Off Topic Post @ Monday August 15, 2005 10:57 am by WunderKraut
Update: Check out the comments for all the math you need to know.
Quiz time boys and girls. Who here remembers my post about two and a half weeks ago about putting flow monitors into manholes? Give up? Ok, you can find it here.
Anyway. Today was the day that we were going to pull the meters out and see how the data looked. Well, the first one came out ok. The data looks good. I have to figure out how to get all the info I want and how to put it into usable form, but it is all there. The second manhole we ran into a problem. We pulled the monitor out of the hole and I noticed that it was off. No problem, the batteries run out quicker than you would like. But it looked like they stopped running a week after we put it in. Again, no problem. But then I start looking at the data and I noticed that the only thing it measured was depth of flow.
As you guys all know, you need four things to measure the flow rate (gallons per minute or gpm) in a pipe. First you have to know the type of pipe. This is a concrete pipe. Second you have to know the shape of the pipe. No problem, it is a 15â€pipe. Third you need to know the depth of flow. Check, I have that. And fourth you need the flow rate (ft/s). Doh! It did not measure that! I think the sensor is broken.
Oh sure, I could send the survey crew out to get the invert of the upstream manhole and this manhole. That coupled with the distance between the two manholes would give the slope of the pipe. Working the equation backwards you can use shape, slope and depth to find the flow rate. Great that is what I will do!
But…
That would assume a free outfall at the manhole I am monitoring. If the next manhole downstream became surcharged then the water would back up into the manhole I am monitoring. If that happened, the depth of flow would go up, but the actual flow rate could drop close to zero. So assuming a free outfall would give flow rates that are too high and I do not know if the line becomes surcharged. A high depth of flow coupled with a low flow rate are sure signs of a manhole being surcharged. But, the only way I can salvage ANY of this data is to find the slope and to assume a free outfall.
Not exactly how I wanted to start my day.
11 Responses to “I-AM-DEPRESSED”
Huh?! OK can you say that in English now?
It is actually rather simple. It is just Manning’s Equation:
V = CR^(2/3)S^(1/2)
C = 1.486/n
Notation
C Chezy Coefficient
n Manning’s n
R Hydraulic Radius (ft)
S Channel Slope (V:H)
V Velocity (ft/sec) Manning’s Formula
Having the velocity, the depth of flow and the cross sectional area you can find the flow rate with this formula:
V(velocity ft/sec)*60(sec/min)*A(area in flow ft^2)*7.48(gallons/ft^3) = gallons/min
This assumes a free outfall, i.e. no surcharge. It is a good approximation for my data and as things stand, I do not have much of a choice.
Whatever!!! Can someone say GEEK!:)
Is that the best come back you have?
Actually…..Yes. I guess I’m not as witty as you are! Although, I think….wait…Yea, I can see your head from here!
Y’all are funny.
Ouch. I am wounded
Math is hard! When I saw “V=CR^(2/3)S^(1/3)” and figured that it was a sewage drain, my mind translated: “Velocity equals two-thirds of the crap times one-third of the s***.” And we all thought that Norton was just some dope who worked in the Sanitation Department! He was an unsung genius.
Wait until you have to draw a proportional sample. 50 years ago we used a characterized weir plate and a characterized sampling chamber.
As with any flow measurement, calibrate!
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