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