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A weir 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 A dam is a barrier that stops or restricts the flow of surface water An example of surface water is Lake Kinney. Surface water is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tast ...

, likely originating from Middle English ''were'', Old English ''wer'', derivative of root of ''werian,'' meaning "to defend, dam".

# Function

Commonly, weirs are used to prevent
flooding A flood is an overflow of water that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, co ...

, measure water discharge, and help render rivers more
navigable A body of water, such as a river, canal or lake, is navigable if it is deep, wide and calm enough for a water vessel (e.g. boats) to pass safely. Such a navigable water is called a ''waterway'', and is preferably with few obstructions against dire ...
by boat. In some locations, the terms
dam A dam is a barrier that stops or restricts the flow of surface water An example of surface water is Lake Kinney. Surface water is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tast ...

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 dam in Wales Wales ( cy, Cymru ) is a country that is Countries of the United Kingdom, part of the United Kingdom. It is bordered by England to the Wales–England border, east, the Irish Sea to the north and west, and the Bristol Channel to ...

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 Hydrology (from Greek: ὕδωρ, "hýdōr" meaning "water" and wikt:λόγος, λόγος, "lógos" meaning "study") is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water ...
and engineers a simple method of measuring the
volumetric flow rate In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...
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: :$Q = C L H^n$ Where: * ''Q'' is the
volumetric flow rate In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...
of fluid (the discharge) * ''C'' is the
flow coefficientThe flow coefficient of a device is a relative measure of its efficiency at allowing fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are ...
for the structure (on average a figure of 0.62). * ''L'' is the width of the
crest Crest or CREST may refer to: Buildings *The Crest (Huntington, New York) The Crest is a historic house on Eatons Neck in Suffolk County, New York. Although on the land mass of Eatons Neck, the house today is within the jurisdiction of the Incor ...
* ''H'' is the height of
head Head Sport GmbH is an American-Austrian headquartered in . It owns the American tennis racket brand Head. Head GmbH is a group that includes several previously independent companies, including the original "Head Ski Company" (founded in the in ...

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.

## Flow over a V-notch weir

The flow over a V-notch weir (in ft3/s) is given by the Kindsvater-Shen equation. :$Q =\frac \sqrt C_e \tan\frac \left(h + k\right)^\frac$ Where: * ''Q'' is the
volumetric flow rate In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...
of fluid in ft3/s * ''g'' is the acceleration due to gravity in ft/s2 * ''Ce'' is the flow correction factor given in * ''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

## 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 Salmonidae is a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to maintain the we ...
), but it also can be useful as a method of preventing
invasive species Kudzu, a Japanese vine species invasive in the southeast United States, growing in Atlanta, Georgia (U.S. state), Georgia An invasive species is an introduced species, introduced organism that becomes overpopulated and negatively alters its n ...
moving upstream. For example, weirs in the
Great Lakes The Great Lakes also called the Great Lakes of North America or the Laurentian Great Lakes, is a series of large interconnected freshwater lake A lake is an area filled with water, localized in a basin, surrounded by land Land ...

region have helped to prevent invasive
sea lamprey The sea lamprey (''Petromyzon marinus'') is a parasitic lamprey native to the Northern Hemisphere. It is sometimes referred to as the "vampire fish." Description The sea lamprey has an eel-like body without paired fins. Its mouth is jawless, ro ...

from colonising farther upstream.

## Watermills

Mill ponds are created by a weir that impounds water that then flows over the structure. The energy created by the change in height of the water can then be used to power waterwheels and power sawmills, grinding wheels, and other equipment.

## Flood control and altering river conditions

Weirs are commonly used to control the flow rates of rivers during periods of high discharge.
Sluice gates A sluice ( ) is a water channel controlled at its head by a gate. A mill race, Leat, leet, flume, penstock or Mill race, lade is a sluice channelling water toward a water mill. The terms sluice, sluice gate, knife gate, and slide gate are used in ...

(or in some cases the height of the weir crest) can be altered to increase or decrease the volume of water flowing downstream. Weirs for this purpose are commonly found upstream of towns and villages and can either be automated or manually operated. By slowing the rate at which water moves downstream even slightly, a disproportionate effect can be had on the likelihood of flooding. On larger rivers, a weir can also alter the flow characteristics of the waterway to the point that vessels are able to navigate areas previously inaccessible due to extreme
currents Currents or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stream), c ...
or
eddies In fluid dynamics, an eddy is the swirling of a fluid and the reverse current (water), current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the o ...
. Many larger weirs will have construction features that allow boats and river users to "shoot the weir" and navigate by passing up or down stream without having to exit the river. Weirs constructed for this purpose are especially common on the
River Thames The River Thames ( ), known alternatively in parts as the River Isis, is a river that flows through southern England Southern England, or the South of England, also known as the South, is an area of England consisting of the southernmos ...
, and most are situated near each of the river's 45
locks Lock may refer to: *Lock and key, a mechanical device used to secure items of importance. *Lock (water navigation), a device for boats to transit between different levels of water Arts, entertainment, and media *Lock (film), ''Lock'' (film), a ...
.

# Issues

## Ecology

Because a weir impounds water behind it and alters the flow regime of the river, it can have an effect on the local
ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms In biol ...
. Typically, the reduced river velocity upstream can lead to increased
siltation Siltation, is water pollution Water pollution (or aquatic pollution) is the contamination of water bodies ( Lysefjord) in Norway Norway ( nb, ; nn, ; se, Norga; smj, Vuodna; sma, Nöörje), officially the Kingdom of Norway, is ...
(deposition of fine particles of
silt Silt is granular material A granular material is a conglomeration of discrete solid, macroscopic scale, macroscopic particles characterized by a loss of energy whenever the particles interact (the most common example would be friction when gra ...
and clay on the river bottom) that reduces the water oxygen content and smothers invertebrate habitat and fish
spawning Spawn is the eggs and sperm Sperm is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, "female" reproductive cell and a smaller, "male" one). Animals produce ...
sites. The oxygen content typically returns to normal once water has passed over the weir crest (although it can be hyper-oxygenated), although increased river velocity can scour the river bed causing erosion and habitat loss.

## Fish migration

Weirs can have a significant effect on
fish migration Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres. Fish Fish are , , -bearing animals that lack with . Include ...
. Any weir that exceeds either the maximum height a species can jump or creates flow conditions that cannot be bypassed (e.g., due to excessive water velocity) effectively limits the maximum point upstream that fish can migrate. In some cases this can mean that huge lengths of breeding habitat are lost, and over time this can have a significant impact on fish populations. In many countries, it is now a legal requirement to build
fish ladder A fish ladder, also known as a fishway, fish pass, fish steps, or fish cannon is a structure on or around artificial and natural barriers (such as dams, Lock (water transport), locks and waterfalls) to facilitate Fish migration#Classification, di ...

s into the design of a weir that ensure that fish can bypass the barriers and access upstream habitats. Unlike dams, weirs do not usually prevent downstream fish migration (as water flows over the top and allows fish to bypass the structure in that water), although they can create flow conditions that injure juvenile fish. Recent studies suggest that navigation locks have also potential to provide increased access for a range of biota, including poor swimmers.

## Safety

Even though the water around weirs can often appear relatively calm, they can be extremely dangerous places to boat, swim, or wade, as the circulation patterns on the downstream side—typically called a
hydraulic jump in Wales. A hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occ ...
—can submerge a person indefinitely. This phenomenon is so well known to canoeists, kayakers, and others who spend time on rivers that they even have a rueful name for weirs: "drowning machines". The Ohio DNR recommends that a victim should "tuck the chin down, draw the knees up to the chest with arms wrapped around them. Hopefully, conditions will be such that the current will push the victim along the bed of the river until swept beyond the boil line and released by the hydraulic." The Pennsylvania State Police also recommends to victims, "curl up, dive to the bottom, and swim or crawl downstream". Escaping a low-head dam As the hydraulic jump entrains air, the buoyancy of the water between the dam and boil line will be reduced by upward of 30%, and if a victim is unable to float, escape at the base of the dam may be the only option for survival.

# Common types

There are many different types of weirs and they can vary from a simple stone structure that is barely noticeable, to elaborate and very large structures that require extensive management and maintenance.

A broad-crested weir is a flat-crested structure, where the water passes over a crest that covers much or all of the channel width. This is one of the most common types of weir found worldwide.

## Compound

A compound weir is any weir that comprises several different designs into one structure. They are commonly seen in locations where a river has multiple users who may need to bypass the structure. A common design would be one where a weir is broad-crested for much of its length, but has a section where the weir stops or is 'open' so that small boats and fish can traverse the structure.

## V-notch

A notch weir is any weir where the physical barrier is significantly higher than the water level except for a specific notch (often V-shaped) cut into the panel. At times of normal flow all the water must pass through the notch, simplifying flow volume calculations, and at times of flood the water level can rise and submerge the weir without any alterations made to the structure.

## Polynomial

A polynomial weir is a weir that has a geometry defined by a
polynomial equation In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It h ...
of any order ''n''. In practice, most weirs are low-order polynomial weirs. The standard rectangular weir is, for example, a polynomial weir of order zero. The triangular (V-notch) and
trapezoid In Euclidean geometry Euclidean geometry is a mathematical system attributed to Alexandrian Greek mathematics , Greek mathematician Euclid, which he described in his textbook on geometry: the ''Euclid's Elements, Elements''. Euclid's meth ...

al weirs are of order one. High-order polynomial weirs are providing wider range of Head-Discharge relationships, and hence better control of the flow at outlets of lakes, ponds, and reservoirs.

*
Crump weir Edwin Samuel Crump Order of the Indian Empire, CIE (born 6 July 1882, died 5 March 1961) was an English civil engineer specialising in hydraulics. Early life and education Crump was born in Wolverhampton, Staffordshire, England, the youngest ch ...
*
Fishing weir A fishing weir, fish weir, fishgarth or kiddle is an obstruction placed in tidal waters, or wholly or partially across a river, to direct the passage of, or trap fish. A weir may be used to trap marine fish in the intertidal zone The interti ...

*
Drop structure of Leasburg Diversion Dam, part of the Rio Grande Project, is an example of a vertical hard basin, the most basic form of a drop structure, designed to dissipate energy. A drop structure, also known as a grade control, Aquatic sill, sill, or weir, ...
* Fixed-crest dam *
International Control Dam The International Control Dam, also known as the International Control Structure, operated by Ontario Power Generation, is a weir that controls the water diversions from the Niagara River The Niagara River ( ; french: rivière Niagara) is a ri ...

* * * * * *