingleton said:
Agreed, I have always thought that RYA tidal calculations are unrealistic. But, have you pr anyone here assessed the predictions at National Tidal and Sea Level Facility
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Sure the RYA / Almanac predictions are just that. They deal with averages but can't take into account environmental factors such as atmospheric pressure, storm surges etc.

As long as we remember this and accept it then no problem.
 
ex-Gladys said:
A phrase I use to Theory class students is that Tide tables are predictions...Just like Astrology!
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The astronomical part of tidal predictions - including basin resonance effects - are highly predictable. But what the business about secondary ports forgets is that the predictions are only valid for places with tide gauges and a long sequence of measurements (several years at least). Interpolating between tide gauges is makes a lot of assumptions, not least that basin resonance effects will be the same. On top of that you have meteorological effects, which are probably the cause of the extreme mismatch between prediction and observation that the OP noted. But the basic astronomical tidal prediction AT A TIDAL GAUGE is very good. Perhaps not as good as the prediction of orbital parameters in the Solar System, but certainly good for tens of years.
 
Certainly I suspect on the SE of the UK and Belguim, NE French coast, that weather is the driving force for tidal surges. Otherwise the beach huts wouldn’t have gone walkabout in the spring last year.
 
AntarcticPilot said:
The astronomical part of tidal predictions - including basin resonance effects - are highly predictable. But what the business about secondary ports forgets is that the predictions are only valid for places with tide gauges and a long sequence of measurements (several years at least). Interpolating between tide gauges is makes a lot of assumptions, not least that basin resonance effects will be the same. On top of that you have meteorological effects, which are probably the cause of the extreme mismatch between prediction and observation that the OP noted. But the basic astronomical tidal prediction AT A TIDAL GAUGE is very good. Perhaps not as good as the prediction of orbital parameters in the Solar System, but certainly good for tens of years.
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Absolutely!
 
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