Flow through perforated pipe

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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 are per meter. The maximum flow rate through a perforated pipe (Qmax, p, L/s) may be calculated:

Where:

  • L is the length of perforated pipe (m)
  • 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 (m) (e.g. ponding/mulch/filter media/choker layer),


Example calculation[edit]

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: Then the maximum flow rate per planter is calculated: