Machines Theory

 Understanding notches and weirs

In fluid mechanics, notch and weir is an important topic that cannot be overlooked. These devices are used to regulate the flow of fluid of different sizes and different methods. Notches and weirs are classified based on some considerations which result in different types.

Today you’ll get to know the definition, applications, function, components, diagram, classifications, types, and working principles of notch and weir. You’ll also get to know the difference between this notch and weir.

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Notch and weir


What are notch and weir?

A notch is an opening provided in the side of a tank in a way that the opening extends even above the free surface of the liquid in the tank. It is designed to measure the flow of water from a tank. On the other hand, a weir is also a notch but it is made on a large scale. Weirs are notches cut in a dam to discharge the surplus quantity of water. The primary difference between a notch and a weir is their quantity or scale at which they measure liquid.

A notch is like a large orifice with no upper edge. The water flows over the notch or weir while water passes through an orifice. Discharging of water from an orifice is called a jet, but is known as nappe or vein when the sheet of water is discharged by a notch or weir. The upper surface of notches or weirs through which the water flows are known as Crest or Sill.  So now let get to understand these two-term Crest and Nappe.

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Nappe or Vein is the layer of fluid flowing above the weir or below the notch. Crest or Sill is the bottom edge of a notch or the top edge of weir.

Applications of notch and weir

In fluid mechanics, notch and weir are application that is often considered because they help in the measurement of fluid. There is no difference in the function of a notch and a weir only that the level and applications of their liquid measurement are different. The applications of notch include tanks, reservoirs, or any water storage devices that have a passage for water escape. Similarly, a weir is a notch on a large scale used to measure flows of rivers and canals.

Apart from measuring, notch and weir are also used to regulate the discharge of small and large channels. The method of determining the theoretical flow through a notch is the same as that adopted for a large orifice.

Note: the primary function of a notch and weir is to regulate and measure the flow of water.

Diagram of notch and weir:

diagram of notch and weir

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Classifications of notch and weir

Below are the classifications of notch and weir in their various applications.

Notches can classify according to the shape of the opening and according to the effects of the sides on Nappe. These brought about the various types of notches.

According to the shape of the opening:

  • stepped notch
  • Trapezoidal notch
  • Triangular notch
  • Rectangular notch

According to the effects of the sides on nappe:

  • Notch without end contraction or suppressed notch.
  • Notch with end contraction.

On our other hand, weirs can be classified according to the followings:

According to the basis of the shape of opening:

  • Trapezoidal weir (Cipolletti weir)
  • Triangular weir
  • Rectangular weir.

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According to the shape of the crest:

  • Ogee shaped weir
  • Broad crested weir
  • Narrow crested weir
  • Sharp crested weir

According to the effect of sides on nappe:

  • Weirs without end contraction
  • Weirs with end contraction.

Types of notch and weir

Notch types:

Below are the various types of notch:

  • Rectangular notch
  • Triangular notch or V-notch
  • Trapezoidal notch

Weir types:

Below are the various types of weir:

Proportional weir:

The proportional types of weirs are designed that the discharge over the weir is proportional to the head of water over the crest. The crest of the weir is horizontal.

Cipolletti weir:

These types of weirs are known for rectangular weir with two end contractions.

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Submerged weir:

In a submerged weir, the water level on the downstream side is also above the crest of the weir.

Anicut or raised weir:

This is a masonry dam provided across a river to raise the water level on the upstream side to a sufficient extent. This is often used in the dry season, so that water can be carried by gravitation to places where it would barely reach.

Broad crested weir:

The broad crested weir types have very broad sill which allows the flow of water over the sill. This flow can be compared to the flow of water in a channel.

Ogee weir:

In ogee weir, the solid boundary of the weir exactly coincides with the bottom surface of the nappe of the sharp-crested weir under the designed head.

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Separating weir:

These types of weirs are arrangements provided in the case of town water supply, where it is necessary to divert the discolored floodwater from the supply channel. At moderate discharge, the water drops over the lip C into a culvert D which communicates with the supply channel. However, during floods, the velocity of flow will be greater because of the greater depth. This causes the water to leap across the opening into the water channel.

Difference between notch and weir in tabular form:


A notch may be defined as an opening provided in the side of the tank or vessel such that the liquid surface in the tank is below the top edge of the opening.A weir may be defined as any regular obstruction in an open stream over which the flow takes place.


Small structureLarge structure
Made of metallic plates.Made of concrete/bricks.
Measure small flow rate.Measure large flow rate.

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Working principles of notch and weir

The working of notch and weir are less complex and can be easily understood. Notches are small in size and they are usually made of metal plate whereas, weirs are of large size and are made of masonry or concrete. The video below gives you an in-depth understanding of how notches and weirs work.

Watch video:


Notches and weirs are devices used to regulate the flow of fluid of different sizes and different methods. That is all for the article, where we discussed the definition, applications, function, components, diagram, classifications, types, and working of notch and weir. We also explained the difference between this notch and weir.

I hope you get a lot from this post, if so, share it with other students. Thanks for reading, see you next time!

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