Flow through media

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 medium, or by the properties of the perforated pipe.

The maximum flow rate through a bed of filter media (Q_{max, m}, L/s) may be calculated: $Q_{max,m}={\frac {K_{m}\times A_{p}\times \left({\frac {\sum d}{d_{m}}}\right)}{3600}}$

Where:

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

Example calculation

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. The maximum flow through the medium will be calculated and a comparison made with the maximum flow through the pipe to see....: $Q_{max,m}={\frac {25mm/hr\times 12\ m^{2}\times \left({\frac {1.1\ m}{0.75\ m}}\right)}{3600\ s/hr}}=0.12\ L/s$

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

A perforated pipe used to assist the draining of soils.

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 parameter that describes the capability of a medium to transmit water.

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

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

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.