How Long Does The Severn Flow.

[Rev. Green]

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.

[Hopkirk]

A giant game of Pooh sticks is needed.

[jim360]

I would have a go at working this out but I'm too Bored.

[Khandro]

I have walked with a friend, its banks from the Severn bridge to its source up on Plynlimon, N.Wales & then back down the Wye - both sources are but a stone's throw apart - to the Severn bridge again, two days at a time, once a year staying the first night in a variety of interesting pubs. It took us 20 years!
This is of no use whatsoever to your question, but I hoped you might find it interesting. :0)

[Rev. Green]

If the river was flowing faster than you were walking Khandro then 20 years is an upper limit, and Jim's bore travels at about 17 km/h, which, if it is faster than the flow, gives 21 hours as a lower limit.

[JJ109]

You are going to need the gradient along its length. An "average" gradient may not be sufficient to calculate the speed.

[jim360]

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 three to eight days, where I'd tend towards the higher value. I don't think anyone could improve on this without actually doing an experiment with eg some handy (non-toxic!) dye or some such.

[grumpy01]

The Severn Bore at Minsterworth is something to behold.

[Khandro]

Rivers though, don't flow at the same speed throughout their journey, where it widens it slows, where it narrows it flows faster.

But the most interesting thing said about rivers was by Heraclitus circa 500 BC;

"No man ever steps in the same river twice, for it's not the same river and he's not the same man."

and it's not he same river we walked, but I have many happy memories of it anyway.

[Jonathan-Joe]

Just seen a question on The Chase. Apparently for the Mississippi, water flows 90 days from source to the Gulf of Mexico.

[jim360]

Interesting. It's over 10 times longer than the Severn, so it stands to reason that the flow there will be about 10 times as long as well! So that supports my upper estimate of eight days. :)

[Khandro]

sorry jim, but you can't extrapolate one from t'other;
https://www.worldatlas.com/articles/which-are-the-fastest-rivers-in-the-world.html

[Peter Pedant]

// "No man ever steps in the same river twice, for it's not the same river and he's not the same man."//

big thing for the Athenians - they did the bit about take a plank of the sacred ship Paralios and replace it wiv new - do and do again
At the end you have two ships, which one is the paralios

now my dears I am surprised no one has goggled speed of the severn which is 8 to 13 mph - 3.5-6 m/s
so it is about double Jims

Now the speed of the bore is 16 km / h and sortta goes up
now if the speed of the river were greater than 16 then the bore wouldnt occur 4 m/s
and if the speed were 16 kph or 4 m/s then the bore would be a standing wave

ter daah didnt expect me to say that didja?

golly 354 km in length so 354/16 is 22 h

did anyone else get that ?

physics on the back of an envelope or
why dont penguins feet freeze are two good books

[Peter Pedant]

eight d
no I got one day
( an order of maggers)

[Peter Pedant]

I dunno
estimating the speed of a river by considering the bore and its existence is sordda AB oudda boxie - innit ?

[jim360]

While that's true, Khandro, if you'll notice my earlier answer, I didn't extrapolate anything. I simply said that a typical average river speed is about one metre per second. Ergo, any river water starting out its 354km-long journey along the Severn can be expected to take around 354,000 seconds, or a little over four days.

Of course, this is too precise, so I then varied by assuming that 1m/s average is an upper limit for the average speed of the river, and that 0.5m/s is a reasonable lower limit for the average. This gives a range of four to eight days, which I then quoted as three to eight in case 1m/s is too low. I could as well have said "about a week", but the main point is that I've established with reasonable confidence that it doesn't take weeks for water to travel along the Severn, and it doesn't take mere hours either. This is Fermi Estimation, in case you're interested.

The Mississippi is, of course, a different river, but it's reasonable to assume that most rivers are fairly similarly behaved, as a first approximation. In that case, the fact that it takes water in a river ten times as long as the Severn a shade over ten times longer to travel its length than my upper estimate *supports* the initial estimate, rather than undermines it! Two different estimation techniques have led to the same answer. Pretty good going, in my book, for somebody who knows more or less nothing about rivers.

[jim360]

I should of course say that I didn't extrapolate anything *from the Mississippi* in my first answer. I just used a fact, readily available from the internet, that water speed on rivers ranges from about 0m/s to about 3m/s, and assumed that river water tended to spend more time moving slower than faster.

[Spicerack]

Peter Pedant
//big thing for the Athenians - they did the bit about take a plank of the sacred ship Paralios and replace it wiv new - do and do again
At the end you have two ships, which one is the paralios//

Think you'll find they pinched that idea from Only Fools & Horses, pp.

Time is a man made construct, innit, jim?

[Hopkirk]

Forget Pooh sticks. Chuck some rubber ducks in.

[DTCwordfan]

I could think of others that could go in to test the theory but I'd probably be done for virtual murder.....

[benhilton]

I remember from a few years ago when there were floods in Bewdley and Worcester , an Environment Agency spokesman was being interviewed on Midlands Today (regional tv ) . He said that it takes about 72 hours for heavy rains in the Welsh mountains to work their way down the river to the Midlands . This seems to back up Jim's estimate .