Weir

A weir /ˈwɪər/ is a barrier across a river designed to alter the flow characteristics. In most cases weirs take the form of a horizontal barrier across the width of a river that pools water behind it while still allowing it to flow steadily over the top. Weirs are commonly used to prevent flooding, measure discharge and help render rivers navigable. In some places the crest of an overflow spillway on a large dam may also be called a weir.

There is not a set definition as to what constitutes a "weir" and they are normally defined by their purpose. 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. Unlike a dam, which specifically impounds water, a weir's purpose can be less obvious. They most commonly exist to alter or control river flow characteristics. A particular distinction between dams and weirs is that water flows over the top (crest) of a weir at least some of its length. Common weir types and purposes are outlined below.

Weirs allow hydrologists and engineers a simple method of measuring the volumetric flow rate in small to medium-sized streams 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. The calculation relies on the fact that fluid passes through the critical depth of the flow regime in the vicinity of the crest of the weir. If water is not carried away from the weir, it can make flow measurement complicated or even impossible.

The discharge can be summarised as:

Where

When used for flow measurement, the weir crest must be free of rust or nicks. Roughness of any form makes the weir discharge more water than indicated by standard discharge equations or tables. The nappe must flow clearly over the top of the crest and not not cling to the vertical face of the weir as this can lead to discharge errors of as much as 25%.

As weirs are a physical barrier they can impede the longitudinal movement of fish up and down a river. This can have a negative effect of fish species that migrate as part of their breeding cycle (e.g., Salmonids), but can also be useful as a method of preventing invasive species moving upstream. Although in most cases not the weirs primary purpose, weirs in the Great Lakes region have helped to prevent invasive Sea lamprey from colonising further upstream. Weirs in this context may also sometimes be referred to as 'low-head dams'.

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