Above Ground Stormwater Piping

[martin888888]

Hello,
We are proposing 600' of fused HDPE (SDR 17) pipe that will be placed above ground for storm water conveyance. Due to ground conditions a bury is not possible. From the inlet we are looking at a initial 200' run of pipe at a 4% slope, with a slope of 11% to 15% in the remainder of the pipe. Pipe will be entirely gravity with no high spots in the pipeline. Does anyone see a reason that pipe could not flow full or even be surcharged? I have heard not having air in the pipe could cause issues and even possible damage to the pipe. I really don't see this as an issue.

 

[BigInch]

If it don't rain, it's not going to flow full, maybe not even at all. Those slopes are steep for gravity flow. The pipe will need a very large storm flow to flow full. At lower flows they will of course not flow full. Flowing full or not isn't necessarily a problem, at least you have a continuously increasing downslope, but flowing too fast can result in erosion of the pipe.

Technology is stealing American jobs. Stop H1-Bs for robots.

 

[martin888888]

Thanks for the response. The initial 4.5% pipe will not have capacity for our design flows and will flow full under large (25yr) storm events. As you said the steeper pipes have plenty of capacity. I guess there is concern that the condition of not having air in the pipeline will cause issues.

 

[LittleInch]

What issues?

Your flow will be difficult to calculate as water flow increase and at times you could have a full pipe at the start but 2 phase flow as the slope increases. Could get some vibration and surging at the end but otherwise not an issue. Why not run two puppies in parallel for the initial 4% section?

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Also: If you get a response it's polite to respond to it.

 

[pennpiper]

martin888888
Some questions:
a) Where in the world is this project located?
b) Why is it necessary to run the pipe above ground?
c) How do you intend to stabilize the pips in place?
d) How do you intend to protect the pipe from damage(people or nature related)?

Sometimes its possible to do all the right things and still get bad results

 

[martin888888]

Thanks pennpiper. Its located in Iowa and we have anchor details and parameters for protection. Main issue we are working through is effect of air lock on the system. Which I believe has no effect.

 

[BigInch]

In a continuous sloping pipe in one direction where can you get an air lock.

Technology is stealing American jobs. Stop H1-Bs for robots.

 

[LittleInch]

https://en.wikipedia.org/wiki/Air_lock

"Pipe will be entirely gravity with no high spots in the pipeline."

A profile would help us here a lot.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

200' run of pipe at a 4% slope, with a slope of 11% to 15% in the remainder of the pipe. Profile would appear to be down, down and more down.

Technology is stealing American jobs. Stop H1-Bs for robots.

 

[bimr]

It is unusual to use flexible pipe for gravity flow applications as it is not possible to maintain constant slope. It will be difficult to prevent high and low spots. HDPE pipe laid on top of the ground also tends to move around.

Is this a temporary application?

 

[dhengr]

Martin888888:
That pipe will certainly move longitudinally due to temperature changes, expansion and contraction, particularly under sunny conditions, and when empty. And thus, it will probably tend to move around laterally too. Flowing water will at least tend to modulate the temps. and add some weight to the pipeline.

 

[martin888888]

Great points, thank you guys. The main issue here however is if the HDPE pipe flows full will it put it in a vacuum situation that could possibly cause cavitation of the pipe.

 

[BigInch]

Vacuum is only possible if you try to close upstream of a segment that is flowing full.

Technology is stealing American jobs. Stop H1-Bs for robots.

 

[bimr]

The flow in your gravity pipe will be limited by either the entrance conditions or the exit conditions, whichever is lower.

If you size the pipe so that there is no backwater, cavitation should never occur.

 

[cvg]

the problem with stormwater is that you will likely have sediment, trash and debris as well as widely varying flow rates and lots of entrained air. debris could possibly block the pipe enough to cause a hydraulic jump and seal the pipe resulting in full flow. resulting full flow will draw in air. insufficient air or a bubble could cause blow back and surging. that could cause the pipe to jump around a bit if it is not anchored well. a large enough pipe will probably never run full, a smaller one might depending on the flow rate. inlet structures and manholes usually provide the necessary venting.