Upsizing storm pipe to increase velocity
Upsizing storm pipe to increase velocity
(OP)
I am reviewing a drainage assessment report in which the following statement is made.
"Larger pipes are able to self-clean due to their ability to achieve a minimum cleansing velocity of 2.5 feet per second based on the engineering principals (sic) behind storm drain design."
It seems to me that this statement is incorrect. If the pipes are assumed to flow full, it would be correct.
But let's say the hydrology indicates a flow of 2 cfs to an inlet. If my slope is limited to 0.1%, going from an 15" pipe to a 36" pipe will not raise the velocity in the pipe. I am assuming the hydraulic grade line here equals the pipe slope.
Any thoughts?
"Larger pipes are able to self-clean due to their ability to achieve a minimum cleansing velocity of 2.5 feet per second based on the engineering principals (sic) behind storm drain design."
It seems to me that this statement is incorrect. If the pipes are assumed to flow full, it would be correct.
But let's say the hydrology indicates a flow of 2 cfs to an inlet. If my slope is limited to 0.1%, going from an 15" pipe to a 36" pipe will not raise the velocity in the pipe. I am assuming the hydraulic grade line here equals the pipe slope.
Any thoughts?





RE: Upsizing storm pipe to increase velocity
V = (1.486/n)(R^.67)S^.5
Where AR^.67 = Qn/1.49S^.5
Do the math here and answer him.
Mike McCann
MMC Engineering
RE: Upsizing storm pipe to increase velocity
"HGL equals pipe slope" is also a dangerous assumption
RE: Upsizing storm pipe to increase velocity
The velocity is inversely proportional to d^2, so as the diameter increases, the velocity decreases for constant flow.
Further, if your slope is limited by the pipe geometry, then using a smaller diameter will allow you to locally increase the slope, thus increase the velocity to a greater extent than other variables.
jgailla....your report writer is wrong...but you already knew that. Hope things are going well.
RE: Upsizing storm pipe to increase velocity
cvg, good point about the HGL equals pipe slope assumption. I always do calcs off the HGL as is correct for Manning's, but for this discussion it opens up a can of worms that I didn't want to get into.
Ron, things are finally going well. I had been doing carpentry for over a year now while looking for engineering work. I'm working for Darrell, so I'm pretty happy about the way things are going right now.
RE: Upsizing storm pipe to increase velocity
RE: Upsizing storm pipe to increase velocity
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Upsizing storm pipe to increase velocity
I'll have to pull a book and bang out some numbers to verify. I'm assuming constant Q and s with varying D. I'll try to do it at lunch.
RE: Upsizing storm pipe to increase velocity
RE: Upsizing storm pipe to increase velocity
When I posted my original response, I was thinking that if a pipe was flowing full, increasing its diameter slightly would create a condition where the flow in the partially-full larger pipe would be in the area where V/Vfull is greater than 1 and that this would offset the larger diameter, but that the effect would quickly fade and that would be that. What I didn't consider is that Vfull for a larger-diameter pipe at a given slope is larger than Vfull in a smaller-diameter pipe. This second point seems to extend over a large range of diameters.
Starting with numbers close to those in the OP, a 15" pipe @ 0.001 has a Qfull of 2.21 ft3/s with a corresponding Vfull of 1.80 ft/s (n=0.012).
A partially-full 18" pipe @ 0.001 would convey 2.21 ft3/s at a depth of 0.85 ft with a corresponding V of 2.14 ft/s (19% increase).
24" Pipe: d= 0.73 ft, v=2.13 ft3/s (18% increase)
36" Pipe: d= 0.63 ft, v=2.05 ft3/s (14% increase)
48" Pipe: d= 0.58 ft, v=1.98 ft3/s (10% increase)
72" Pipe: d= 0.52 ft, v=1.87 ft3/s (4% increase)
96" Pipe: d= 0.48 ft, v=1.80 ft3/s
The Flowmaster results are at this link:
http://fi
RE: Upsizing storm pipe to increase velocity
RE: Upsizing storm pipe to increase velocity
Using a flow of 2.21 ft3/s as it is close to the 2 ft3/s mentioned in the OP and conveniently is equal to the flowing full capacity of a 15" RCP @ 0.001.
There are two possible depths the 15" pipe conveys 2.21 ft3/s, d/D=1.0 and d/D=0.81 (1.01 ft). The corresponding velocities at these depths of flow are 1.80 ft/s and 2.08 ft/s.
A partially-full 18" pipe @ 0.001 would convey 2.21 ft3/s at a depth of 0.85 ft with a corresponding V of 2.14 ft/s (3% increase)
21" Pipe: d= 0.78 ft, v=2.14 ft/s (3% increase)
24" Pipe: d= 0.73 ft, v=2.13 ft/s (2% increase)
27" Pipe: d= 0.70 ft, v=2.11 ft/s (1% increase)
30" Pipe: d= 0.67 ft, v=2.09 ft/s
33" Pipe: d= 0.65 ft, v=2.07 ft/s
RE: Upsizing storm pipe to increase velocity
- Is it a circular (not ovoid/egg shaped) pipe?
- Is the gradient staying the same? They are not proposing to relay a long length of pipeline at a steeper gradient?
- Are the materials similar?
RE: Upsizing storm pipe to increase velocity
It was somewhat of a blanket statement in the draft report, so there was no reference to squash pipe or using different materials.
I not sure what they intended to say, but the statement has been removed from the report.