True capacity of a pipe?
True capacity of a pipe?
(OP)
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
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





RE: True capacity of a pipe?
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.
RE: True capacity of a pipe?
"...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
RE: True capacity of a pipe?
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.)
RE: True capacity of a pipe?
"...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
RE: True capacity of a pipe?
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.
RE: True capacity of a pipe?
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.