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→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==
==To calculate the invert of the underdrain from the base of the reservoir==
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 (''d<sub>s, max</sub>'') can be calculated as follows:
<math>d_{s, max}=\frac{f'\times '}{0.4}</math>
{{Plainlist|1=Where:
*''f''' = Design infiltration rate (mm/hr), and
*''t'' = [[Drainage time]] (hrs), e.g. 96 hours, check local regulations for drainage time requirements.
*''0.4'' = Void ratio of [[reservoir gravel|clear stone]]}}
<math>d_{max} = \frac{f' \times t}{V_{R}}</math>
<math>d_{max} = \frac{f' \times t}{V_{R}}</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where: