Thetownend.com

...solve this little conundrum I've just encoutered whilst revising...

A River, 20m wide and 4m deep drains a field that is 3000km square in which the average rain fall is
48cm/Metre Square/Year. One quarter of the rainfall returns to the atmosphere via evaporation but the remainder drains into the river. What is the average speed of the river current?

Speed = distance/time

You sure the field is 3,000 square Kilometres !
It's a bloody big field !

Not sure if this will end anywhere but..

3,000km^2 = 3,000,000,000m^2

48cm = 0.48m

Volume of water that falls annually = 0.48 x 3,000,000,000 = 1,440,000,000m^3

Only 25% exits via the river = 0.25 x 1,440,000,000 = 360,000,000m^3 per year.

So we've got a volume and a period. Just need to think about the rest.

3/4 exits via the river 1/4 evaporates

Just a little bit smaller than Wiltshire in fact.

i dont think this can be done from that info. what level is it? ie gcse, A / AS level?

ok, 0.75 x 1,440,000,000 = 1,080,000,000m^3 per year

1 year = 31,536,000 seconds

1,080,000,000 / 31,536,000 = 34.25m^3/sec

just under 1 mile an hour

I make it 1.54 mph. The river's current that is. ???

Average Velocity = Flow Rate / Cross Sectional Area

= 34.25m^3/sec / 80m^2

0.428m/s

or 0.96 mph

Its Degree level Engineering, and I believe I owe Brighton Red a pint...well done.

Einstein or what well done BR.....silly question as usual but dosn,t the strength of the wind have any bearing on the flow of a river?

yes - I agree - that's what I said

bloody hell mr red, you seem like a right thick bumpkin cunt when i have ever been in your company :)

I'm flying the flag for basic maths on this one: :sherlock:



Field = 3,000,000,000 m2 (3,000 km2)
Rainfall = 0.48m per m2 per year

0.48 x 3,000,000,000 = 1,444,000,000 m3 of rainfall
0.75 of that finds its way into the river = 1,080,000,000 cubic metres of water per year

River crosssection = 80m2 (20 x 4)

so 1,080,000,000 / 80 = 13,500,000 metres of water flows through that river crosssection per year

so the speed = 13,500,000 / 365days / 24hours / 60minutes / 60seconds = 0.428 metres per second


Degree level engineering it aint. :suicide:

Would it be a constant though.

Surely when the water in the field is deeper, the increased pressure would force it into the river quicker therefore increasing the flow.

A lot of it is hypothetical, obviously there is no field that is 3000k square and in real life there would be a lot of other contributing factors. I was revising fluid flow so thats all the question encompassed.

In which case the correct answer would be: "how the fook do you expect me to answer the question without knowing all of the parameters".

It's a bit like asking "How long is a piece of string"

Im a University and it was a tutorial question given to me, you do the maths..

That's why it's average flow Einstein ;)

you're a university I'm a library, go figure...

Living up to your name then Plagiarist?  ;)