Colebrook-White equation
This pipe friction equation is generally considered to be the most accurate available for determining head losses and flowrates. The following equation can be used to obtain velocities:
Where g is gravitational acceleration (9.80 m/s2 in Australia),
D is pipe diameter (m),
S is the slope of the total energy line (m/m),
k is the pipe wall roughness (m), and
u is the kinematic viscosity (m.m/s).
Unfortunately, a direct expression is not available to calculate a head loss or hydraulic grade line slope. The equation must be solved by iterative calculations, but this is easily accomplished.
Values for the roughness height k are:
(Sources: Hydraulics Research (1994) Tables (and Charts) for the Hydraulic Design of Pipes, Thomas Telford, London Standards, Australia (1978), AS2200-1978 Design Charts for Water Supply and Sewerage, Sydney)
The Colebrook-White Equation can be applied to open channel flows, although some difficulties might be encountered.
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