
Only smoothwalled plastic pipes are recommended for underdrains. They maintain higher flow rates than nonsmooth walled corrugated pipes and are less prone to be a habitat for mosquitoes.
The number of pipes needed for the underdrain system is determined using the following 5step process.
 Determine flow rate through the soil media (see factsheet for calculation).
 Apply a factor of safety: range from 2 to 10, to the flow rate. That is, the pipe design will carry at least 2 to 10 times the amount of water that would flow through the media. This is underdrain design flow, Q.
 Use the following equation: D = 16 X (Q * n * s^0.5)^(3/8), Where D = Diameter of single pipe, n = roughness factor (recommended to be 0.011), s = internal slope (recommended to be 0.5%). Units: Q (cfs), D (in).
 The only unknown is D. This is the diameter of a single pipe that could carry all the water were it to be the only underdrain. Pipe diameters are typically either 4 inches or 6 inches. The table below converts "D" (in inches) to an equal number of 4 or 6 inch underdrains at 0.5% slope.
 It is strongly recommended to have at least 2 underdrains, even if only one is needed. This prevents system failure if one pipe inadvertently clogs.

