People & Places

Why don't sewers overflow every time it rains?

Cities use separate drains, storage spaces and overflow plans to handle rain before pipes are overwhelmed. In many places, rainwater goes into storm drains while toilet and sink water goes into separate sewer pipes. Where older systems combine them, engineers add large pipes, tanks, tunnels or basins that temporarily store extra water during storms. Grates, slopes and channels also move rain away from streets.