Orifice Outlet allows an Orifice plate control to be modelled.

The Orifice outlet uses Torricelli's equation to calculate the flow when the orifice is surcharged. When unsubmerged, the flow through the orifice is calculated using a circular shaped weir formula.


Diameter - Enter the diameter of the orifice. If you are working to a maximum allowable discharge a Calculator is provided to help size the control.

This gives two additional parameters:

  • Design Depth - Enter the maximum depth of water that you are designing to. This should be entered as a depth above the invert level of the control and will vary depending on the depth of the stormwater control and the desired freeboard.
  • Design Flow - Enter the required flow when the water level reaches the design depth. This should be the maximum desired discharge through this control. Be aware that if the water level is lower than the design depth, the flow may be lower than the design flow and if the water level exceeds the design depth the design flow may be exceeded.

Coefficient of Discharge - Enter the discharge coefficient of the orifice.

A coefficient of 0.6 may be used for square edged inlets (see 1). The coefficient may be increased to 0.95 if the inlet is well rounded using a rounding radius 15% of the orifice diameter (see 2). It should be decreased to 0.5 if the orifice is formed by a pipe protruding into the flow.

Invert Level - The invert level of the orifice.


When orifice is submerged the flow rate is calculated using Torricelli's equation:


Ao = Area of the orifice opening 

He= The effective head seen by the orifice.

Cd = coefficient of discharge. A default value of 0.6 is suitable for sharp edged orifices but can be changed.

Weir behaviour occurs when the inlet water level is below the top of the orifice opening:


Cd = Discharge coefficient

b = equivalent weir width

h = Depth of water above the invert level of the orifice.


More details can be found in the SWMM Reference Manual Volume II - Hydraulics which is available Here.