HD Modular Geocellular structures are a form of attenuation and infiltration system designed to store excess water in circumstances where heavy rainfall or snowmelt runoff water, means that the volume of water flowing usually in gravity from a site is too high to go immediately into off-site/network drainage. Water that is ‘rejected’ from the outfall flows back up the system and into the geocellular structure, where it is stored until the allowable discharge rate from site permits the stormwater to join the off-site network or water body