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How Long Does The Severn Flow.

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Rev. Green | 20:49 Wed 03rd Nov 2021 | Science
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How long in time (not distance) does it take a river like the Severn to flow from its source to the sea? Of course, this is impossible to answer: some molecules will travel faster than others; some may evaporate; the river may flow faster when in spate. However, there must be a rough answer for how soon one millilitre will reach the bar (no pub jokes, please) after a litre of water is released at the source.


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River flow speeds are typically somewhere between 0 and 3 metres per second. So if you assume 1 m/s as a reasonable average (since I only care about order of magnitude), then it would take roughly four days for something to go from the Severn's source to its mouth. I'd assume that (roughly) double this is the correct upper limit. So the correct range is around...
10:10 Thu 04th Nov 2021
Anyone know where the Severn ends and the sea starts?
^ Penarth. Long way from the Severn Bridge.
// I could think of others that could go in to test the theory but I'd probably be done for virtual murder..... //

Wanting to throw people into the river is surely one of the Severn signs of evil.
jim: No disrespect to the OP, but it really is a non-question, a 'how long is a piece of string' type question, unanswerable.

I live on the bank of the beautiful river Neckar & watch it every day throughout the seasons, in the summer droughts it flows slowly, but in the late winter it is most exiting to stand on my village bridge after the snow is thawing upstream in the Black Forest & watch the torrent containing huge lumps of ice & logs hurtling by at high speed.

You could place a hypothetical ping-pong ball at the source of the Severn & never get the same result twice, e,g. what if after the first hour of its journey there was an enormous storm over Bewdley?

And jim, my above reference to Heraclitus wasn't entirely facetious, I think modern scientists could learn something from the ancients;

// No disrespect to the OP, but it really is a non-question, a 'how long is a piece of string' type question, unanswerable. //

It depends on the answer you're requiring, I guess. Earlier it was mentioned that it takes 90s days for water to flow from the Mississippi's source to its mouth. Presumably, this is not literally true and represents only an average (and then an approximation at that!), based either on measurements of the river's flow rate at various points or on some other experiment, or maybe even on the same Estimation technique I was using on the Severn. But it's still a reasonable answer all the same. So I don't agree that it's "unanswerable". You can say with some confidence that it takes "a few days" for water to flow down the Severn. Not weeks, and not a mere handful of hours. Presumably it'll fluctuate, eg after heavy rainfall the flow will speed up, especially upstream, but that doesn't change the usefulness of an average/ballpark figure.

So I disagree with your assertion that it's "unanswerable".
jim, //So I disagree with your assertion that it's "unanswerable".//

and what I'm saying is the question is unanswerable without the qualifier of 'when?'.
The OP asks for a "rough answer" & that's what you have given, I just think it's so rough as to be rather meaningless, as it could easily be 100% out.
Incidentally, my (possibly naive) answer to Heraclitus is that he can't see the wood for the trees. I of course agree that you don't literally step into the same river twice, but I don't agree that you can say nothing useful about the river as an ensemble. The analogy I am instantly reminded of in physics is Statistical Mechanics, or Thermodynamics in general, ie "how do we describe a gas, made from countless trillions upon trillions of tiny particles constantly interacting?" The answer is to care less about what the individual particles are doing, about which you can never have complete knowledge, and start focusing on what the gas is doing as a whole. You can measure its pressure, its volume, over average properties, and it turns out that way so much useful information lies.

Or, in your analogy, the individual ping-pong ball's time to travel tells you little, but there's nothing to stop you dropping dozens or hundreds, and measuring the average journey time (and then repeating the experiment in different seasons, before or after rainfall, etc).

Don't disagree that studying ancient Greek philosophers is interesting, though. Thanks for the video!
// I just think it's so rough as to be rather meaningless, as it could easily be 100% out.//

I guess I'm biased because there are times in my job where an error of less than 100% is actually a decent achievement! Measuring the world is difficult. It's even harder if you give up :P
jim, I think that shower up in Glasgow could do with Heraclitus' presence. :0)
no - yes I am with Jim on this

is there a speed? yes - what is it? between one and three ( units)
that is not good enough therefore there is no speed

is NOT something I wd subscribe to

is there a blue frog with red stripes and three eyes - no there is not. Describe one. I cant, there isnt one
IS one I would subscribe to

but boys and gurlz - this is semantics and not physics
in my opinion
In the evidence based medicine debate twenty years ago
one of the Great Thinkers er thought people should read Plato
Everyone thought he had lost it

People thought everything cd be answered by randomised controlled trials - but covid teaches us ( 20 y later ) it cant
Safety trials need large volumes of those who are just vaccinated.

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