What is a V-notch weir

Weir - Węgorzewo

A weir on the Humber River near Raymore Park in Toronto, Ontario, Canada

A. Weir / wɪər / or dam installed is a barrier across the width of a river that changes the flow properties of the 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 from lakes, ponds, and reservoirs. There are many weir structures, but water usually flows freely over the top of the weir ridge before dropping to a lower level.


There is no standard definition of what constitutes a weir, and an English dictionary defines a weir simply as a small dam, probably derived from Middle English, Old English wer, deriving from the root of werian, which means "to defend", dam " .


Usually weirs are used to prevent flooding

A common difference between dams and weirs is that water flows at least part of its length over the top (the ridge) of a weir or below it. Accordingly, the apex of an overflow on a large dam can be called a weir. Weirs can vary in size both horizontally and vertically, with the smallest being only a few inches high, while the largest can be many meters high and hundreds of meters long. Some common defense purposes are listed below.

Flow measurement

Weirs provide hydrologists and engineers with a simple method of measuring volume flow in small to medium-sized streams / rivers or in industrial unloading points. Since the geometry of the top of the weir is known and all the water flows over the weir, the water depth behind the weir can be converted into a flow rate. However, this can only be achieved in places where all the water flows over the top of the weir ridge (as opposed to the sides or through pipes or sluices) and in places where the water that flows over the ridge comes from the structure is discharged. Failure to meet these conditions can make flow measurement complicated, inaccurate, or even impossible.

The runoff calculation can be summarized as follows:

In which:

However, this calculation is a generic relationship and specific calculations are available for the many different types of weirs. Flow gauges need to be well maintained to keep them accurate.

Flow through a V-notch weir

The flow over a V-notch (in ft / s) results from the Kindsvater-Shen equation.

In which:

Control of invasive species

Since weirs are a physical barrier, they can impede the longitudinal movement of fish and other animals up and down a river. This can negatively impact fish species that migrate as part of their breeding cycle (e.g. salmonids), but can also be useful in preventing invasive species from moving upstream. For example, weirs in the Great Lakes region have helped prevent invasive lamprey from settling further upstream.


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

Flood protection and changed river conditions

A lock gate-based weir at Bray Lock on the Thames, downstream in the background, are smaller secondary "overflow" weirs. Two small boats are also visible, which are held against the overflow weir and were washed against it during a particularly high runoff due to meltwater from the winter cold wave 2018.

Weirs are commonly used to control the flow rates of rivers during periods of high drainage. Sluice gates (or in some cases the height of the weir ridge) can be modified to increase or decrease the volume of water flowing downstream. Weirs for this purpose are usually located in front of towns and villages and can either be automated or operated manually. By slowing down, even slightly, the rate at which water moves downstream, it can have a disproportionate effect on the likelihood of flooding. In larger rivers, a weir can also alter the flow characteristics of the waterway so that ships can navigate areas that were previously inaccessible due to extreme currents or eddies. Many larger weirs have design features that allow boats and river users to "shoot" the weir and navigate by going up or down the river without leaving the river. Weirs that were built for this purpose are particularly common on the Thames. Most are located near the river's 45 locks. .


In times of high river flow in the 19th century A porphyry stone weir on a stream in the Alps would allow significantly more water to flow over it.


Since a weir accumulates water behind it and changes the flow regime of the river, this can have an impact on the local ecology. Typically, the reduced flow velocity upstream can lead to increased siltation (deposition of fine particles of silt and clay on the river floor), which reduces water oxygen levels and suffocates the invertebrate habitat and the spawning grounds of the fish. Oxygen levels usually return to normal once water has passed over the weir (although it may be oxygenated), although increased flow rates can scrub the riverbed and cause erosion and habitat loss.

Fish migration

Weirs can have a significant impact on fish migration. Any weir that either exceeds the maximum height a species can jump or creates flow conditions that cannot be bypassed (e.g. due to excessive water speed) effectively limits the maximum upstream point at which fish can migrate . In some cases, this can mean that large amounts of breeding habitat are lost and over time this can have a significant impact on fish populations.

In many countries today, it is a legal requirement to incorporate fish ladders into the design of a weir to ensure that fish bypass the barriers and gain access to upstream habitats. Unlike dams, weirs usually do not prevent downstream fish migration (as water flows over the top and allows fish to bypass structure in that water), although they can create flow conditions that injure juvenile fish. Recent studies suggest that navigation barriers can also provide improved access to a range of biota, including poor swimmers.


While the water around weirs can be frequent, they appear relatively calm and can be extremely dangerous places to go boating, swimming, or wading, as the circulation patterns on the downstream side - usually known as a hydraulic jump - can keep a person underwater indefinitely . This phenomenon is so familiar to canoeists, kayakers, and others who spend time on rivers that they even have a rueful name for weirs: "drowning machines". In this situation, the Ohio DNR recommends that a victim "straighten their chin down, knees to their chest with arms folded. Hopefully the conditions are such that the flow pushes the victim along the bed of the river until he is The Pennsylvania State Police also recommends that victims "curl up, dive to the bottom, and swim or crawl downstream." Since the hydraulic jump takes air with it, the buoyancy of the water between the dam and the boiling line is reduced by up to 30%. If a victim cannot levitate, escaping along the dam floor may be the only way to survive.

Common types

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

Broad comb

A broad ridge weir is a flat structure in which the water flows over a ridge that covers most or the entire width of the canal. This is one of the most common types of weirs in the world.


A composite weir is any weir that includes several different designs in one structure. They often appear in places where a river has multiple users who may need to bypass the structure. A common design would be one in which a weir is broad-crested for much of its length but has a section where the weir stops or is "open" for small boats and fish to traverse the structure.


A notch weir is one where the physical barrier is significantly higher than the water level, with the exception of a certain notch (often V-shaped) in the panel. With normal flow, all of the water must pass the notch, which simplifies the calculation of the flow volume. During floods, the water level can rise and leave the weir submerged without any changes to the structure.


A polynomial weir is a weir whose geometry is defined by a polynomial equation of any order n. In practice, most of the weirs are low order polynomial weirs. The standard rectangular weir is, for example, a polynomial weir of order zero. The triangular (V-notch) and trapezoidal weirs are of the first order. Higher order polynomial weirs offer a wider range of head-discharge relationships and thus better control of the flow at the outlets of lakes, ponds, and reservoirs.

See also



Works cited

further reading

External links

Wikimedia Commons contains media too Weirs .
Look up Weir in Wiktionary, the free dictionary.