Open main menu

LID SWM Planning and Design Guide β

Bioretention: Variations

Types of bioretention by underground design
Type Gravel layer Underdrain Liner Mechanisms Schematic
Rain gardens - - - These are the simplest construction, often used by residents or community groups. Volume reduction is through infiltration and evapotranspiration. Rain Garden Schematic.png
Infiltrating bioretention yes - - This is a highly desirable type of bioretention where the soils permit infiltration at a great enough rate to empty the facility between storm events. Volume reduction is primarily through infiltration to the underlying soils, with some evapotranspiration. As there is no outflow from this BMP, it is particularly useful in areas where nutrient management is a concern to the watershed. Full infiltration.png
Partially infiltrating bioretention yes yes - Including an underdrain in the gravel storage layer help to empty the facility between storm events, even over ‘tight soils’. The drain discharges to a downstream point, which could be an underground infiltration facility. Limited volume reduction is gained through infiltration and evapotranspiration. By raising the outlet of the discharge pipe the bottom portion of the BMP can only drain through infiltration. This creates a fluctuating anaerobic/aerobic environment which promotes denitrification. Increasing the period of storage has benefits for promoting infiltration, but also improves water quality for catchments impacted with nitrates. A complimentary technique is to use fresh wood mulch, which also fosters denitrifying biological processes. Partial infiltration.pngPartial with storage.png
Biofilters
(non-infiltrating)
yes yes yes This type may be required over contamination hot-spots or in very dense urban areas with a lot of other underground infrastructure. The design includes an impermeable base and sides, so that volume reduction is made only through evapotranspiration. This type of cell can be constructed above grade in any waterproof and structurally sound container, e.g. in cast concrete or a metal tank. Planter.png