Flow through media

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Practices which infiltrate surface runoff through an engineered soil or filter media, and discharge through an underdrain include stormwater planters and some forms of bioretention. The maximum flow rate from the BMP may be limited by the hydraulic conductivity of the mmedium, or by the properties of the perforated pipe.

The maximum flow rate through a bed of filer media (Qmax) may be calculated:

Where:

  • Km is the hydraulic conductivity of the filter media (mm/hr),
  • Ap is the area of the practice (m2),
  • Σ d is the total depth of bioretention components over the perforated pipe (mm) (e.g. ponding/mulch/filter media/choker layer), and
  • dm is the depth of the filter media (mm).

Example calculation[edit]

A stormwater planter with footprint of 8 x 1.5 m is planned to received runoff from an adjacent rooftop. The initial design for the planters includes 750 mm depth of filter medium, 50 mm rock mulch, and a further ponding of 300 mm. The underdrain pipe will be embedded into high performance bedding or similar, with a strip of geotextile over the top to prevent migration of the filter media into the pipe. The lab test states that the medium has a hydraulic conductivity of 25 mm/hr.

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Q_{max}=25 mm/hr\times 12 m^{2}\times \left (\frac{\1.1 m}{0.75 m} \right )\times 3.6 \times 10^{-3}}