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Created page with "<p>The depth of an infiltration facility is dependent on the native soil infiltration rate, porosity (void space ratio) of the gravel storage layer and th..."
<p>The depth of an infiltration facility is dependent on the native soil infiltration rate, porosity (void space ratio) of the [[Reservoir gravel|gravel storage layer]] and the targeted time period to achieve complete drainage between storm events. The depth of the reservoir for designs <em>without an underdrain</em> can be calculated using the following equation:</p>
<strong>d = (i × t<sub>s</sub>)/ V<sub>r</sub></strong>
<br>
<p>Where:
<br> d = Stone reservoir depth (mm)
<br> q = Infiltration coefficient for native soils (mm/hr) (See [[Infiltration#Planning|Infiltration]] for details on measurement and safety factors)
<br> V<sub>r</sub> = Void space ratio for aggregate(typically 0.4 for 50 mm clear stone)
<br> t<sub>s</sub> = Time to drain (48 hour recommended)
<br>
</p>
<p>Notes:
<ul>
<li>The infiltration coefficient (q) used above should incorporate a safety correction factor </li>
<li>On highly permeable soils (e.g., infiltration rate of ≥ 45 mm/hr), a maximum depth of 2 m is recommended to prevent soil compaction and loss of permeability from the mass of overlying stone and stored water.</li>
<li>For designs that include an underdrain, the above equation can be used to determine the maximum depth of the stone reservoir below the invert of the underdrain pipe.</li>
</ul>
</p>
----
<p>Once the depth of the stone reservoir is determined the water quality volume (WQV), can be used to determine the area needed using the following equation:</p>
<strong>A<sub>f</sub> = WQV /(d × V<sub>r</sub>)</strong>
<p>Where:
<br>A<sub>f</sub> = Area of facility (m<sup>2</sup>)
<br>WQV = Water quality volume (m<sup>3</sup>)
<br>d = Stone reservoir depth (m)
<br>V<sub>r</sub> = Void space ratio for aggregate used (typically 0.4 for 50 mm clear stone)
<p>The ratio of impervious drainage area to infiltration area should be between 5:1 and 20:1 to limit the rate of accumulation of fine sediments and clogging.</p>
<strong>d = (i × t<sub>s</sub>)/ V<sub>r</sub></strong>
<br>
<p>Where:
<br> d = Stone reservoir depth (mm)
<br> q = Infiltration coefficient for native soils (mm/hr) (See [[Infiltration#Planning|Infiltration]] for details on measurement and safety factors)
<br> V<sub>r</sub> = Void space ratio for aggregate(typically 0.4 for 50 mm clear stone)
<br> t<sub>s</sub> = Time to drain (48 hour recommended)
<br>
</p>
<p>Notes:
<ul>
<li>The infiltration coefficient (q) used above should incorporate a safety correction factor </li>
<li>On highly permeable soils (e.g., infiltration rate of ≥ 45 mm/hr), a maximum depth of 2 m is recommended to prevent soil compaction and loss of permeability from the mass of overlying stone and stored water.</li>
<li>For designs that include an underdrain, the above equation can be used to determine the maximum depth of the stone reservoir below the invert of the underdrain pipe.</li>
</ul>
</p>
----
<p>Once the depth of the stone reservoir is determined the water quality volume (WQV), can be used to determine the area needed using the following equation:</p>
<strong>A<sub>f</sub> = WQV /(d × V<sub>r</sub>)</strong>
<p>Where:
<br>A<sub>f</sub> = Area of facility (m<sup>2</sup>)
<br>WQV = Water quality volume (m<sup>3</sup>)
<br>d = Stone reservoir depth (m)
<br>V<sub>r</sub> = Void space ratio for aggregate used (typically 0.4 for 50 mm clear stone)
<p>The ratio of impervious drainage area to infiltration area should be between 5:1 and 20:1 to limit the rate of accumulation of fine sediments and clogging.</p>