Bioretention: Internal water storage

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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[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 bioretention cell permit[2]. However, any depth of IWS would be expected to make some improvement on water retention and infiltration.

  1. Brown RA, Hunt WF, Kennedy SG. Designing Bioretention with an Internal Water Storage (IWS) Layer.; 2011.
  2. Lynn TJ, Yeh DH, Ergas SJ. Performance of Denitrifying Stormwater Biofilters Under Intermittent Conditions. Environ Eng Sci. 2015;32(9):150710132840004. doi:10.1089/ees.2015.0135.