Industry News
The Truth About Manning's "n" Values
Laboratory "n" values for any pipe material do not take into account in-service conditions such as: bends, manholes, trash, slime deposits etc..
Typical Concrete pipe design applies a 20% correction factor to the theoretical lab "n" value of .010, yielding a suggested design "n" value of .012.
HDPE pipe will require even higher suggested design values once the in-service conditions, deflections and buckling are considered.
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HYDRAULICS
|
TYPE
OF PIPE |
LAB
VALUES
(UTAH STATE) |
SUGGESTED
DESIGN VALUE |
CONCRETE
(RCP) |
.010 |
.012 |
PLASTIC
(HDPE) |
.009-.015 |
.019 |
METAL
(CMP) |
.019 |
.024 |
HDPE pipe manufacturers claim their pipe is smoother than other type of pipe. Depending on which brochure you pick up, the "n" value that is listed may be anywhere from .010 to .012.
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HDPE promotes superior hydraulics?? | However, HDPE "smooth-lined" pipe is very susceptible to buckling and deflections. Once installed, the inner liner will often develop corrugations similar to CMP. These corrugations will require even higher design values once the in-service conditions, deflections and buckling are considered.
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