In piped stormwater drainage systems pits serve several purposes. They act as inlets for stormwater, points where pipes can conveniently change their size, slope or direction, and inspection and maintenance openings.

An important distinction is between on-grade pits and sag pits.  Pits of the former type are located on a slope, so that any flows that bypass the pit move away and travel on to the next pit. Sag pits are located in a depression or sag, so that water cannot readily escape.  If the capacity of the pit is not sufficient to accept all flows arriving at the pit, stormwater ponds over the pit until it becomes high enough to cross some barrier such as the crown of a road.

Pits can have various shapes and types of inlets; in Australian separate stormwater systems they are usually rectangular boxes constructed of in-situ or precast concrete. The main inlets used are kerb inlets, slots located in the kerb face, or grates. A sump (a bottom of a pit lower than the invert of the outlet pipe) may be provided to catch sediments, but most Australian pits do not provide this.