A question about tides

[Mirelle]

We've just had some of the biggest tides of the year; the proverbial equinoctial springs, and this set me to thinking.

We all know what tidal streams do off the coast because the Hydrographer and his staff tell us.

Very broadly, tides run harder at Springs than at Neaps and they run hardest at half tide.

We all also know that, in estuaries with extensive mud flats and saltings, a Spring tide runs hardest on the early ebb and almost as hard on the last hour of the flood, not at half tide, because the saltings and flats only cover during the last stages of the rise of the tide.

Now, does this mean that in an estuary the half tide rate is always the same, because the same volume of water is shifting - the acreage of water at half tide must always be the same, apart from barometric pressure considerations, be the tide springs or neaps?

Or is the rate at springs higher because the tide is rising or falling faster?

I suppose the answer should be the latter, but is it?

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[peterb]

The rate at springs is higher. As you say, the acreage at half tide must be the same at springs as at neaps, but the depth is changing more quickly at springs than at neaps. That means that water must be coming in at a greater rate.

It's not necessarily true that the greatest rate will be at half tide. As an extreme example, think of a large lake connected to the sea by a narrow channel. If the lake is large enough and the channel narrow enough, then the amount of water flowing through the channel in one tide will be so small that the level in the lake hardly changes. The flow through the channel will depend on the height difference between the lake and the sea, and this will be at its greatest at high and low waters. Maximum inwards stream at high water, maximum outgoing stream at low water, stream slack at half tide. I know it's an extreme case, but it does help to explain why maximum stream doesn't necessarily come at half-tide.

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[MainlySteam]

Interesting question.

I would suspect that, again very broadly speaking, the tidal current in estuaries at half tide is greater at springs and lesser at neaps. Why I think that is - well I don't really know! But -

The submarine and tidal zone physical relief stays the same but the tidal forces change between the tide types. That difference in tidal force between the tide types applies throughout the range of the tide (except at stand when I assume there is no force).

The estuary may see that tidal force directly ie the estuary has unimpeded flow to the open sea so sees the force as the water in the estuary being just part of the tidal bulge itself, or indirectly if it has restricted flow to the open sea ie it sees the force as a height differential between the tidal bulge in the open sea and itself.

Also, if in the springs the range is greater in the estuary than in lesser tides, which I assume it commonly is, then a greater volume of water has to flow to fill/drain that range. Conversely for neaps.

I would suspect these things together means the speed of the spring/neap tidal current is different throughout the tidal range when compared to an ordinary tide (except at stand of course), rather than the half tide current staying the same and the beginning and end of the range seeing a faster/slower one.

The only real different influence I can see may be end effects at springs if at the extremes of the range the spring part of the tidal zone slopes more steeply than the remainder, or if the flats drop off into channels at the bottom of the range. But that is outside of the "very broadly speaking" case.

John

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[qsiv]

Absolutely the latter - it's the rate of change of height that governs the flow rate. The half tide issue is broadly correct - but there are well documented exceptions that are not to do with narrow channels, rather they are due to harmonic (like standing) waves. A good example is around the channel Islands where the tide is slack close to half tide - as such such a time is significant locally and is often referred to as half tide up and half tide down.

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[Mirelle]

Thanks, that is nice and clear. Indeed, the case I was thinking of is an estuary with wide flats which cover at the top of springs, and a bar at the mouth. Lying overnight near the mouth, but not in the full run of the tide, I noted the following speeds on the log whilst at anchor:

HW 0.0kt
+1 2.3kt
+2 1.3kt
+3 1.1kt
+4 0.7kt
+5 0.2kt
+6 0.0kt
LW 0.0kt

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[MainlySteam]

That looks like as if as the flats uncover the volume of water needed to flow out the constriction of the bar from about mid tide in order to lower the water level in the estuary becomes much less so that the head differential between inside the estuary and outside the bar is lowered and hence the driving force from about mid tide is also lessened. With the lower driving force the speed of the current reduces.

I would take a guess and assume that by HW 5 much of the flats are uncovered and perhaps mainly channels and smaller expanses of water left (I am no oceanographer though). If I have guessed that right then you can put it down to the excellence of your description!

If that is so then I would expect the effect of rapidly reducing tidal current speed from HW to be reduced at neaps and exaggerated at springs.

I have no familiarity with Southampton but I understand that there is an unusually long stand there - the reason for which I do not know. Here in Wellington, NZ, the range is only a little over 1 m and that is so up around the West Coast of North Island in the Cook Strait region. It is the same across the top of South Island also until in the West one gets to Tasman Bay (the big bay that Nelson is in), and the range suddenly becomes around 4 m. Nelson Harbour extends into an estuary - we will be there on our yacht at least by sometime around March/April next year and your interesting observation has prompted me to make sure I see what happens current wise there (if you hear of an anchored yacht run down in the harbour entrance you will know you had a part in it!)

John
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[Mirelle]

Thanks, John. You are quite right about the shape of the estuary.

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[DeeGee]

Your figures certainly support your argument, which appears to run along the lines <font color=red> "the stream in the feeder channel runs faster when emptying/filling a big bucket than later when it is emptying/filling a little bucket" </font color=red>where the bucket represents the total volume of the floodplane at any instance?

At intermediate levels of tide, incl half-tide, the slope of the curve determines the rate, which will be higher anywhere, relatively, BUT still susceptible to the bucket argument, if flats are uncovered at such intermediate levels.

Nice observation. How about the tide-times in the centre of the stream vs the banks?

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[Evadne]

Re:Southampton

I'm not an oceanographer either, but I have met a few. The long stand at Southampton, sometimes a double high depending on the phase of the moon, is I think, due to the double arms of the solent feeding into Southampton water. Put simply, as the HW "wave" passes eastwards between the two entrances you get water flowing out of the western arm but in through the eastern one. Once HW reaches the eastern end the tide is flowing out through both arms, the water in and out of southampton water depends roughly on the sum of these two flows. (Numerous complications due to the relative size of the east and west solent, not to mention the harbours and rivers within the solent itself.)
Another weird place I used to sail from is Rye, E. Sussex which is an otherwise normal looking river estuary where the flood is stronger than the ebb. Don't know why, though it may have something to do with reflections from Dungeness and the narrowing of the English channel at that point.
UK Tideophiles can find more fascinating facts at <A target="_blank" HREF=http://www.nbi.ac.uk/>http://www.nbi.ac.uk/</A>.


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[MainlySteam]

Re:Southampton

Dave

Thanks for that explanation.

John

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[AndrewB]

Re:Southampton (Solent tides)

It's a plausible explanation but unfortunately not correct. It would not explain why the double-high occurs all along the coast from Poole to Chichester, or indeed other irregularities like the slight double-low at Portland. See <A target="_blank" HREF=http://www.weston.org.uk/tides.htm>HERE</A> for a fuller explanation.

I believe the true reason was originally discovered by Lord Kelvin (as in temperature) about 120 years ago.

 

[Evadne]

Re:Southampton (Solent tides)

Thanks, that is a very well written precis of a complicated subject.

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[Twister_Ken]

Re:Southampton (Solent tides)

If Lord Kelvin discovered temperature, presumably that's why they named a brand of fridge after him - Kelvinator.

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[MainlySteam]

Re:Southampton (Solent tides)

Thanks Andrew, for the very interesting article.

John

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