Orifices in open channels are usually circular or rectangular opening in vertically bulkhead through water flows. The edge of opening is sharp and often constructed of metal.
Free flow orifices:
It can be used to measure comparatively small stream like the flow into border strips, or check basin. They consist of sheet of iron, steel, or aluminum plate that contains accurately machined circular opening ranging from 2.5 cm to 7.5 cm. A plastic scale may be fixed directly on upstream flow of orifices with zero coinciding with the center of orifice.
Submerged orifices may be divided in to two types:
1) Those having orifices of fixed dimension.
2) Those in which the height of opening may be varied.
The discharge through standard submerged orifices may be obtained using above equation.