Flat Pipes 0 Percent Grade 1

[mattmitts]

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.

 

[Twinkie]

Throw it in hydrocad as pipe storage....you need to model this as pipe storage, manning's isn't really applicable. The dimensions (volume) of your trench will certainly come into play here.

 

[jgailla]

Manning's S is the slope of the hydraulic grade line, not the slope of the pipe.

 

[cvg]

you may want to calculate the capacity of the slots in your pipe. they may limit the flow, especially if partially blocked or plugged. The depth of the pipe trench will come into play since the head is directly related to the flow capacity through the orifices.

 

[fel3]

For most of us, I would think, the most common application for Mannings Equation is UNIFORM open channel flow. However, if there is flow, it cannot be uniform flow because the HGL will have to be steeper than the zero-slope pipe invert. You have "gradually varied flow." jgailla is correct about S being the slope of the HGL and not the slope of the pipe invert.

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

 

[dicksewerrat]

I would raise the upstream end about 8 inches just to get some movement. Is the trench level?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[pinkpig]

use HY-8 to do your calcs, which can handle zero slope culvert as I remember. as others pointed out, the S in Manning's equation is not bed/culvert slope, it is HGL slope or more strictly, energy grade line slope.

 

[psmart]

As others have stated, it's the slope of the energy grade line that matters, not necessarily the slope of the pipe. When you model a culvert in HydroCAD, the flow is based on the difference between the headwater and tailwater, adjusted for entrance losses and frictional (Mannning's) losses. This allows you to model a culvert with zero slope, or even a reverse slope.

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

http://www.hydrocad.net

 

[CivilEngAus]

To be honest I wouldn't bother as it is hard to justify the time given the size of the trench.

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.