Manning n for Corrugated Metal Culverts

In Reply Refer To:                                  June 22, 1993
Mail Stop 415


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 

                     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 

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