Storm overflows are used during heavy rainstorms to protect properties from flooding and to prevent sewage from overflowing into streets and homes.
They are part of an older type of sewer system called a combined sewer system. These sewer systems carry both surface water (run-off from roof gutters, patios, driveways and some highways) and the foul water from homes and industry together in one pipe. The combined sewage is then transported to a water recycling centre to be treated.
During a storm event, heavy or prolonged rainfall can rapidly increase the flow in a combined sewer and cause it to become overwhelmed. Storm overflows are designed to release excess storm water into rivers or the sea when this happens.