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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]].
Practices which infiltrate surface runoff through an engineered soil or [[filter media]], and discharge through an [[underdrain]] perforated pipe include [[stormwater planters]], and partial- and no-filtration 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 from the BMP may be limited by the hydraulic conductivity of the medium, or by the properties of the perforated [[pipe]]. See [[Flow through perforated pipe]] page for guidance on estimating the maximum flow rate of a perforated pipe.
The maximum flow rate through a bed of filter media (''Q<sub>max, media</sub>'', m<sup>3</sup>/s) may be calculated as:
The maximum flow rate through a bed of filter media (''Q<sub>max, media</sub>'', m<sup>3</sup>/s) may be calculated as:
As the maximum flow rate exceeds the downstream maximum permissible flow, the design must be amended. In order of preference, some options include:
As the maximum flow rate exceeds the downstream maximum permissible flow, the design must be amended. In order of preference, some options include:
#Increasing the size of the downstream pipe to accommodate a greater flow rate;
#Increasing the size of the downstream pipe to accommodate a greater flow rate;
#Reducing the depth of the filter media bed in agreement with a Landscape Architect, and increasing pretreatment (as filter bed < 600 mmm will provide less treatment); or
#Reducing the depth of the filter media bed in agreement with a Landscape Architect, and increasing pretreatment (as filter bed < 600 mm deep will provide less water quality treatment); or
#Reformulating the bioretention filter media in collaboration with the soils provider, to reduce the hydraulic conductivity.
#Reformulating the bioretention filter media in collaboration with the soils provider, to reduce the hydraulic conductivity.