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The equation for the depth of the stone bed is as follows:
The equation for the depth of the stone bed is as follows:
−<math>d= \frac{[Q_c\times R + P - q\times T ]}{Vr}</math>
+<math>d= \frac{[Q_c\times R + P - q'\times t ]}{V_{R}}</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*''d'' = Stone bed depth (m)
*''d'' = Stone bed depth (m)
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*''R'' = A<sub>c</sub>/A<sub>p</sub>; the ratio of contributing drainage area (''A<sub>c</sub>'') to permeable paving area (''A<sub>p</sub>''). Note that the contributing drainage area (''A<sub>c</sub>'') should not contain pervious areas.
*''R'' = A<sub>c</sub>/A<sub>p</sub>; the ratio of contributing drainage area (''A<sub>c</sub>'') to permeable paving area (''A<sub>p</sub>''). Note that the contributing drainage area (''A<sub>c</sub>'') should not contain pervious areas.
*''P'' = Rainfall depth (m)
*''P'' = Rainfall depth (m)
−*''q'' = Infiltration rate for native soils (m/day)
+*''q''' = 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 0.35 for 50 mm dia. stone)}}
+*''V<sub>R</sub>'' = Void ratio for stone bed (typically 0.35 for 50 mm dia. stone)}}
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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} = \frac{q \times t}{Vr}</math>
+<math>d_{max} = \frac{q' \times t}{Vr}</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
−*''dr<sub>max</sub>'' = Maximum stone reservoir depth (m)
+*''d<sub>max</sub>'' = Stone reservoir depth (m)
−*''q'' = Infiltration rate for native soils (m/hr)
+*''q''' = Infiltration coefficient for native soils (m/hr)
−*''Vr'' = Void space ratio for aggregate used (typically 0.35 for 50 mm clear stone)
+*''V<sub>R</sub>'' = Void space ratio for aggregate used (typically 0.35 for 50 mm clear stone)
*''t'' = Time to drain (design for 48 hour time to drain is recommended)}}
*''t'' = Time to drain (design for 48 hour time to drain is recommended)}}