Bioretention: Internal water storage

An internal water storage zone (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 directly adjacent and buried. The effect of this is to create a water-logged, low-oxygen environment within the lower part of the bioretention cell. This promotes infiltration by holding the water in place over the native soil for longer. The types of microbes that grow in a low-oxygen environment are also better at completing the removal of nitrogen compounds from the water. An effect that is enhanced by using a low-nutrient carbon source in the bioretention filter media.

Saturated soil conditions are not good for the health of the plants on the surface of the bioretention cell, so a minimum depth of 30 cm planting zone must be included at the top of the cell. This is achieved by having 30 cm elevation difference between the top of the outlet and the surface of the cell. Nitrogen removal rates have been 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. 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.

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

Penetration of water through the ground surface.