What is Manning's n for concrete?

For a concrete-lined channel with a trowelled finish the typical value is 0.013 (range 0.011 to 0.015); unfinished concrete is 0.017 (0.014 to 0.020). Use the upper end where the surface is rough, damaged or carries sediment, and the lower end only for new, smooth, well-jointed work.

What is Manning's n for a grass-lined channel?

An excavated earth channel with short grass and few weeds is typically 0.027 (0.022 to 0.033). For channels designed around a vegetal lining, n depends on the grass type, its height and the flow depth; FHWA HEC-15 computes it from the vegetation properties and the applied shear rather than from a fixed table value.

What n should I use for corrugated metal or PVC pipe?

Corrugated metal pipe is typically 0.024 (0.021 to 0.030); PVC is 0.010 (0.009 to 0.011). Manufacturers publish values for their own corrugation profiles, and many road authorities prescribe a design value: use those where they apply.

How much does Manning's n affect the flow depth?

For a wide channel the normal depth varies with n to the power 0.6, so a 10 % error in n changes the depth by about 6 % and the velocity by about the same amount in the other direction. Narrower sections respond in a similar way. Run the calculation at the low and high end of the range when the result sits close to a freeboard or capacity limit.

Does Manning's n change with depth?

For most hard linings it is close to constant. It changes markedly for vegetated channels, for very shallow flow over rough beds and for overbank flow through vegetation, where n falls as the depth increases. For natural channels the Cowan method builds n from a base value plus adjustments for irregularity, obstructions, vegetation and meandering.

Manning's n Table: Roughness Coefficients for Channels and Pipes

Manning's equation and n

Manning's equation relates the mean velocity of uniform flow to the channel's hydraulic radius, its bed slope and a roughness coefficient:

V = \frac{1}{n} R^{2/3} S^{1/2} (SI) V = \frac{1.49}{n} R^{2/3} S^{1/2} (US customary)

n carries the effect of everything that resists the flow: the lining material, surface irregularity, vegetation, obstructions and changes in cross-section. It is the least certain input to most open-channel calculations, which is why the tables give a range rather than one number. Start from the typical value, move towards the maximum for poor maintenance, vegetation growth, debris or a rough finish, and towards the minimum only for new, clean, well-finished linings.

Most rows follow Chow (1959), Table 5-6. The riprap, gabion and pipe rows are typical design values; where a road authority or a manufacturer prescribes a value for your lining or pipe, use theirs. Use these values with the Manning's equation calculator, the gradually varied flow calculator or the culvert calculator.

Tables

Jump to: Excavated Channels · Lined Channels · Natural Channels · Pipes

Excavated Channels

Lining or channel description	Minimum	Typical	Maximum
Earth, clean, recently completed	0.016	0.018	0.020
Earth, clean, after weathering	0.018	0.022	0.025
Earth, with short grass, few weeds	0.022	0.027	0.033
Earth, dense weeds, deep channels	0.025	0.032	0.040
Earth, gravel bottom, clean sides	0.022	0.025	0.030
Earth, cobble bottom, clean sides	0.030	0.035	0.040
Dragline-excavated, no vegetation	0.025	0.028	0.033
Dragline-excavated, light brush	0.035	0.050	0.060
Rock cut, smooth and uniform	0.025	0.035	0.040
Rock cut, jagged and irregular	0.035	0.040	0.050
Gravel bottom, rubble sides	0.023	0.033	0.036

Lined Channels

Lining or channel description	Minimum	Typical	Maximum
Concrete, trowel finish	0.011	0.013	0.015
Concrete, float finish	0.013	0.015	0.016
Concrete, unfinished	0.014	0.017	0.020
Concrete, gunite, good section	0.016	0.019	0.023
Concrete, gunite, wavy section	0.018	0.022	0.025
Concrete on excavated rock	0.017	0.020	0.024
Asphalt, smooth	0.013	0.013	0.015
Asphalt, rough	0.016	0.016	0.018
Brick, glazed	0.011	0.013	0.015
Brick, in cement mortar	0.012	0.015	0.018
Metal, smooth steel	0.011	0.012	0.014
Metal, corrugated	0.021	0.025	0.030
Masonry, cemented rubble	0.017	0.025	0.030
Masonry, dry rubble	0.023	0.032	0.035
Dressed stone / ashlar	0.013	0.015	0.017
Riprap, D50 = 150mm	0.030	0.035	0.040
Riprap, D50 = 300mm	0.035	0.040	0.050
Gabion	0.025	0.028	0.032

Natural Channels

Lining or channel description	Minimum	Typical	Maximum
Clean, straight, no rifts or pools	0.025	0.030	0.033
Clean, straight, stones and weeds	0.030	0.035	0.040
Clean, winding, some pools/shoals	0.033	0.040	0.045
Sluggish reaches, weedy, deep pools	0.050	0.070	0.080
Very weedy, heavy timber	0.075	0.100	0.150
Mountain streams, gravel/cobbles	0.030	0.040	0.050
Mountain streams, cobbles/boulders	0.040	0.050	0.070
Floodplain, short grass	0.025	0.030	0.035
Floodplain, high grass	0.030	0.035	0.050
Floodplain, cultivated, no crop	0.020	0.030	0.040
Floodplain, cultivated, mature crop	0.030	0.040	0.050
Floodplain, scattered brush	0.035	0.050	0.070
Floodplain, dense brush	0.070	0.100	0.160
Floodplain, dense trees	0.080	0.120	0.200

Pipes

Lining or channel description	Minimum	Typical	Maximum
PVC	0.009	0.010	0.011
HDPE, smooth interior	0.009	0.010	0.012
HDPE, corrugated interior	0.018	0.022	0.025
Concrete pipe	0.011	0.013	0.017
Corrugated metal pipe (CMP)	0.021	0.024	0.030
CMP, paved invert	0.018	0.020	0.022
Cast iron, coated	0.010	0.013	0.014
Cast iron, uncoated	0.011	0.014	0.016
Steel, welded, new	0.010	0.012	0.014
Steel, riveted	0.013	0.016	0.017
Vitrified clay (VCP)	0.011	0.014	0.017

Composite sections

Where the bed and the banks have different linings, an equivalent n for the whole section is needed. The Horton–Einstein method assumes the mean velocity is the same in each part of the section:

n_{e} = [\frac{\sum _{i} P _{i} n _{i}^{3/2}}{P}]^{2/3}

where P_i is the wetted perimeter with roughness n_i and P the total wetted perimeter. For a main channel with vegetated floodplains, divide the section into subsections and add their conveyances instead; a single n across the floodplain understates the capacity of the main channel.

Natural channels: the Cowan method

For natural streams, Cowan (1956) builds n from a base value for the bed material and additive adjustments, multiplied by a factor for meandering:

n = (n_{b} + n_{1} + n_{2} + n_{3} + n_{4}) m

with n_b for the bed material, n₁ for surface irregularity, n₂ for variation in cross-section, n₃ for obstructions, n₄ for vegetation and m for the degree of meandering. Arcement and Schneider (1989) give the adjustment values and photographs of calibrated reaches, and are the usual reference for floodplain n. The natural-channel rows above are useful as a cross-check on the result.

Manning's n table