Polypipe Polystorm Mid Range Water Storage Unit 1m x 500mm x 400mm is used to retain large volumes of water to create an underground water tank. It receives rainwater collected from the roof gutter system or surface drains. The tank can be modified to either release the water within set discharge limits or slowly release the water back into the surrounding soil. The unit has a 61 tonne strength limit so can be used in high traffic areas.