Manning’s Roughness Coefficient - CivilWeb Spreadsheets

Manning’s Roughness Coefficient (n)

Manning’s Roughness Coefficient is a measurement of the average roughness of the wetted perimeter of a pipe or conveyance. This allows the Manning formula to correct for the effect of this roughness slowing down the water as it moves through the pipe or conveyance.

Typical values are given below;

Channel	Manning's n Values
Channel	Min	Normal	Max
Concrete or asbestos-cement pipe	0.011	0.013	0.015
Smooth walled helical spiral rib pipe	0.012		0.013
Corrugated metal subdrain	0.017	0.019	0.021
Plastic pipe (Smooth)	0.011		0.015
Plastic pipe (Corrugated)		0.024
Metal pipe or pipe arch with helically wound corrugations		0.024
Wrought iron	0.012	0.014	0.015
Cast iron	0.010	0.013	0.014
Steel pipe, welded	0.010	0.012	0.014
Brick	0.012	0.015	0.017
Vitrified clay sewer	0.011	0.014	0.017
Sanitary sewer, coated with sewage slime, with bends and connections	0.012	0.013	0.016
Concrete gutter, troweled finish		0.012
Asphalt pavement		0.016
Concrete lined channel	0.013	0.013	0.015
Grouted riprap	0.028	0.030	0.040
Bare soil	0.020	0.020	0.023
Rock cut	0.025	0.035	0.045
Grass, some weeds	0.025	0.030	0.033
Dense weeds or aquatic plants	0.030	0.035	0.040
Dense weeds, high as flow depth	0.050	0.080	0.120

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