A drainage cell comprises a backing plate defining a plurality of holes, a series of conical tubular structures having side walls and a base extending from the backing layer, the tubular structures being in fluid communication with at least some of the holes in the backing plate thereby allowing fluid to pass through the holes into a generally tubular structure. Sides of the tubular structures define apertures permitting fluid flow out of the tubular structures. Typically, the open area of the tubular structures is at least 20% of the area of the side walls of the tubular structures, and most preferably 70 to 75% of that area. Providing apertures in the side walls of the tubular structures significantly improves the rate of flow of the water through the cell and creates a higher internal drain void volume area.
