Culvert Diameter Based on Q

[jeb6294]

I'm doing some quick calculations to size some culverts. I've come up with a Q for my three areas and I was trying to find what diameter pipe would be needed for some road crossings.

My boss gave me an equation he uses...Q=2.674d^0.4...to solve for d in feet. Stick 2ft in there and the capacity of a 2-foot pipe is 3.5cfs? Did he give me the wrong equation or what?!?!

Usually I use Eqn. 19.16 in my CERM but that needs n and s which I don't have (this is very preliminary). I can fake it and get something that seems to make sense.

My boss is really good at this stuff so I'd like to get some sort of confirmation before I tell him his equation doesn't work.

 

[Eckybay]

try inches instead of feet for d

 

[jeb6294]

Using inches instead of feet still doesn't work...also Q is in cfs so it's definitely supposed to be feet.

Apparently it is supposed to be a derivation of the inlet control equation (Q=CA(2gH)^0.5) where H is just assumed to be the pipe diameter

 

[MA_PE]

there's obviously a lot of assumptions going on in that simplification. If I were you I'd ask your boss where the equation came from. Either he's made some fundamental assumptions to the basic theoretical equation based on his experience or someone else has and derived the simplifed formula.

Ask, the worst that could happen is that you learn something.

My 0.02

 

[Guest]

^^^ I completely agree with MA_PE. I took a few minutes to check CERM, Gupta, Chelapati, and Publication No. FHWA-NHI-01-020 (HYDRAULIC DESIGN OF HIGHWAY CULVERTS). I did not see a simplification like the one that you presented in any of those texts.

The larger concern is the simplifications because obviously there are a number of underlying assumptions to simplify the equation to that form. Without know what simplifications have been made or if they are valid for the situation you are presented with, you want to be able to explain (or defend) your #'s if anyone asks.



JR

 

[jeb6294]

Okay, I think I got it. It looks like it's a simplification of Eqn. 19-106 (pipe under inlet control so no slope or roughness) in my CERM when you assume the pipe is flowing just full, i.e. H=D

Q=CA(2gH)^0.5

Subsitute in H=D and A=((PI)D^2)/4 and solve for D

Depending on what constant you use for C, I come out with something pretty close to:

Q=2.674D^2.5 ---> D=(Q/2.674)^0.4

I think he just got a little ahead of himself and accidentally inverted the 2.5 a little early.

 

[VA_ENGR]

You may want to compare your answer with the results you get from using the FWHA charts for culverts. Depending on what state you are in, your DOT may also have those charts.

 

[Old as Dirt Geo]

If you are planning on using RCP culvert pipe here is a great resource from the American Concrete Pipe Association. There are some good easy to follow design examples as well as tables and charts you may find useful

http://www.concrete-pipe.org/designdata.htm

 

[NCcarguy]

This is interesting......Using FlowMaster, I get using a 2' Concrete pipe flowing full at 0.08% slope, still has a velocity of 2.04fps gives you a capacity of 6.40cfs....I also used the orifice calculation to see if it would need a tremendous amount of head to push that much water, and it doesn't......so I think the capacity of a 2' pipe would be almost double what you originally got????

could anyone check this? I'm getting better and better at this stuff, but I still find I'm missing something from time to time.