For practices with the underdrain perforated pipe elevated off the bottom of the storage reservoir, the active storage component is the depth of water in the storage reservoir below the invert of the underdrain perforated pipe that can reliably drained within the specified drainage time:<br> | For practices with the underdrain perforated pipe elevated off the bottom of the storage reservoir, the active storage component is the depth of water in the storage reservoir below the invert of the underdrain perforated pipe that can reliably drained within the specified drainage time:<br> |