Q = Cd × A × √(2gh). For free outfall: h = HWE − centroid. For submerged flow (TWE above invert): h = HWE − TWE.
Orifice Regime
Orifice flow equations apply when the headwater surface is above the crown (top) of the opening. When headwater is below the crown, use a weir equation instead.
Discharge Coefficient
Cd ranges from about 0.60–0.65 for sharp-edged orifices. Rounded or re-entrant inlets use different values. See the Engineering Toolbox or the HEC-RAS Hydraulic Reference Manual for guidance.
Submergence
When TWE is above the opening invert, this calculator automatically applies the submerged orifice equation using h = HWE − TWE. When TWE is at or below the invert, free outfall is assumed and h = HWE − centroid.
May we keep one digit in this browser to remember that we have already counted this page? It records nothing about you and nothing you type. Without it we cannot tell your second visit from somebody else's first.