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<math>d_{max} = \frac{f' \times t}{V_{R}}</math> {{Plainlist|1=Where:*''d<sub>max</sub>'' = Stone reservoir depth (m) *''f''' = Infiltration coefficient for native soils (m/hr) *''V<sub>R</sub>'' = Void space ratio for aggregate used (typically 0.35 for 50 mm clear stone) *''t'' = Time to drain (design for 96 hour time to drain is recommended)}} The value for native soil infiltration rate (f') 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.
→To calculate the invert of the underdrain from the base of the reservoir
*''0.4'' = Void ratio of [[reservoir gravel|clear stone]]}}
*''0.4'' = Void ratio of [[reservoir gravel|clear stone]]}}
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(f') used in the above equations should be the [[design infiltration rate]] that incorporates a safety correction factor. On highly permeable soils (e.g., infiltration rate of 45 mm/hr or greater), a maximum stone reservoir depth of 2 metres is recommended to prevent soil compaction and loss of permeability from the mass of overlying stone and stored water.
On highly permeable soils (e.g., infiltration rate of 45 mm/hr or greater), a maximum stone reservoir depth of 2 metres is recommended to prevent soil compaction and loss of permeability from the mass of overlying stone and stored water.
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When sizing the area of permeable paving based on the contributing drainage area, the following equation may be used:
When sizing the area of permeable paving based on the contributing drainage area, the following equation may be used: