How Orifices and Weirs  Work in SWMM 5.   The inputs to the orifice flow  calculator are

Step 1

If the head in the orifice or the rules for  the  orifice cause the orifice to  be closed then the flow is zero.

Step 2

If the inlet depth to the  orifice is  less then the  critical depth then the orifice behaves as  a weir

If f  is less than 1 then  the  orifice  flow = cWeir * f^1.5