This might be specific to very old cities, but New York City has a combined sewage and storm water system. Sewage ("black water") and rainwater from the street

This might be specific to very old cities, but New York City has a combined sewage and storm water system. Sewage ("black water") and rainwater from the streets goes through the same output processing. When it rains a lot, the system can't cope, and the water (sewage and all) is discharged into the Atlantic.