Hydraulic Design of Highway Culverts - DOT On-Line Publications

Substituting Hf/L for S and rearranging Equation (35) results in Equation (4b).
The Manning’s n value in Equation (35) is based on either hydraulic test results or resistance
values calculated using a theoretical equation such as the Darcy equation and then converting
to the Manning’s n. As is seen from Equation (34), the Manning’s n varies with the conduit size
(hydraulic radius) to the 1/6 power and has dimensions of ft 1/6 . Therefore, for very large or very
small conduits, the Manning’s n should be adjusted for conduit size. Most hydraulic tests for
Manning’s n values have been conducted on moderate size conduits, with pipes in the range of
2 to 5 ft in diameter or on open channels with hydraulic radii in the range of 1 to 4 ft. For large
natural channels, backwater calculations are used to match observed water surface profiles by
varying the Manning’s n. The resultant Manning’s n accounts for channel size and roughness.
Using a constant value of Manning’s n regardless of conduit size or flow rate assumes that the
Manning’s n is a function of only the absolute size of the wall roughness elements and is
independent of conduit size and Reynolds number. This assumption is best for rough conduits
where Reynolds number has little influence and the inherent variation with conduit size to the
1/6 power holds true. Thus, the Manning equation has found wide acceptance for use in natural
channels and conduits with rough surfaces. For smooth pipes, other empirical resistance
equations, such as the Hazen-Williams equation, are more often used.
Extensive tables of Manning’s n values are provided in references (23) and (31). For natural
channels, the designer is referred to Table 11 in Appendix D as well as to references 16, 17,
and 18. Manning’s n values for commonly used culvert materials are discussed in the following
sections.
B. Concrete Pipe Culverts
Concrete pipes are manufactured (pre-cast) using various methods, including centrifugally spun,
dry cast, packerhead, tamp, and wet cast (63). The interior finish (wall roughness) varies with
the method of manufacture. For instance, the tamped process generally results in a rougher
interior finish than the wet cast process. The quality of the joints and aging (abrasion and
corrosion) also affect the hydraulic resistance of concrete pipe. Laboratory tests on tamped
pipe (24 to 36 in., average to good joints) resulted in Manning’s n values of about 0.009 (64).
These values are increased to 0.011 to 0.013 based on field installation and aging. Suggested
values of Manning’s n for concrete pipes are shown in Table 10.
C. Concrete Box Culvert
The hydraulic resistance of concrete box culverts is based on the method of manufacture,
installation practices, and aging. Concrete box culverts are either pre-cast or cast-in-place. For
pre-cast boxes, the smoothness of the walls, the quality of the joints, and aging affect the
Manning’s n values. For cast-in-place boxes, the quality of the formwork, construction
practices, and aging are factors. Suggested Manning’s n values range from 0.012 to 0.018 for
concrete box culverts (23).
202