A surcharge pit is one that is intended to overflow. It may have an outlet pipe that is smaller in diameter than its inlet pipe(s), or it may have no outlet pipe at all. The latter situation occurs in localised pipe systems that are intended to carry flows under roads and then to surcharge into a gutter. They are called 'converters' in New South Wales, 'under and overs' in Queensland and 'bubble up' pits in Victoria.
The first case is modeled in DRAINS in a straightforward way, and there are no special requirements. The pit overflow can be directed to another pit or node.
Where there is no outlet pipe from a surcharge pit, it can be modelled as a simple node, with its overflow being directed to a downstream destination.