Along a sewer pipeline, there are hydraulic structures and control devices, such as manholes, weirs, and orifices where the flow is often rapidly varied rather then gradually varied in space. The Saint-Venant equations are not applicable at these locations since the gradually varied flow assumption in the Saint-Venant equations derivation is no longer valid. Instead these locations are treated as hydraulic internal boundaries; usually alternative empirical internal boundary equations are used for these internal local computational reaches (a computational reach is a link between two computational sections).

Typical internal boundaries are: