Mannings
Mannings is the most popular equation for stormwater design because it is simple to apply and it generally provides an acceptable level of accuracy. In calculations involving Hardie Pipes the use of the Mannings equation may give reasonable flow estimates if an allowance is made for the variation of the roughness coefficient with pipe size and flow velocity.

Where
V = Velocity (m/s)
n = Manning roughness coefficient
R = Hydraulic radius = D/4 (m)
S = Hydraulic gradient (m/m)

Note: The flow chart (to the right) has been prepared using the Manning's formula.

Hydraulic chart
(Click to enlarge)