An attenuation tank is an underground storage structure that holds rainwater run-off temporarily and releases it at a restricted rate through a flow control device. It is used where run-off cannot simply infiltrate and where the receiving sewer or watercourse cannot accept the full peak flow from a development.
Development increases the amount of hard surface on a site, so rainfall that once soaked into the ground runs off quickly instead. Attenuation compensates by storing that additional volume and discharging it slowly, so the peak flow leaving the site is no greater than before development — often limited to a greenfield run-off rate specified by the local authority.
Tanks are typically built from modular plastic crates or large diameter pipes, with a hydrobrake or orifice plate controlling the outflow. Maintenance access and silt management are important, because a blocked flow control defeats the design.