# Flow through perforated pipe

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In practices such as bioretention, bioswales or stormwater planters, the flow limiting component is usually the filter media. See Flow through media page for guidance in estimating the maximum flow rate through bioretention filter media. In underground practices such as infiltration trenches, infiltration chambers, and exfiltration trenches, the perforated pipe that delivers water to the practice will be the primary flow limiting component.

Manufacturers of perforated pipe are often able to provide the open area per meter length. Where this information is not directly available, the open area can be calculated by measuring the perforations and summing the open area per meter. The maximum flow rate through a perforated pipe (Qmax, pipe, m3/s) is calculated similarly to flow through an orifice:

${\displaystyle Q_{max,pipe}=L\times B\times C_{d}\times A_{o}{\sqrt {2\cdot g\cdot h_{max}}}}$

Where:

• L is the length of perforated pipe (m)
• B is the clogging factor (between 0.5 (for matured installation) and 1 (for a new perfectly performing BMP)),
• Cd is the coefficient of discharge (usually 0.61 for the sharp edge created by relatively thin pipe walls),
• Ao is the total open area per unit length of pipe (m2/m),
• g is acceleration due to gravity (m/s2), and
• hmax is the total head of water within bioretention components over the perforated pipe (m) (i.e. ponding + mulch + filter media + choker layer).

### Example calculation

A part used roll of 100 mm diameter perforated pipe will be used for a stormwater planter project, where each planter will be 8 meters long. The initial design for the planters includes 750 mm depth of filter medium, 50 mm rock mulch, and a further ponding of 300 mm. Upon inspection the pipe is found to have perforations of 8 mm x 1.5 mm on six sides, repeated every 3 cm along the pipe. To calculate the maximum flow rate from each planter, first the open area of the pipe must be calculated in m2/m: ${\displaystyle {\frac {0.008\ m\times 0.0015\ m\times 6}{0.03\ m}}=0.0024\ m^{2}/m}$ Then the maximum flow rate per planter is calculated: ${\displaystyle Q_{max,pipe}=8\ m\times 0.5\times 0.61\times 0.0024\ m^{2}/m{\sqrt {2\cdot 9.81\ m/s^{2}\cdot 1.1\ m}}=0.027\ m^{3}/s}$

### Note

While it is recognised that many smaller perforations present a greater resistance to flow than fewer, larger perforations[1], calibrating for this is quite complex [2][3]. The practice of approximating, using the open area of the pipe per unit length, is commonplace, and we are unaware of ill effects arising[4][5][6][7].