Orifices are used to model outlet and diversion structures in drainage systems which are typically openings in the wall of a manhole, storage facility, or control gate. They are internally represented in SWMM as a link connecting two nodes. An orifice can have either a circular or rectangular shape, be located either at the bottom or along the side of the upstream node, and have a flap gate to prevent backflow.
Orifices can be used as storage unit outlets under all types of flow routing. If not attached to a storage unit node, they can only be used in drainage networks that are analyzed with Dynamic Wave flow routing.
The flow through an orifice is computed based on the area of its opening, its discharge coefficient, and the head difference across the orifice.
The height of an orifice's opening can be controlled dynamically through user-defined Control Rules. This feature can be used to model gate openings and closings.
The input parameters for the Orifice are listed in the table below:
| Field | Description |
| Name | User-assigned orifice name. |
| Start Node | Name of node on the inlet side of the orifice. |
| End Node | Name of node on the outlet side of the orifice. |
| Type | Type of orifice (SIDE or BOTTOM). |
| Shape | Orifice shape (CIRCULAR or RECT_CLOSED). |
| Height | Height of orifice opening when fully open (feet or meters).Corresponds to the diameter of a circular orifice or the height of a rectangular orifice. |
| Width | Width of rectangular orifice when fully opened (feet or meters) |
| Inlet Offset | Depth or elevation of bottom of orifice above invert of inlet node (feet or meters). |
| Discharge Coeff. | Discharge coefficient (unitless). A typical value is 0.65. |
| Flap Gate | YES if a flap gate exists which prevents backflow through the orifice, or NO if no flap gate exists. |
| Time to Open/Close | The time to open a closed (or close an open) gated orifice in decimal hours. Use 0 or leave blank if timed openings/closings do not apply. Use Control Rules to adjust gate position. |