Cities that installed their sewage collection systems before the 1930s typically used single piping systems to transport both urban runoff and sewage. This type of collection system is referred to as a combined sewer system (CSS). The cities rationale when combined sewers were built was that it would be cheaper to build just a single system. In these systems a sudden large rainfall that exceeds sewage treatment capacity is allowed to overflow directly from storm drains into receiving waters via structures called combined sewer overflows'.
Storm drains are typically installed at shallower depths than combined sewers. This is because combined sewers were designed to accept sewage flows from buildings with basements, in addition to receiving surface runoff from streets.
About 860 communities in the US have combined sewer systems, serving about 40 million people. New York City, Washington, D.C., Seattle and other cities with combined systems have this problem due to a large influx of storm water after every heavy rain event. Some cities have dealt with this by adding large storage tanks or ponds to hold the water until it can be treated. Chicago has a system of tunnels, collectively called the Deep Tunnel, underneath the city for storing its stormwater. Many areas require detention tanks or roof detention systems that temporarily hold runoff in heavy rains and restrict outlet flow to the public sewer. This lessens the risk of overwhelming the public sewer in heavy rain. An overflow outlet may also connect higher on the outlet side of the detention tank. This overflow prevents the detention tank from completely filling. By restricting the flow of water in this way and temporarily holding the water in a detention vault or tank or by rooftop detention, public sewers are less likely to overflow.