Manning n for Corrugated Metal Culverts
In Reply Refer To: June 22, 1993
Mail Stop 415
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 93.17
Subject: Manning n for Corrugated Metal Culverts
Several types of corrugated metal now used for culvert pipe are
not discussed in Techniques of Water-Resources Investigations
(TWRI), Book 3, Chapter A3, Measurement of Peak Discharge at
Culverts by Indirect Methods. Laboratory studies conducted by
Utah State University for the National Corrugated Steel Pipe
Association provide n values for the new types of corrugation.
These studies have caused the Federal Highway Administration to
revise culvert roughness tables in the manual, Hydraulic Design of
Highway Culverts (Hydraulic Design Series No. 5), and provides
sufficient basis to revise n values for multiplate culverts as
given in TWRI, Book 3, Chapter A3, pages 10 and 11. The values
given herein should be used for all future culvert computations.
The Office of Surface Water (OSW) also recommends that previous
computations for flow through multiplate culverts be reviewed if
the following conditions are met:
1. The n value used in the computation differs by
0.003 or more from the value in this memorandum, and
2. discharges from types 2, 3, 4, or 6 computations using
n values from TWRI, Book 3, Chapter A3, or from ratings
based on such computations, have been published.
Ratings that were based on the old n values and are still in use
should be reviewed and revised if use of the revised n values
change any part of the rating by 5 percent or more. Published
discharges do not need to be revised unless they meet the criteria
for revisions given in Novak (1985, p. 103-104, WRD data reports
preparation guide) and the water-surface elevations and field
conditions on which the computation is based provide a high degree
of reliability to the computed discharge. The following material
supersedes the discussion in Standard riveted section and
Multiplate section in the part of the manual entitled "Corrugated
Metal" under Roughness Coefficients on pageJ10 of TWRI, Book 3,
Chapter A3.
Corrugated Metal
Corrugated pipes and arches are made in riveted, spiral, and
structural-plate styles. The riveted and spiral styles are used
in small pipes of less than 9-foot diameter. Spiral corrugations
have the same pitch and depth as that used in riveted
construction, but the plates are wound to form a continuous pipe.
Because of its greater strength, structural-plate (also called
multiplate) commonly is used for pipes that are more than 6 feet
in diameter. Multiplate is made in sheets that are bolted
together.
Standard Riveted Sections
The corrugated metal most commonly used in riveted pipes and
arches has a 2 2/3-inch pitch with a rise of 1/2 inch. This is
frequently referred to as standard corrugated metal. According to
laboratory tests, n values for full pipe flow vary from 0.0266 for
a 1-foot-diameter pipe to 0.0224 for an 8-foot-diameter pipe for
velocities normally encountered in culverts. The American Iron
and Steel Institute recommends that a single value of 0.024 be
used in design of both partly-full and full-pipe flow for any size
of pipe. This value may be satisfactory for many computations of
discharge. However, more precise values are given in the
accompanying table, which shows values derived from tables and
graphs published by the Federal Highway Administration for culvert
design and that apply to both annular and spiral corrugations, as
noted in the table. Values from this table should be used by
U.S.JGeological Survey offices in computation of discharge through
culverts.
Riveted pipes are also made from corrugated metal with a 1-inch
rise and 3-, 5-, and 6-inch pitch. Experimental data show a
slight lowering of the n value as pitch increases. The n values
for these three types of corrugation are also given in the table.
Structural Plate (Multiplate)
Structural-plate metal used in multiplate construction has much
larger corrugations than does that used in riveted pipes.
Multiplate construction is used with both steel and aluminum. The
steel has a 6-inch pitch and a 2-inch rise; aluminum has a 9-inch
pitch and a 2.5-inch rise. Tests show somewhat higher n values
for this metal and type of construction than for riveted
construction. Average n values range from 0.035 (steel) or 0.036
(aluminum) for 5-foot-diameter pipes to 0.033 for pipes of 18 feet
or greater diameter. The n values for various diameters of pipe
are given in the following table.
Revised Roughness Coefficients for Corrugated Metal (May 1993)
Pipe | n value for Indicated Corrugation Size
Diameter | | Structural-plate
ft | Riveted Construction | Construction
| Corrugation, Pitch x Rise, inches
|2-2/3 x 1/2 3 x 1 5 x 1 6 x 1 6 x 2 9 x 2-1/2
Annular Corrugations
1 0.027
2 0.025
3 0.024 0.028 0.025
4 0.024 0.028 0.026 0.024
5 0.024 0.028 0.026 0.024 0.035 0.036
6 0.023 0.028 0.026 0.024 0.035 0.035
7 0.023 0.028 0.026 0.023 0.035 0.034
8 0.023 0.028 0.025 0.023 0.034 0.034
9 0.023 0.028 0.025 0.023 0.034 0.034
10 0.022 0.027 0.025 0.023 0.034 0.034
11 0.022 0.027 0.025 0.022 0.034 0.033
12 0.027 0.024 0.022 0.033 0.033
16 (a)0.026(a)0.023(a)0.021
18 (a)0.033
21 (a)0.033
Spiral Corrugations
4 0.020 Use values for annular
5 0.022 corrugations for all other
6 0.023 corrugation sizes and pipe
7 0.023 diameters.
Range of pipe diameter in feet commonly encountered with the above
indicated corrugation size:
<9 3-13 5-13 3-13 5-25 5-25
(a)Extrapolated beyond Federal Highway Administration curves.1
Note: n values apply to pipes in good condition. Severe
deterioration of metal and misalignment of pipe sections
may cause slightly higher values.
1See page 16 HDS-5 for extrapolation.
Charles W. Boning, Chief
Office of Surface Water
WRD DISTRIBUTION: A, B, FO, PO