An underdrain system having a bottom formed of concrete. The concrete bottom includes a plurality of depressions formed therein. A porous plate is operably associated with at least one of the depressions. An anchor assembly is provided for anchoring the porous plate to the concrete bottom in an efficient and cost effective manner. Preferably, the anchor assembly permits the porous plate to be readily replaced. An optional insert may be used to enhance the distribution of the flow of fluid through the porous plate.