Hydraulic Design of Highway Culverts - DOT On-Line Publications

Curves are shown for 2-2/3 by 1/2 inch, 3 by 1 inch, 6 by 1 inch, 6 by 2 inch, and 9 by 2-1/2 inch
corrugations. A curve has also been developed for annular 5 by 1 inch corrugated metal
conduits.
To use Figure B-3, enter the horizontal scale with the circular conduit diameter and read the
Manning's n from the curve for the appropriate corrugation.
Figure B-3--Manning’s n versus Diameter for Corrugated Metal Conduits
E. Composite Roughness
Corrugated metal culverts are often fabricated using different materials for portions of the
perimeter. Examples are corrugated metal arches with unlined bottoms and corrugated metal
box culverts with concrete bottoms. In order to derive a composite Manning’s n value for the
above situations, a common practice is to derive a weighted n value based on the estimated
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