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→Design
===Storage reservoir===
===Storage reservoir===
Should be a minimum of 300 mm deep, and filled with granular material of washed 25 or 50 mm diameter crushed angular stone (max. wash loss of 0.5%). Include organic material derived from untreated wood (e.g. chips, mulch or shavings) to enhance nitrogen removal. Also see [[Bioretention: Internal water storage]]. See [[Bioretention: Sizing]] for reservoir sizing guidance, including depth and area of surface ponding and reservoir volume needed to capture and store the design storm.
Should be a minimum of 300 mm deep, and filled with granular material of washed 25 or 50 mm diameter crushed angular stone (max. wash loss of 0.5%). Include organic material derived from untreated wood (e.g. chips, mulch or shavings) to enhance nitrogen removal. Also see [[Bioretention: Internal water storage]]. See [[Bioretention: Sizing]] for reservoir sizing guidance, including depth and area of surface ponding and reservoir volume needed to capture and store the design storm.
===Underdrain===
*Recommended where native soil infiltration rate is less than 15 mm/h (hydraulic conductivity < 1x10-6 cm/s) and needed for non-infiltrating, filtration only designs.
*Typically comprised of a length of perforated pipe embedded near the top of the storage reservoir, with an overlying choker layer of medium-sized aggregate, and structures to provide inspection and maintenance access.
*Alternatively the perforated pipe could be installed on the reservoir bottom and connected to an upturned pipe assembly or riser.
*May include an optional flow restrictor (e.g., orifice cap or valve) on the underdrain outlet pipe or outlet storm sewer, to provide erosion control and optimize infiltration while meeting the required drainage time.
*Continuously perforated, smooth interior HDPE or PVC pipe with diameter ≥ 200 mm to reduce freezing risk and facilitate access by camera and cleaning equip. Solid pipe leaders from inlet structures should extend 300 mm into the storage reservoir before transitioning to perforated pipe. Transitions back to solid pipe to connect to outlet storm drain system.
* See [[Underdrains]] for further guidance.
===Plants===
===Plants===