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<math>db= [Qc\times R + P - i\times T ] / V</math>
<math>db= [Qc\times R + P - i\times T ] / V</math>
where:
where:
{{plainlist|
{{plainlist|}}
*''db'' = Stone bed depth (m)
*''db'' = Stone bed depth (m)
*''Qc'' = Depth of runoff from contributing drainage area, not including permeable paving surface(m)
*''Qc'' = Depth of runoff from contributing drainage area, not including permeable paving surface(m)
*''i'' = Infiltration rate for native soils (m/day)
*''i'' = Infiltration rate for native soils (m/day)
*''T'' = Time to fill stone bed (typically 2 hr)
*''T'' = Time to fill stone bed (typically 2 hr)
*''Vr'' = Void ratio for stone bed (typically 0.4 for 50 mm dia. stone) }}
*''Vr'' = Void ratio for stone bed (typically 0.4 for 50 mm dia. stone)}}
Note that the contributing drainage area (Ac) should not contain pervious areas.
Note that the contributing drainage area (Ac) should not contain pervious areas.
*''i'' = Infiltration rate for native soils (m/hr)
*''i'' = Infiltration rate for native soils (m/hr)
*''Vr'' = Void space ratio for aggregate used (typically 0.4 for 50 mm clear stone)
*''Vr'' = Void space ratio for aggregate used (typically 0.4 for 50 mm clear stone)
*''ts'' = Time to drain (design for 48 hour time to drain is recommended)
*''ts'' = 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