Open channel flow is the flow of a liquid via a conduit having a free surface. Many actual examples exist, both manmade (Weirs, flumes, floodgates, waterways, drainage channels) and natural (rivers, streams, estuaries). The typical dam is an example of a weir, which is a channel obstacle over which the flow must divert. Weirs are also used to gauge flow and control artificial channels.
A sharp crested notch is a vertical flat plate with a sharp edge which is positioned across the channel such that the fluid must flow past the sharp edge and subsequently into the pool downstream of the weir or notch plate as is shown in Fig. The kind of notch is determined by the form of flow region in the plane of the notch plate. A rectangle and a V notch, as shown in fig below, are the two most common shapes. All should be thoroughly ventilated and not drowned out.
The difference between notch and weir is that, weir is a concrete structure that is placed in an open channel, over which liquid flow occurs, while notch is a metal plate which is inserted at the end of a narrow channel. Occasionally notches are smaller in size in comparison to weir. The top edge of a weir or the bottom edge of a notch is called a crest or sill, over which the water flows. A rectangular notch is called a suppressed notch if its crest length is equal to width of the channel. However, if the width is shorter than the channel width, the effect of end-contraction is considered.