Stormwater Calculations - Mathematics and Statistics for the Environment - page 784

TABLE 25.6B
Roughness Coefficient ( n ) for the Manning Equation—
Pipe Flow

n Value Range

Material From To

Coated cast-iron 0.010 0.014

Uncoated cast-iron 0.011 0.015

Vitrified sewer pipe 0.010 0.017

Concrete pipe 0.010 0.017

Common clay drainage tile 0.011 0.017

0.023 0.026

Corrugated metal (2-2/3 × 1/2)

0.026 0.029

Corrugated metal (3 × 1 and 6 × 1)

0.030 0.033

Corrugated metal (6 × 2 structural plate)

Source: Adapted from Brater, E. and King, H., Handbook of Hydraulics ,
6th ed., McGraw-Hill, New York, 1976.

TABLE 25.6C
Roughness Coefficient ( n ) for the Manning Equation—
Constructed Channels

n Value Range

Lining Material From To

Concrete lined 0.012 0.016

Cement rubble 0.017 0.025

Earth, straight and uniform 0.017 0.022

Rock cuts, smooth and uniform 0.025 0.033

Rock cuts, jagged and irregular 0.035 0.045

Winding, sluggish canals 0.022 0.027

Dredged earth channels 0.025 0.030

Canals with rough stony beds, weeds on earth banks 0.025 0.035

Earth bottom, rubble sides 0.028 0.033

Small grass channels

Long grass, 13-inch 0.042 —

Short grass, 3-inch 0.034 —

Source: Adapted from Brater, E. and King, H., Handbook of Hydraulics ,
6th ed., McGraw-Hill, New York, 1976.

• Channel low occurs where low converges in gullies, ditches or swales, and natural or
artificial water conveyances (including storm drainage pipes). Channel flow is assumed
to exist in perennial streams or wherever there is a well-defined channel cross-sec-
tion. The Manning equation is used for open channel flow and pipe flow, and usually
assumes full flow or bank-full velocity. Manning coefficients can be found in Table
25.6B-D for open-channel flow (natural and artificial channels) and closed channel
flow. Coefficients can also be obtained from standard textbooks such as Open Channel
Hydraulics (Chow, 1959) or Handbook of Hydraulics (King and Brater, 1976).