Orifices (SWMM)

Orifices are used in a SWMM network to model outlet and diversion structures in the network. Orifices can be either a circular or a closed rectangular shape and can be located at the bottom or the side of an upstream node, and may also contain a flap gate to prevent blackflow. If an orifice contains a flap gate, a control rule can also be used to adjust its position.

The flow through an orifice is calculated based on its opening area and discharge coefficient, and the head difference across the orifice. The equations used to determine flow through an orifice are described in the SWMM5 User Guide and the SWMM Reference Manual, Volume II, Hydraulics. Like EPA-SWMM5, under low-head conditions, orifice calculations in ICM are treated similarly to weirs.