Roughness Coefficients

The following table provides roughness coefficients for conduits.
Recommended Culvert Conduit Roughness Coefficients
Type of Conduit
n-Value
Concrete Box
0.012
Concrete Pipe
0.012
Smooth-lined metal pipe
0.012
Smooth lined plastic pipe
0.012
Corrugated metal pipe
0.015-0.027
Structural plate pipe
0.027-0.036
Long span structural plate
0.031
Corrugated metal (paved interior)
0.012
Plastic
0.012-0.024