Notches and Weirs | Definition, Types, Differences, Applications

In this article we will have discussion on Notches and Weirs. We will first go through the definition of each and then we will try to understand different types of both. At the end of our discussion we will understand the differences between both of them.

1. What is a Notch?

A notch is a kind of opening (throughout the width) to the fluid flow, in which fluid flows through the bottom of notch. It is used to regulate the flow of fluid.

The bottom end of the notch is generally made sharp so that it offers minimum resistance to the fluid flow (generally water).

A notch is generally made up of a metal plate and is used to regulate fluid flow at a small scale.

2. What is a Weir?

A weir is a concrete or masonry structure which is located at the bottom side (throughout width) in the path of fluid flow. The fluid (generally water) flow above the weir.

A weir is generally used to regulate flow of water at a large scale, like in rivers or canals.

A weir has considerable width along the length of fluid flow.

Before moving further in our discussion on notches and weirs let us first understand two important terms, which will help us in understanding types of both. These terms are Nappe and Crest.

3. What is Nappe (or Vein)?

The layer of fluid flowing above the weir or below the notch is known as Nappe (or Vain).

4. What is Crest (or Sill)?

The bottom edge of notch or the top edge of weir is known as Crest (or Sill).

Now we can move our discussion further on the types of Notches and Weirs.

5. Types of Notches

Notches can be further classified on the basis of shape of opening and on the basis of effects of the sides on Nappe (or Vein).

5.1. According to the shape of opening, notches can be classified as

• Stepped notch
• Trapezoidal notch
• Triangular notch
• Rectangular notch

5.2. According to the effects of the sides on Nappe (or Vein), notches can be classified as

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

6. Types of Weirs

Weirs can be classified on the basis of shape of opening, on the basis shape of Crest (or Sill) and on the basis of effects of sides on the Nappe (or Vein).

6.1. On the basis of shape of opening, weirs can be classified as

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

6.2. According to the shape of the Crest, weirs can be classified as

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

6.3. According the effect of sides on nappe (or vein), weirs can be classified as

• Weirs without end contraction
• Weirs with end contraction

Now to have better clarity on notches and weirs, we should understand differences between them.

7. Differences between notches and weirs

Below are the differences between notches and weirs in tabular form.

	Notch	Weir
1	Used to regulate discharge of small channels	Used to regulate discharge of large channels, like canals and rivers
2	They are smaller in size	They are bigger in size
3	They are usually made of metallic plate	They are usually made of concrete
4	Fluid flows below the notch	Fluid flows above the Weir

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