===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 (''d<sub>s, max</sub>'') can be calculated as follows:
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: