Flow through perforated pipe

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Manufacturers of perforated pipe are often able to provide the open area per meter length.

Where:d is the coefficient of discharge (0.61 for a sharp edged orifice),

  • B is the clogging factor (between 0.5 to calculate a for matured installation and 1 to calculate 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)
  • Σ d is the total depth of bioretention components over the perforated pipe (mm) (e.g. ponding/mulch/filter media/choker layer),


Example calculation[edit]

A part used roll of 100 mm diameter perforated pipe appears long enough to use for a stormwater planter project. 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 x 1.5 mm on six sides, repeated every 3 cm along the pipe. To calculate the maximum flow rate, first the open area per meter is calculated: Then: