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