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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.
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*''t'' = Time to fill stone bed (typically 2 hr)
*''t'' = Time to fill stone bed (typically 2 hr)
*''V<sub>R</sub>'' = Void ratio for stone bed (typically 0.4 for 50 mm dia. [[reservoir aggregate|clear stone]])}}
*''V<sub>R</sub>'' = Void ratio for stone bed (typically 0.4 for 50 mm dia. [[reservoir aggregate|clear stone]])}}
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.
===To calculate the invert of the underdrain from the base of the reservoir===
===To calculate the invert of the underdrain from the base of the reservoir===
*''0.4'' = Void ratio of [[Reservoir aggregate|clear stone]]}}
*''0.4'' = Void ratio of [[Reservoir aggregate|clear stone]]}}
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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: