True capacity of a pipe? 4

[Quest101]

When analyzing the capacity of a pipe, when do we use Manning's equation, solving for Q, given (S, n, A, R) and when do we use other equations, say the culvert equations for different head conditions?
Also. Would we ever use Manning's to solve for the capacity of an outlet pipe ( pond culvert ) in a detention basin?

Doesn't capacity increase with head (HW), thus using Manning's would be underestimating the capacity of the pipe?

Thank you very much

 

[cousink]

Those are good questions. We recently applied a couple pipe capacity analyses for a site design. To estimate the capacity of a pipe and channel, we used the equation Q=1.49/n*R^(2/3)A*S^(1/2), which, of course, gives us the maximum discharge for the pipe or channel, for whenever it's flowing full.

To design the culvert beneath a driveway, as an outlet pipe, we analyzed a pipe using a common culvert analysis method. Based on the D, S, Q, n, HW, TW, and L, this method pegged the expected flow into one of six flow types, controlled by either inlet or outlet conditions. It estimated the water surface profile inside the pipe and determined how full the pipe is flowing at the expected discharge, TW and HW. It let us know whether the expected flow is subcritical or supercritical, which is important to prevent hydraulic jumps from occuring in undesirable locations. It also let us know whether the water is carried either entirely by the culvert or over the wall and driveway.

 

[ACtrafficengr]

Mannings often overestimates pipe capacity. Due to turbulence at the inlet, less water can enter the pipe than it can carry. A smooth interior pipe won't allow more water to flow than a CMP, you'll just get higher exit velocity.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[francesca]

I remember having a similar discussion with a very seasoned engineer who had spent most of his career as a city engineer of a town with over 100,000 residents. I argued that you should never decrease the pipe diameter downstream in a sewer system, even if the grade increases and the downstream pipe can carry more water. He didn't think it was be a problem, and had the experience to back that belief up. Now channeling in manholes could help channel the water into the pipe eliminating a degree of the entry/exit losses, I suppose, but it still doesn't make sense to me.

You can do the same analysis of a detention basin outlet that cousink used on the driveway pipe. Presumably detention basin design software does this analysis when calculating the discharge rate, though I've never seen an output that tells me if/where an hydraulic jump occurs, neither in Hydraflow Hydrographs, PondPac nor StormCAD. (Though my experience with the latter two software packages is very limited.)

 

[ACtrafficengr]

Thanks for the reminder, Francesca. I should have said "for pipes where exit conditions don't control capacity (usually short ones)."

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[gbam]

Manning's only estimates the "capacity" of a pipe's flow rates that have an energy gradient equivalent to the slope of the pipe. It does not account for exterior/minor headlosses like enterance or exit losses.

I only use Manning's for preliminary sizing of storm drain pipes, never culverts. I typically use inlet control nomographs for my initial sizing of culverts.

 

[bpattengale]

One of my college professors once told me anyone can take an equation and get an answer to a problem. It doesn't mean the answer is valid or correct, but they just got an answer.

A lot of the equations we use as engineers are emperical equations. They only apply under certain conditions and constraints. The reason we're engineers is because we have the education and experience to know what equations and assumtions are valid.

For example Manning's Equation only applies to fully turbulant uniform flow in water with a free surface(gravity flow). We assume for practical reasons flow in a pipe or a channel is uniform, in reality this isn't the case. However in most cases the difference between what is actually occuring and our assumtions is negliable.

If you ever have a question about whether an equation is valid look at how it was derived.

As far as your question that capacity increases with headwater requires assumptions that the pipe is operating under inlet control or pressure flow. With those assumptions Mannings equation is not valid. Its like comparing apple to oranges it just does not make any sense.