Flat Pipes 0 Percent Grade
Flat Pipes 0 Percent Grade
(OP)
Florida Engineer here. It is common for us to have shallow or nearly flat grades. Something I've never been able to wrap my head around is how to size pipes for this situation. I know that water will flow in a flat pipe is it has somewhere to go.
For example,
I am installing a small 6" pipe, say n=0.024, in an exfiltration trench. The pipe is being installed at 0% slope. The pipe is 50' long. The invert of the pipe is 18" below grade and the pipe is a dead end at one end and the other end is plumbed into a drainage structure. I'm assuming the drainage structure is a free flow condition. The water level never rises above the pipe invert.
The trench is open to air. So stormwater runoff drains thru the rock into the perforated pipe, then thru the pipe to the drainage structure.
How do you calculate the capacity of the pipe in these situations? I've never really had a good grasp on how you do this. I would sometimes use the formula
Q = 1.489 * A * R^(2/3) * S ^(1/2) / n
And use a really small number for S like 0.001.
In the case I mentioned above would the slope be 18" rise in 50' of run?
Looking for some other perspectives on this. I've attached a sketch.
For example,
I am installing a small 6" pipe, say n=0.024, in an exfiltration trench. The pipe is being installed at 0% slope. The pipe is 50' long. The invert of the pipe is 18" below grade and the pipe is a dead end at one end and the other end is plumbed into a drainage structure. I'm assuming the drainage structure is a free flow condition. The water level never rises above the pipe invert.
The trench is open to air. So stormwater runoff drains thru the rock into the perforated pipe, then thru the pipe to the drainage structure.
How do you calculate the capacity of the pipe in these situations? I've never really had a good grasp on how you do this. I would sometimes use the formula
Q = 1.489 * A * R^(2/3) * S ^(1/2) / n
And use a really small number for S like 0.001.
In the case I mentioned above would the slope be 18" rise in 50' of run?
Looking for some other perspectives on this. I've attached a sketch.





RE: Flat Pipes 0 Percent Grade
RE: Flat Pipes 0 Percent Grade
RE: Flat Pipes 0 Percent Grade
RE: Flat Pipes 0 Percent Grade
When the HGL and invert are not parallel, the hydraulic radius continuously varies. This makes it impossible to apply Mannings Equation one time to the entire pipe. The techniques for solving gradually varied flow are much more involved than for uniform flow, so I would use software instead of grinding through it by hand.
If your pipe will be flowing full (i.e. under some head, even if not very much), then you can apply Mannings again because the hydraulic radius would be constant for the entire length of the pipe. Manning can be used just like Hazen-Williams, which is more common for water distribution systems.
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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill
RE: Flat Pipes 0 Percent Grade
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Flat Pipes 0 Percent Grade
RE: Flat Pipes 0 Percent Grade
However, regarding the OP, it's not clear that we're talking about pipe/culvert flow in this case. Since this is a perforated pipe draining a trench, the flow could be limited by the perforations. You could also develop some head inside the trench, which would increase the capacity of the perforations and the pipe itself.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Flat Pipes 0 Percent Grade
Is it possible to lower the invert of the pipe where it connects into the pit? I'd also suggest putting some fall along the pipe even if it is only 0.2% or so.