What is weir?

The term "weir" refers to a masonry or concrete construction that is constructed over the channel (such as the river) to alter its characteristics in water flow. Weirs are designed to be an obstruction to the flow of water. They are typically used to determine the flow rate in volumetric units as well as to stop flooding, and allow rivers to be navigable.


A symmetrically-placed rectangular notch in an vertical thin (metallic) plate that is parallel to both sides as well as the the bottom of a straight-channel is referred to as a rectangular sharp weir at the crest.

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Types of Weirs


Weir that is rectangularly suppressed, in which the weir runs throughout the channel such it's length, L is the same as what the size of the channel is.

The discharge of the rectangular line that is suppressed can be calculated in the following manner:

Q = 1.84 B H3/2


Q is the flow rate of water in m3/sec.

The length B of the channel (and the width of the channel) in millimeters,

The head is H above the weir in.


A rectangular weir that is contracted is a kind of weir that the weir only covers one portion of the channel which means that the length of the weir (L) differs in comparison to the length of the channel.

The flow rate of the notch, which is a rectangular shape, can be calculated using the following formula:

Q = 1.84(L - 0.2H)H3/2


Q is the flow rate of water in m3/sec.

It is defined as the distance of the pipe in millimeters and

The head is H that is over the weir in.

B is the length of the channel measured in millimeters B is the width of the channel in m

Hmax is the highest expected head that can be expected over the weir, measured in millimeters.

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The data from calculations of flow rates for a rectangle weir may be utilized in a variety of ways. Flood control and water management practices and policies are usually based on such information. Flow data is used to determine if the hydropower project is viable or economically viable. Data on water flow can be beneficial in environmental impact studies particularly in the determination of how weirs or other structures impact the ecology of a stream or river. The irrigation and other water usage projects can also benefit from this kind of information.


The V-shaped notch can be described as a vertical thin plate which is placed perpendicularly to the shoulders. the V-notch that is located at in the middle of the straight channels can be described as sharp ridge. The line that separates the line angle must be vertical and located at an equal distance between the two sides. It is the V-Notch sharp-crested line is among of the most accurate discharge measuring tools that are suitable for an array of currents. In the literature of the world, V-Notch Sharp-Crested Weir is usually called "Thomson Weir'.

Triangular or V-notch Weirs measure lower flows more accurately that horizontal ones. The V-notch is typically an opening of 90 degrees and the sides are angled 45 degrees to the horizontal. Since the V-notch has no long crest, it is possible that lower flows are represented as the drop height, which is lower than for the rectangular weir. For the triangular or V-notch type of weir, the discharge may be represented in the following manner:

q = 8/15 cd (2 g)1/2 tan(th/2) h5/2

in which

Th = V-notch angle

H=head of weir

Cd= discharge constant for the weir - has to be established

G is 9.81 (m/s2 ) Gravity


The V-Notch Weir system is based on the principle of water gravity discharge on a rectangular or triangular Notched steel plate.

Common applications are Monitoring long-term of dam dams Drainage systems within tunnels and dams * Artesian wells and springs

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