A weir is an opening in the side walls of a tank. It is same as an orifice without having an outer boundary. If the head is reduced the liquid flows with its level below the top of the orifice. The wall above the liquid level is superfluous and can be removal.

The difference between a large orifice and weir is that liquid flows through the orifice while it flows over the weir. The flow of liquid coming out of orifice is called jet while that comes through the weir is called 'nape, sheet or vein'.

There is no difference between a notch and a weir, except that notch is a small structure and has a sharp edges. Weir, on the other hand, is generally is an over flow structure. With broad crested, built across an open channel.

It is built across a river in order to raise water on the upstream and to allow excess water to flow over its entire length to the downstream side. Weirs are used for measuring the rate of flow of water in rivers or stream.

The relation between H and h fo rmaximum discharge is, h =(2/3)H

theretical discharge, Q_t = (1.705)LH^2/2 in m3/sec

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