Orifices

Orifices are usually circular or rectangular openings in the wall of a tank or in a plate normal to the axis of the conduit. Orifices can be oriented in a variety of ways, such as side outlet or bottom outlet. Drainage and Utilities can also treat an orifice as one of the controlling elements in a detention pond composite control structure; other controlling elements within a composite control structure include weirs, risers and culverts.

Orifices are treated the same as weirs to be internal boundaries except that the flow equation of an orifice is used to calculate the discharge. There are different flow conditions in an orifice and the calculation of the discharge through the orifice is different:

The discharge through the orifice is proportional to the 0.5 exponent of the head if the orifice is fully submerged.
A weir equation is usually used for unsubmerged conditions of the orifice.
Special treatment is necessary for a smooth transition between unsubmerged and submerged conditions due to the calculating equation switch.

The orifice discharge coefficients typically range between 0.6 and 0.7 (English units). Without knowing the orifice specifics, a default value of 0.65 is commonly used.