Flow through perforated pipe

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

${\frac {Q_{max,p}=L\times B\times C_{d}\times A_{o}{\sqrt {2\cdot g\cdot \sum d}}}{1000}}$

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

• 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),
• C_d is the coefficient of discharge (usually 0.61 for the sharp edge created by relatively thin pipe walls),
• A_o 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

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: ${\frac {0.008\ m\times 0.0015\ m\times 6}{0.03\ m}}=0.0024\ m^{2}/m$ Then the maximum flow rate per planter is calculated: $Q_{max,p}={\frac {8\times 0.5\times 0.61\times 0.0024\ m^{2}/m{\sqrt {2\cdot 9.81\ m/s^{2}\cdot \sum 1.1\ m}}}{1000}}=27\ L/s$

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

A vegetated practice that collects and treats stormwater through sedimentation and filtration. Contributions to water cycle/water balance are through evapotranspiration only; no infiltration.

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.