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→Design
Bioretention can be designed to be inline (accepts all flow from the drainage area and conveys large event flows through an overflow outlet) or offline (allows only the design storm runoff storage volume to enter the facility). Overflow structures must be sized to safely convey large event flows out of the facility.
Bioretention can be designed to be inline (accepts all flow from the drainage area and conveys large event flows through an overflow outlet) or offline (allows only the design storm runoff storage volume to enter the facility). Overflow structures must be sized to safely convey large event flows out of the facility.
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===Filter Media Bed===
Recommended filter media bed depth is 0.5 to 1.0 m, but in constrained applications pollutant removal benefits may be achieved by beds as shallow as 0.3 m. Plant-specific depths recommended are: 0.3 m to support grasses and perennials, 0.6 m to support shrubs and 1.0 metre to support trees.
Filter bed must be installed level, with maximum side slopes of 1:3 or 33%.
Maintaining a 75 to 100 mm layer of double shredded wood mulch is recommended on portions of the filter bed surface not covered by plantings. Benefits include soil moisture preservation, suppression of weed growth and enhanced water treatment. Erosion-prone inlets may require stone instead of mulch.
[[Filter media]] should come pre-mixed from an approved vendor, be free of stones or debris greater than 50 mm diameter.
*Blend A - Use when drainage rate is the priority (when I:P1 ratio ≥15:1). Consists of 3 parts sand to 1 part organic material/additives.
*Blend B – Use when water quality treatment (phosphorus, metals) is the priority and more diverse planting options are desired. Consists of 3 parts sand to 2 parts topsoil to 1 part organic material/additives.
See [[Filter media]] for detailed guidance on different blends, required performance criteria, and specifications and laboratory tests for blend components and the filter media itself.
===Plants===
===Plants===