Low head dam

A weir /ˈwɪər/ is a barrier across the horizontal width of the river that alters the flow characteristics of the water and usually results in a change in the vertical height of the river level. There are many designs of weir, but commonly water flows freely over the top of the weir crest before cascading down to a lower level.

There is no set definition as to what constitutes a weir and the English Dictionary simply defines a weir as a small dam, likely originating from Middle English were, Old English wer, derivative of root of werian, meaning "to defend, dam".

Weirs are commonly used to prevent flooding, measure discharge and help render rivers navigable. In some locations the terms dam and weir are synonymous, but normally there is a clear distinction made between the structures. A dam is usually specifically designed to impound water behind a wall, whilst a weir is designed to alter the river flow characteristics. A common distinction between dams and weirs is that water flows over the top (crest) of a weir or underneath it for at least some of its length. Accordingly the crest of an overflow spillway on a large dam may therefore be referred to as a weir. Weirs can vary in size both horizontally and vertically, with the smallest being only a few inches in height whilst the largest may be hundreds of metres long and many metres tall. Some common weir purposes are outlined below.

Weirs allow hydrologists and engineers a simple method of measuring the volumetric flow rate in small to medium-sized streams/rivers or in industrial discharge locations. Since the geometry of the top of the weir is known and all water flows over the weir, the depth of water behind the weir can be converted to a rate of flow. However, this can only be achieved in locations where all water flows over the top of the weir crest (as opposed to around the sides or through conduits/sluices) and at locations where the water that flows over the crest is carried away from the structure. If these conditions are not met it can make flow measurement complicated, inaccurate or even impossible.

The discharge calculation can be summarised as:

Where

However this calculation is a generic relationship and specific calculations are available for the many different types of weir. Flow measurement weirs must be well maintained if they are to remain accurate.

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Wikipedia