Underdrain n. : An underground drain or trench with openings through which the water may percolate from the soil or ground above.

Underdrain v. t. : To drain by forming an underdrain or underdrains in; as, to underdrain land.

Example:


Taxonomy
underdrain, Leopold, pipe, balls, filter media, backwashing, Hoehn, tile, diagram, cells, Wheeler, ceramic balls, drainage pipe, collection pipes, cycle, filter, spaces, used with permission.

Defination from the web:
underdrain system is the pipe lateral which is displayed in the top animation (Animation courtesy of Mark Gans) ... Although this type of underdrain has been used successfully in the past, it has been phased out over the years in newer plants do to the fact ...