Changes

 

<p>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 environment within the bottom of the bioretention cell. <br>

 

This has two primary benefits:

<p>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.</p>

   

<li>Additional infiltration is promoted by holding the water in place over the native soil for longer, </li>

<p>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. </p>

<li>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.</li>

</ol>

  </p>

<p>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<ref>Brown RA, Hunt WF, Kennedy SG. Designing Bioretention with an Internal Water Storage (IWS) Layer.; 2011.</ref>.  

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<ref>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.</ref>.

However, any depth of IWS would be expected to make some improvement on water retention and infiltration. </p>

<references />