Question of the Week

Timing the Tides

Sun, 16th Mar 2008

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Roy Lightning asked:

I often do a lot of walking and so I like to know when the tide is right out because I like to walk out as far as I possibly can. When I was looking at the tide times I thought, "how do they get it so accurate?" It will say something like Wells Bar, low tide 14:02. The question was really how do they get it accurate like that and who needs it that accurate because I certainly don't?


Dr Ken George, University of Plymouth, Institute of Marine Studies.

Ill answer the question, how can one predict tides so precisely? The answer is they are so dependent upon the Sun, Earth, Moon system which is a system which changes extremely slowly. Namely over tens or even hundreds or millions of years. In a lifetime there is very little change. Therefore the tides are the most predictable natural phenomena on Earth. Indeed, the only thing which is going to stop you predicting tides accurately for lets say a hundred years time is silting or dredging of harbours. You can predict tides years in advance, you can publish tide tables for next year, 2009 and even a decade hence if we so wish. Only about 98% of the energy in the sea level variation is predictable. The bit which is not predictable is caused by storms. People will be very aware of that in the last two days we have had a severe storm striking the south of the UK. In many places the tide rose and the sea level rose, to be more precise, owing to the storm.

Why do we need to know tides with such precision? There are two main areas which we are concerned with. One is coastal flooding how high your tides can bee predicted and the second one is for the passage of ships over shallow water, over shoals. Its very important that they know the height of the tide over a shoal so that they have sufficient clearance under the keel. If they have insufficient clearance then they will scrape the bottom or in the worst case actually go aground and be stuck.


The bay of Fundy at high tide, 1972.

The bay of Fundy at low tide, 1972.

The Bay of Fundy at High Tide

The Bay of Fundy at Low Tide


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There are probably hundreds of examples of why people need to know times of high and low tide, like getting ships that have run aground, afloat again.

One interesting example of tides is in the Sound Of Mull where they have a large quarry near Lochaline. The sound of Mull is very tidal and they used to have the fastest ship loading in Europe (I'm not sure if this is still the case) so ships could come in with the tide and leave with the tide. Otherwise they would would have use a lot of fuel to get out or wait 12-13 hours.

turnipsock, Tue, 11th Mar 2008

I would have though is less important to know the exact time of low and high tide, as to know the time when the tide will reach above, or below, a certain point; and this will determine whether you can float at a given level, or whether a certain section of beach will be exposed.  Thus, that point might be reached at a different time relative to high tide for a spring tide as for a neap tide.

As for calculating the exact moment of high or low tide, I would have thought that would not be too difficult on a section of coast simply exposed to an ocean, simply by knowing position of the moon and sun; but I would have thought it more complex maybe is an estuary, or some region of coast with lots of complex currents. another_someone, Tue, 11th Mar 2008

Once you know the exact time of high and low tide, you can then use various tables to calculate tide hight at any given time. If you ever do any sailing qualifications, there are loads of questions asking you things like what is the earlist time you can get under a certain bridge if you vessel is such and such a hight.

The tides don't follow the moon exactly, there is a slight delay as the moon is pulling the water around, a bit like slip angle in electric motors. Also, the highest tides are about three days after the New and Full moon. turnipsock, Tue, 11th Mar 2008

Yes, but the point I was trying to make was that one can probably calculate the time of high tide with greater precision that the hight at high tide, and so knowing the exact time at which you can travel under a bridge is probably more limited by the imprecision of your knowledge of the height of the tide than the imprecision of the time of high tide.

Yes, but the delay in open ocean is still fairly simple compared with trying to work out when high tide will happen when water is coming at you from multiple angles (e.g. in parts of south-east England, where you have tides coming down the North Sea, and in from the ocean along the English Channel).

As an example of some complexities:
another_someone, Tue, 11th Mar 2008

There is still the effect of resonance in large bodies of water (oceans) which introduces a phase shift and just knowing Lunar and Solar positions is not enough.
In fact, of course, the state of the tide in open oceans is not of much interest to anyone; once beyond the continental shelf the water is 'well deep'.
As for coastal waters, the tidal flow tends to be of interest, at least as much as the depth, if your vessel is a slow one. They call it 'sailing' but it's really 'tiding' for most cruising yachts which may well be doing no more than two or three Kt through the water.
Many inexperienced sailors rely just on the depths marked on charts, which are pretty much the minimum you are likely to find (except in a strong anticyclone - possible, or in very bad weather - unlikely for a beginner), when passage planning. But they can't avoid the effects of tidal flow, unless they are in a gas guzzling power boat.
It amazes me that people have so little respect for what goes on at sea; ignorance is certainly not bliss out there. lyner, Thu, 13th Mar 2008

We in the British Isles and coastal Europe are so familiar with our large tides that we tend to forget how weird the tides in this area really are.  In fact the North Sea has two tidal "gyres"  or spots out at sea where it is high tide all the time and the tides effectively circulate around that point.  The comments about the Southampton double tide fail to point out one of the reasons why the tides are in opposition between Dover and Lands end is that  the tide coming down the North Sea is delayed differently from that coming up the channel.

My wife and I were once keen surfers but lived just west of London and made use of this tidal feature for many years.  The best time to surf is generally on the rising tide because the waves are stronger then. Now because of the four week cycle of the moon when the tide is rising in the morning one weekend it is falling the next weekend.  Two good spots to go surfing quite close to west London are Porthcawl in South Wales and West Wittering on the south coast.  When it is high tide in Porthcawl it's low tide in West Wittering.  If you go on a day trip its convenient if the tide is rising during the afternoon so one week this would be true for Porthcawl and the next week this would be true for West Wittering so we could always find a spot where the tides were right as long as there were waves. Soul Surfer, Sat, 15th Mar 2008

There are quite a lot of places around the west coast of Scotland that are very tidal, the Gulf of Corryvreckan for instance, which has the third largest whirlpool in the world.

The Cuan sound, to the South of Seil Island, is another interesting channel, one where small yachts travel through often. If you were to attempt this with the tide running in the opposite direction at 7-8 knots you wouldn't get very far.

Submarines use the tides to get through the Straights Of Gibralta, they drift through with the tide and engines off. If the tide turns, they drop an anchour and wait for the tide to turn again.

Before we had GPS, knowing the tidal flows was important in order to calculate your position. If calculated your position with a fix at one point, then you sailed for a few hours in a certain direction, you would have to add in the effect of the tide before you could plot your position.

An appreciation of the tides is still useful. If you were sailing from England to France it would be silly just to aim for your destination if the tide was pushing you to starboard for six hours, you could end up miles of target. Using a GPS would still get you there, but you would be better to calculate the most efficient course.

turnipsock, Sat, 15th Mar 2008

The predicted height is not always accurate because the height of the water is affected by the atmospheric pressure (tide higher during a low-pressure weather system) and the wind (wind can cause the water to "pile up". Obviously these cannot be forecast with any great accuracy when the tide-tables are compiled. I understand that around the UK coastline these effects can add up to 0.5-1.0metre onto the height of the basic prediction. techmind, Mon, 17th Mar 2008

It's essential on some passages if you sail. Even in a fast motorboat you can take longer and use more fuel if you set your autopilot at a distant waypoint rather than choosing a proper heading. A tide, taking you  west as you go south will cause you to follow a curved course if the boat is always steering 'at' the target. A proper choice of heading over a 12hr passage will ensure you minimise or even eliminate the tidal effect. Most of the time you are not actually heading for the destination at all. lyner, Thu, 20th Mar 2008

I notice, no factor of the seabed in the calculation of tides surely the angle or shape of the rise to the shore has some contributing factor, why for instance are there variations in depth of tides at different locations?
Estuaries and bays also being a factor as the tides rise to fill these areas.

AlphBravo, Sat, 29th Mar 2008

The only way to calculate tides properly is to do lots of measurements at the location where the tide is to be calculated. Even then, as has been mentioned, it will be affected by the weather conditions. I believe most tide tables, at least for British ports, are done exactly this way. Tidal calculations based on harmonic models are much less accurate.

Tides are affected strongly by the Coriolis force resulting from the earth's rotation. This is why, in the Northern hemisphere, Northern coastlines have much higher tidal ranges than Southern coastlines. Compare Dover with the Channel Isles for example. graham.d, Sun, 30th Mar 2008

tide predictions are not based on mechanical / physical theory but on observations in a specific area; the measurements are affected by such things as local seabed topography, as graham-d says. If you do enough measurements you can get enough 'coefficients' for harmonic analysis. You still need to know the other factors like atmospheric pressure and wind.
Racing sailors get to know how to get the best of the tides - both to push their luck on short cuts over shallow areas and in using tidal streams to advantage. Far too tiring for a nice day's cruising! Where's that nice pub again? lyner, Mon, 31st Mar 2008

who cares the world is my trashcan... ?, Thu, 27th Nov 2008

I am working on a project that deals with marine (vessel) casualties, specifically barge groundings. I was wondering if any of you would know if it's possible to pinpoint an area that is a "hot spot" for groundings. I gathered information for 13 months and two places are indicative of most groundings: Bolivar Penninsula Area & Brazos Flood Gates (Both are in near Galveston, TX). I spoke with a scientist from NOAA and he said that MLLW is what it's used to create nautical charts. My question is how can I use this data to prove that groundingsa are in fact happening in this two areas vice the rest of the Gulf Intracoastal Waterway? Boricua de Ponce, Thu, 4th Dec 2008

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