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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: |
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| <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)}} |
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