Darcy-Weisbach Formula

For all pipes, the Hydraulic Institute and many engineers consider the Colebrook equation most reliable for evaluating the friction factor.

Where

The Reynolds Number is defined as:

Where

The trick to solving the Colebrook equation is to select a friction factor that balances the left and right sides of the equation. Since the Reynolds number is dependent on viscosity, the temperature of runoff is of concern. The following table provides a range of viscosity of water at various temperatures.

Specific Gravity and Kinematic Viscosity of Water
Temperature (deg F)	Spec Gravity	Kin. Viscosity (ft2/sec)
40	1.000	1.664
50	1.000	1.410
60	0.999	1.217
70	0.998	1.059
80	0.997	0.930
90	0.995	0.826
100	0.993	0.739
110	0.991	0.667
120	0.990	0.610
150	0.980	0.475

StormShed3G™ currently assumes a default temperature of 68 deg F.