For practices with the underdrain perforated pipe elevated off the bottom of the storage reservoir, infiltration water storage is only provided by the depth of storage reservoir below the invert of the underdrain perforated pipe, and only the portion that can reliably drain within the specified drainage time. So the infiltration water storage of the practice can be calculated as:
For practices with the underdrain perforated pipe elevated off the bottom of the storage reservoir, infiltration water storage is only provided by the depth of storage reservoir below the invert of the underdrain perforated pipe, and only the portion that can reliably drain by infiltration within the specified drainage time. So the infiltration water storage of the practice can be calculated as: