To develop a continuous discharge rating relation for an orifice structure, it is necessary to handle flow situations in which the orifice opening is not fully submerged. For area-based orifice calculations, Drainage and Utilities performs a straightline interpolation, setting the flow by multiplying the full flow, Q_t, (at unsubmerged head, H_t) by the ratio of actual H to H_t.

Heads are measured from the opening invert or from the controlling tailwater, whichever is greater.