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A weir /wɪər/ or low head dam is a barrier across the width of a river that alters the flow characteristics of water and usually results in a change in the height of the river level. They are also used to control the flow of water for outlets of lakes, ponds, and reservoirs. There are many weir designs, but commonly water flows freely over the top of the weir crest before cascading down to a lower level.

## Etymology

There is no single definition as to what constitutes a weir and one 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".[1][2]

## Function

A broadcrest weir at the Thorp grist mill in Thorp, Washington, USA

Commonly, weirs are used to prevent flooding, measure water discharge, and help render rivers more navigable by boat. In some locations, the terms dam and weir are synonymous, but normally there is a clear distinction made between the structures. Usually, a dam is designed specifically 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 many metres tall and hundreds of metres long. Some common weir purposes are outlined below.

### Flow measurement

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 or 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:

${\displaystyle Q=CLH^{n}}$

Where:

• Q is the volumetric flow rate of fluid (the discharge)
• C is the flow coefficient for the structure (on average a figure of 0.62).
• L is the width of the crest
• H is the height of head of water over the crest
• n varies with structure (e.g., 3/2 for horizontal weir, 5/2 for v-notch weir)

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.[3][4]

### Flow over a V-notch weir

The flow over a V-notch weir (in ft3/s) is given by the Kindsvater-Shen equation.[5]

${\displaystyle Q={\frac {8}{15}}{\sqrt {2g}}C_{e}\tan {\frac {\theta }{2}}(h+k)^{\frac {5}{2}}}$

Where:

• Q is the volumetric flow rate of fluid in ft3/s
• g is the acceleration due to gravity in ft/s2
• Ce is the flow correction factor given in Shen 1981, p. B29, Fig 12
• θ is the angle of the V-notch weir
• h is the height of the fluid above the bottom of the V-notch
• k is the head correction factor given in Shen 1981, p. B20, Fig 4

### Control of invasive species

As weirs are a physical barrier, they can impede the longitudinal movement of fish and other animals up and down a river. This can have a negative effect on fish species that migrate as part of their breeding cycle (e.g., salmonids), but it also can be useful as a method of preventing invasive species moving upstream. For example, weirs in the There is no single definition as to what constitutes a weir and one 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".[1][2]

## Function

A broadcrest weir at the Thorp grist mill in Thorp, Washington, USA

Commonly, weirs are used to prevent flooding, measure water discharge, and help render rivers more navigable by boat. In some locations, the terms dam and weir are synonymous, but normally there is a clear distinction made between the structures. Usually, a dam is designed specifically 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 many metres tall and hundreds of metres long. Some common weir purposes are outlined below.

### Flow measurement

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 or 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: