Underdrain Sizing

Only smooth-walled plastic pipes are recommended for underdrains. They maintain higher flow rates than non-smooth 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 5-step process.

1. Determine flow rate through the soil media (see factsheet for calculation).
2. 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.
3. 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).
4. 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.
5. 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.