advanced drainage system - Polypipe

General Hydraulic Design 3.3
Velocities and flow rates for thermoplastic structured walled pipes can be calculated using either the Manning or
Colebrook-White equations. The Colebrook-White equation forms the basis of this guide as it has been shown to
provide accurate results for a wide range of flow conditions and is the method commonly used in the UK.
For circular pipes flowing full, the Colebrook-White equation may be expressed as:
⎧ ks
n = -2√(2gSfD)log10 x + 2.51v ⎫
⎪
⎪
⎩3.7D
D√(2gSfD) ⎭
Where: n = Mean water velocity
g
= Gravity
Sf = Hydraulic gradient, (hf / L)
D
ks
V
= Internal pipe diameter
= Pipe roughness
= Kinematic viscosity
An alternative approach is required when utilising the Colebrook-White equation to determine either the pipe diameter
or hydraulic gradient variables.
Hydraulic gradient (S f)
Hydraulic gradient is governed by the pipe slope.
Pipe roughness (k s)
A mean measurement of the height that surface roughness projects from the pipe wall. Measured in terms of an
equivalent sand roughness.
Sewer type and age will influence the choice of pipe roughness. Except for calculating initial flow conditions,
consideration should also be given to environmental factors, such as sediment and biological slime deposits.
Typical values of roughness (Ks), for use in the Colebrook-White equation, are given in table 3.3.1
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