Hydraulic Design of Highway Culverts - DOT On-Line Publications

F. Spiral Rib Pipe
Spiral rib pipe is smooth walled metal pipe fabricated using helical seams. Roughness
elements include the joints and a helical, recessed rib running spirally around the pipe. Based
on tests of 2 and 3 feet diameter spiral rib pipe (65), the pipe has essentially the same hydraulic
resistance as smooth steel pipe, plus joint and aging effects. The laboratory test results indicate
Manning’s n values of from 0.010 to 0.011. Allowing for aging and higher joint resistance,
Manning’s n values in the range of 0.012 to 0.013 are recommended for design use. In using
these low resistance values, the designer should ascertain that no large roughness elements
such as projecting interior ribs or poor joints are present.
G. Summary
Table 10 summarizes the Manning's n values for materials commonly used in culvert
construction. For the corrugated metal conduits, the specified range of n values is related to the
size of the conduit. For other conduits, the range shown relates to the quality of the conduit
construction. In all cases, judgment is necessary in selecting the proper Manning’s n value, and
the designer is directed to other references for additional guidance in special situations.
207