In alluvial areas where permeable rocks are exposed on the land surface or at very shallow depth, roof top rainwater harvesting can be done through recharge pits. The technique is suitable for buildings having a roof area of 100 sq.m. and are constructed for recharging the shallow aquifers. Recharge Pits may be of any shape and size and are generally constructed 1 to 2 m. wide and 2 to 3, deep which are back filled with boulders (5-20 cm),Gravels (5-10mm) and coarse sand (1.5- 2mm) in graded form. Boulders at the bottom, gravels in between and coarse sand at the top so that the silt content that will come with runoff will be deposited on the top of the coarse sand layer and can easily be removed.
For smaller roof area, pit may be filled with broken bricks/ cobbles. A mesh should be provided at the roof so that leaves or any other solid waste / debris is prevented from entering the pit and a desilting /collection chamber may also be provided at the ground to arrest the flow of finer particles to the recharge pit. The top layer of sand should be cleaned periodically to maintain the recharge rate. By-pass arrangement be provided before the collection chamber to reject the first showers.