of the ribs 8 gives the unit 4 a greater tolerance again referring to fig6 the ribs 8 are located along the length 3 of the water management unit 4 and extend laterally from the side of the unit 4 the width 2 of the ribs is not critical however the dimension will be chosen to keep the overall profile of the unit 4 narrow and to provide the desired extraction capacity suitable dimensions have been given above however the person skilled in the art will appreciate that these are not limiting fig9 shows a cross section view of a water management unit 4 ribs 8 extend laterally from the side of the collection portion 16 the ribs 8 have a triangular cross section with a downwardly angled top surface and water extraction material 14 on the generally horizontal bottom of each rib 8 fig1 shows an expanded view of the water extraction material 14 similarly to the drain belt 10 the material has slots 12 and channels 11 which extract water by capillary action optionally the ribs 8 have triangular cross section with the water extraction material 14 on the bottom side of the triangle the cross section of the ribs 8 provides increased rigidity to the extraction portion 18 so that the extraction material 14 remains in place and facing down when installed and during compaction of the backfill the ribs 8 may also provide increased rigidity to.