A detention cistern collects rainwater and discharges it into the sewer system at a controlled rate. This reduces the burden on drains during periods of peak rainfall. For controlled discharge of the rainwater the detention cistern is fitted with a flow control. This discharges water to the sewer at a rate of 0.5 to 16 litres per second. The water level in the tank rises temporarily during heavy rainfall and falls at the set rate during, and after the rain stops. GRAF detention cisterns are also equipped with an emergency overflow to prevent the tank from overflowing.