Hydraulic Design of Highway Culverts - DOT On-Line Publications

1. Annular Corrugations. In reference (25), resistance factors are developed for the annular
corrugation shapes shown in Figure B-1. Methods are also presented for estimating the
hydraulic resistance of new or untested corrugation types. Those methods have been used to
estimate the resistance of 5- by 1-inch corrugations, shown in Figure B-2, for which no test
results are yet available (61).
Figure B-2--Shape of 5- by 1-inch Corrugation
A series of charts were developed in reference (25) depicting the Manning’s n resistance value
for various corrugation shapes over a range of conduit sizes. The charts show the variation of
Manning's n value with diameter, flow rate, and depth. The curves for structural plate conduits
have discontinuities due to changes in the number of plates used to fabricate the conduits.
Curves are presented for two flow rates, Q/D 2.5 = 2.0 and Q/D 2.5 = 4.0. Under design conditions,
culvert flow rates approximate the Q/D 2.5 = 4.0 curves.
2. Helical Corrugations. In pipes less than about 6 feet in diameter, helical corrugations may
provide lower resistance values. This is due to the spiral flow which develops when such
conduits flow full. As the pipe size increases, the helix angle approaches 90 degrees, and the
Manning's n value is the same as for pipes with annular corrugations.
For partial flow in circular metal pipes with 68 mm by 13 mm (2-2/3 by 1/2 inch) in helical
corrugations, Manning's n should be 11% higher than that for the full flow. In the case of full
flow in corrugated metal pipe-arches with 68 mm by 13 mm (2-2/3 by 1/2 inch), Manning's n is
the same as an equivalent diameter pipe.
3. Design Relationships. Based on the charts of reference (25) for annular and helical
corrugations, Figure B-3 has been developed to assist the designer in the selection of a
Manning’s n value for corrugated metal conduits. The figure is based on certain assumptions
which reduce the complexity of the relationships.
a. The curves are based on Q/D 2.5 = 4.0, which is typical of culvert design flow rates.
b. The discontinuities inherent in the structural plate curves have been ignored in favor of a
smooth curve.
c. The only helically corrugated metal conduit curve shown is for 2-2/3 by 1/2 inch corrugations,
with a 24 inch plate width.
204