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