Another tide question

[DeeGee]

I once read somewhere that the tide can change earlier/later at the banks of a deeper river than in mid-stream.

Can anyone confirm this, and perhaps give an explanation, or a reference? I have to say that I have never been able to confirm this by any consistent observation.

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[Mirelle]

I would not put it as generally as that, but eddies are common at certain states of the tide.

Two well known ones are the eddy that sets onto the scrubbing posts at Brightlingsea from upriver during the last of the flood, and the eddy that does the same thing with the scrubbing posts at Felixstowe Ferry.

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[DeeGee]

No, this wasn't just about back eddies, I am pretty sure this was in a book by Jack Knights or Eric Twiname or even an old book by JDS, in a racing context of course, and an explanation was given, rather as there is a perfectly logical explanation why the tide runs fast in the middle and slow at the edge, and similarly on bends. Maybe I misremember, and it was something else altogether?

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[bedouin]

This is certainly true in enclosed waters like the Solent, where the tide can change near the shore much earlier than in the main stream; but this is probably more like an eddy.

I don't see how it could in any other way

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[AuntyRinum]

Shallower water creating more drag.

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[DeeGee]

Perhaps you could illuminate on your rather succinct posting?

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[GeorgeP]

There are lots of factors that affect tidal streams. However, in general the tide will run more strongly in deeper water (e.g. a channel). This is because the ground drags on water next to it, and only indirectly on water further away. You will find that water tends to run more strongly in the outside of a bend in a river, which is also where you find the deepest water. Momentum of the water will also be a factor here.

Other examples are: tide tends to run more strongly on the ebb in rivers, because it runs with the pressure of fresh water; it will accelerate through narrows like Hurst.

This is different from tidal streams running in opposite directions, which is normally due to obstructions causing eddies.

I hope this helps!

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[DeeGee]

Ah yes, this is true, but I was rather hoping you would address the question as to whether such a mechanism could in some way mean that the tide will turn earlier/later in the banks rather than midstream. I would rather eliminate back-eddies which are another phenomenon.

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[DeeGee]

Further thought prompted by a phrase in your last.... perhaps you have hit the nail on the head? Once upon a time, I could have done the fluid dynamics to work this out, but age..... Momentum may be the key. Could it be that the water in mid-stream has more momentum, so takes longer to slow down, and the shallow banks can turn earlier? Or is the matter to be considered at a microlevel where each parcel of fluid knows nothing of the other parcels around? This does need a young mathematician to work out, unless it has been done already.

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[GeorgeP]

I'm sure momentum has an effect. In Poole Harbour, the tide changes at the entrance an hour earlier than at Wareham. The only thing keeping the tide going in at one end of the harbour while it is already flowing out at the other is momentum.

However, that's over several miles! Whether it can have a noticeable effect across a river beats me!

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[peterb]

I think Brett has the answer. The momentum of the water would keep the flow going past the point at which the tide generating force would normally make it turn. But the shallow water at the edges creates more drag and slows down the inertial flow, allowing the tide to turn at the sides before it turns in the middle.

Another similar point often seen in the River Orwell is that the boats on moorings on opposite sides of the river swing at different times. Usually the boats on the inside of the bends turn first, and sometimes the boats on the outside don't swing until nearly an hour later.

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[Bergman]

I'm no mathematician but it is a fact. Watched many times when the river Ouse has invaded part of my garden. The water flows out of the garden while the centre of the river is still flowing fast in flood. the water at the edge of the river is definately moving downstream while the centre is moving upstream.

Not an eddy because the level is actually falling.

I had put it down to inertia, large mass of water moving quite fast wants to carry on moving even if its going uphill.

By the by the flood tide here is significantly faster than the ebb, and when the river has excess freshwater the effect is to reduce the amount of fall on the ebb tide. In really heavy floods it maintains an almost constant level through the whole cycle.

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[oldharry]

I was taught to visualise the tide as a wave - watching the mouth of a small river on a beach will demonstrate a lot of the apparent anomolies of tidal flows inshore. A wave entering against a strong flow of water will 'creep' up the sides, while in the centre it is held back by the opposing current, until it is high or strong enough to reverse the flow. The opposite happens as the wave starts to ebb, and the centre of the stream starts flowing much more quickly than the sides. Get a nice twisty bit of stream mouth and you can observe all kinds of interesting eddy effects as the heights change.

Most 'eddifying' ....hmmm.

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[jimi]

Made me think back to my surfing days when you'd look for a rip to get seaward on, the incoming waves would bring water in and a strong seaward current forms carrying water back out as it finds its own level. This current obviously tend to form where there is least resistance .ie along a headland etc. Just wonder if a similar mechanism is working here where the incoming rush of water has a slightly higher level in the middle and is "pushing" forward and sideways forming a small "rip" at the water's edge?

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[Trevor_swfyc]

Another theory, last year I went to check my mooring which dries on spring tides +- several days we also had a period of heavy rain during the week in question. The water was rushing out over my wellies so I thought I still have plenty of time to scrape off some barnicles (last time I use cheap antifoul). I then realised that the water was going out and coming in? it would soon cut me off or at least come over my wellies.
Now if we consider that this outflow is in competition with the inflow, lets consider the situation approaching high tide, the momentum of the water at the edge being least would give way to the outflow first and the tide would appear to change in direction even though the depth would still be increasing.

Trevor


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[AuntyRinum]

Nice answer. This is why in the English Channel and Solent (and no doubt elsewhere) the tide is normally less strong inshore than further out to sea.

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[oldharry]

Ahh, Trevor you want to come and look at how our tides misbehave in the Solent: In Spithead the ebb starts to flow west on average 2 hours before HW, but it takes notionally another hour before west flow reaches the western end around Lymington.

The direction of a current is not necessarily an indicator of the rise or fall of the level. Water running out of a channel into a rising tide will continue to flow outward although the level of the sea is rising. We had a rather spctacular demonstrati0on of this in Chichester Harbour when the City of Chichester was flooded so badly a few years ago. The ebb - normally around 3+ knots on the bar, was running at a logged 6 knots, while the next flood made at around 1.5 knots instead of its usual brisk 3+, simply because of the amount of water pouring into the harbour from the inland floods.

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