A notch may be defined as an opening in one side of a tank or a reservoir, like a large orifice, with the upstream liquid level below the top edge of the opening.
Since the top edge of the notch above the liquid level serves no purpose, therefore a notch may have only the bottom edge and sides.
The bottom edge, over which the liquid flows, is known as sill or crest of the notch and the sheet of liquid flowing over a notch (or a weir) is known as nappe or vein. A notch is, usually made of a metallic plate and is used to measure the discharge of liquids.
Types Of Notches
There are many types of notches, depending upon their shapes. But the following are important from the subject point of view.