At unsealed drains, both liquid and air can enter the sewer system. Unlike drains, flow enters the sewer at the level of existing flow. A point at which any two or more pipes meet. The liquid must then fall some distance before flowing further. Manholes are points of access to the sewer system for maintenance reasons.
Air can enter or leave the system through the pickholes.
A new reach exists wherever a manhole, drain, drop, or change in direction occurs.