Changes

Note that the contributing drainage area (Ac) should not contain pervious areas.

Note that the contributing drainage area (Ac) should not contain pervious areas.

For designs that include an underdrain, the maximum depth of the stone reservoir below the invert of the underdrain pipe can be calculated as follows:  

For designs that include an underdrain, the maximum depth of the stone reservoir below the invert of the underdrain pipe can be calculated as follows:  

<math>dr max = [i \times ts] / Vr </math>

<math>dr_{max} = [i \times ts] / Vr </math>

Where:  

Where:  

{{plainlist|

{{plainlist|

*''dr max'' &#61; Maximum stone reservoir depth (m)  

*''dr<sub>max'' &#61; Maximum stone reservoir depth (m)  

*''i'' &#61; Infiltration rate for native soils (m/hr)  

*''i'' &#61; Infiltration rate for native soils (m/hr)  

*''Vr'' &#61; Void space ratio for aggregate used (typically 0.4 for 50 mm clear stone)  

*''Vr'' &#61; Void space ratio for aggregate used (typically 0.4 for 50 mm clear stone)  

*''ts'' &#61; Time to drain (design for 48 hour time to drain is recommended) }}

*''ts'' &#61; Time to drain (design for 48 hour time to drain is recommended)  

The value for native soil infiltration rate (i) used in the above equations should be the design infiltration rate that incorporates a safety correction factor based on the ratio of the mean value at the proposed bottom elevation of the practice to the mean value in the least permeable soil horizon within 1.5 metres of the proposed bottom elevation  

The value for native soil infiltration rate (i) used in the above equations should be the design infiltration rate that incorporates a safety correction factor based on the ratio of the mean value at the proposed bottom elevation of the practice to the mean value in the least permeable soil horizon within 1.5 metres of the proposed bottom elevation