On a typical dry weather day, wastewater from homes, schools and businesses flows through the Metropolitan Sewer District's public sewers without problem.
However, during wet weather, some parts of the sewer system carry large amounts of rainwater in addition to wastewater.
If the combined amount of rainwater and wastewater is more than the pipes can carry, the water stops flowing forward and begins to back up. Excess water may get pushed out of the pipe through manhole lids and through private building sewer pipes (sometimes called a "sewer lateral" or "house tap") that connect buildings to the sewer system.
When this happens, basements and lower levels can get flooded with the sewage/rainwater mixture.
Click to learn more about building sewers.