Bioretention: Internal water storage

An internal water storage zone/reservoir (IWS or IWSZ) is created by including an upturned 90 deg on the outflow pipe to force an elevated outlet. For practical reasons, this is commonly applied within a manhole or other nearby piece of infrastructure, rather than being buried directly adjacent. The effect is to create longer term, water-logged, low-oxygen environment within the bottom of the bioretention cell.
This has two primary benefits:

1. Additional infiltration is promoted by holding the water in place over the native soil for longer,
2. The types of microbes that grow in a low-oxygen environment are also better at completing the removal of nitrogen compounds from the water. This effect is enhanced by using a low-nutrient carbon source in the bioretention filter media and storage reservoir aggregate.

Saturated soil conditions can kill plants in the bioretention cell, so a minimum of 30 cm vadose zone (free draining depth) must be included for healthy rooting at the top of the cell. Nitrogen removal rates are linked to increased depth of IWS, so that a minimum of 45 cm of IWS is recommended where the depth of the native soils and bioretention cell permit. However, any depth of IWS would be expected to make some improvement on water retention and infiltration.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.

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

The natural ground material characteristic of or existing by virtue of geographic origin.

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.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

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

A bioretention BMP featuring an impermeable liner and underdrain that prevents infiltration of runoff into the underlying native soil; provides sedimentation and filtration of urban runoff as it passes through the mulch layer, engineered filter media and vegetation root zone.

A bioretention cell that features an impermeable liner that collects and treats stormwater through sedimentation and filtration only (i.e., no infiltration).