Flow rate - Discharge Vs Time/Distance

[redhumpy]

I remember there used to be some guy on this forum who had some funny letters behind his name, something like PR Eng BS ( I think he was a Public Relations Officer who liked to read the Engineering News and talk BS) - anyways it was damn sad - it would be like Tom adding PhD behind his name to show people he has arrived and is not some riff-Raff from Sasolburg.

Anyways - I wonder if these edumacated people can tell us with more clarity what time it would take for water to reach a specific point in the Vaal river once a gate is opened at the barrage thus increasing the rate.

For argument sake - it is not raining and the current flow at the barrage is 20 (m3/s).
They open a gate and increase the flow to a steady 70 (m3/s)

• How long will it take for the water released to reach a point 50km downstream from the barrage?

Interesting reading on flow rates and measurement in SA


Redhumpy - PhD Gynae. CUM laude

[icstuff]

Cant you force an expert/student in fluid dynamics to model it for you?

[Korrie]

The gradient will be a factor.

[Korrie]

Thanks for the link, interesting stats and photos.

[E.T.]

Redhumpy - PhD Gynae. CUM laude

This made my day!!!!

I hate weddings, but I love my wife. So I have to join her when she takes snaps at a wedding.
So at this one wedding from the script of "The Bride of Chucky" I was seated next to this lady that just has to know everything about everyone. So she kept asking questions about the photos and do we see a lot of weddings and do we this and do we that. I answered politely everytime. And then she asks "So what do you do for a living"? I knew I wasn't going to shake this lady, so I said " No one believes me if I tell them I am an Assassin, so I tell them that I am a Gynecologist. So what would you like me to do for you Today? Do you need someone taken out or do you want to book a Papsmeer? Took the lady a while to get her ducks in a row and then she asked " Are you serious, you just said that you put stickers on a boat". I replied "Yes I do work undercover from time to time".

So back to your question. The flow is 20m3/s, it increases to 70m3/s.
So 50km downstream the water is already flowing at 20m3/s. So to get to the 70m3/s, the floodgate actually adds 50m3/s. So how long will 50m3/s take to get down to the 50km mark?

There is a 1000m in one Kilometer
There is 60 seconds in a minute and 60 minutes in an hour.

So 50m3/s would have moved 3000 meter in one minute.(50m3/s x 60 seconds = 3000 meter/ minute)
And then 180 000 meter in one hour (3000 meter/min x 60 minutes = 180 000m/hour)
So 180 000 meter / 1000 = 180km is what that water would have traveled in just one hour.
So if I then go and break 180km up into 60 minute segments the water travels at 3km/minute.
50km devide by 3km/minute = 16.667 minutes for that water to reach you.

This is how I see it.

Koos Eckard - Assassin (aim small, miss small)

[Reveck Hariram]

Hey E.T

You cant multiply discharge by time to get distance. If you know the velocity then you can do it. Velocity X time gives you distance. But then the velocity never remains the same over the 50km reach of river as there are narrow sections where the velocity is high and there are wide deep sections where the velocity will be very low. It is quite a complicated calculation which many variables to take into consideration. modeling it is a very complicated, date intensive and expensive method and if you are within 50% of the real answer - well done !!!

an easy way to do it is to compare the flows from goosebay canyon and schoemans drift to give you an indication. remember if the discharge is higher the time taken is less.

can I add my Pr Sci Nat to this post ?

[smallstreams.co.za]

I'm just qualified to Google, and apparently I passed that by the skin of my teeth ...

http://easycalculation.com/physics/f...-flow-rate.php

[redhumpy]

50km devide by 3km/minute = 16.667 minutes for that water to reach you.

Where's my red pen.
Don't you think 16.667 hours would be closer than 16.667 minutes?

If you click on the link I added you will see a graph on page 15.
The researcher measured the time it took over a distance of 47km of the Olifants River measured at different flow rates.

At 70m3/s it took 15 hours to reach the 47km distance
At 500m3/s it took 9 hours.

I know the Olifants is not the Vaal but maybe it is closer to the truth?

Maybe to get a control you should measure how long it takes for the flows to drop 50km downstream once they close a gate

Probably the only way to get an estimate is to print out a set of the flow rate charts from the DWAF website over time and do a graph to find the average.

[E.T.]

Mario, based on that link, you are right.
16.667 Hours is more likely to be true.

@Revek, I also believe that you are correct. My calculation is based on something that travels constantly like a person walking 1 meters per second(So that would be 60 meters per minute and 3600 meters per hour if he maintains the 1 meter per second rate throughout the duration of the hour. So that would in effect be 3.6km in 1 hour traveled.)

But you are right, there are a lot of variables here and yes, you are dead right if you have the velocity you would indeed be able to make a more accurate calculation.

[Reveck Hariram]

Barrage_releases.JPG

OK guys. The distance between Goosebay canyon and Schoemansdrift weirs is approx 70km. On the 2nd Dec Goosebay recorded a peak of 91.9 m3/s and Schoemansdrif a peak of 99 m3/s. The time difference between these 2 peaks is 28.2 hours. I assume that the two weirs are recording water level properly.