===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===
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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:
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For designs that include an underdrain, the maximum depth of the storage layer below the invert of the underdrain pipe (''d<sub>s, max</sub>'') can be calculated as follows: