Hydrgraphs - UG Chambers

[jmayo-EE]

Does anyone have an issue with this computation? I have a 12'x500'x5' deep trench with (2) 36" pipes telling me that I have 29,602 CF. I did two hand calcs (one based on hor. & ver. geometry & stage storage based on sectional area) that provide me with about 16,200 C.F.

Can anyone provide some backgound on the Hydrograph calc. My computations account for wall thickness but I do not have 13,000 CF in pipe walls.

I am doing this because I cannot get exfiltration with a manual entry hydrograph.

[Joe-Bouza]

Way over my head Bernouli , but I would trust your hand nad check the computer input for a mistake.

[jmayo-EE]

It's the voids. Hydrographs gives me close to what I expect if I cut my 40% void to 18%.

[Joe-Bouza]

You mean the 12*500*5 volume less the 2 (36) @ 500 *.4?

I get (30,000- (3.58^2pi/4*500*2))= 19,993

19,993*0.4 = 7,974 cf in the voids, no ?

[Joe-Bouza]

http://www.hydrocad.net/chambers.htm

there is a chamber modeling article at the hydrocad site - maybe it will be of some help?

[Joe-Bouza]

REVISED

 12*500*5 volume less the 2 (36) @ 500 *.4?

I get (30,000- (pi[d^21-d2^2]/4*500*2))= 27,000

27,000*0.4 = 10,800 cf in the voids, no ?

[jmayo-EE]

I am home but those numbers are very close to my calcs not Hydrographs.

John

[Matt.Anderson · Accepted solution]

John -

I think your 2 barrels doubles the size, ie if you cut your trench width from 12 feet to 6, and then double the barrels, you have the answer you seek.

When you double the barrels, you've double your trench width as well as the pipe.

Matt Anderson, PE

[jmayo-EE]

Thanks Matt! The program wants a unit trench not the total trench.

[jmayo-EE]

FYI, if you are using Chambers, your pond report in 2010 will list the number of barrels at ten time the expected value.

[annw2]

I just got caught on the underground chamber volume calculation.

I was expecting the width to be the total bed width, not the width for each pipe.  I didn't notice the volumes were off until after Phase I was bid.  I even tried cutting the width to match the rows of pipes and didn't get anything useful.

The worst part was I needed the total bottom surface area to correctly model the infiltration for the NPDES permit.

I wound up with a mess and calculating the volume manually and inputting the basin as contours with an average void ratio.

I am going back to HydroCAD for any future underground basins.