An underdrain system having a bottom formed of concrete. The concrete bottom includes a plurality of generally cone shaped depressions formed therein. A porous plate is operably associated with at least one of the generally cone shaped depressions. The underdrain system further includes an insert having at least one distribution orifice for distributing the flow. The insert is positioned beneath the porous plate and at least partially within the at least one generally cone shaped depression. The insert insures thorough distribution of any fluid passing through the porous plate, i.e., the fluid is evenly distributed over the entire surface area of the porous plate. In an alternative embodiment, an underdrain system is provided including a bottom formed of concrete. The bottom has a plurality of generally cone shaped depressions formed therein. A porous plate is operably associated with at least one of the generally cone shaped depressions. The underdrain system further includes a removable anchoring member for removably anchoring the porous plate to the concrete bottom without penetrating any surface of the concrete bottom.