Bioretention: Internal water storage

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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 being buried directly adjacent. The effect is to create longer term, water-logged, low-oxygen environmentRefers 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. within the bottom of the bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cell.
This has two primary benefits:

  1. Additional infiltration is promoted by holding the water in place over the native soilThe natural ground material characteristic of or existing by virtue of geographic origin. for longer,
  2. The types of microbes that grow in a low-oxygen environmentRefers 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. 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 bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. filter mediaThe 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. and storage reservoirAn 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. aggregateA 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..

Saturated soil conditions can kill plants in the bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cell, so a minimum of 30 cm vadose zone (free draining depth) must be included for healthy rooting at the top of the cell[1].

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 bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cell permit[2].

However, any depth of IWS would be expected to make some improvement on water retention and infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface..

  1. Brown RA, Hunt WF, Kennedy SG. Designing BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. with an Internal Water Storage (IWS) Layer.; 2011.
  2. Lynn TJ, Yeh DH, Ergas SJ. Performance of Denitrifying Stormwater BiofiltersA 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). Under Intermittent Conditions. Environ Eng Sci. 2015;32(9):150710132840004. doi:10.1089/ees.2015.0135.