## Notches

**Introduction: **A notch may be defined as an opening in one side of a tank or a reservoir, like a large orifice, with the upstream liquid level below the top edge of the opening.

Since the top edge of the notch above the liquid level serves no purpose, therefore a notch may have only the bottom edge and sides. The bottom edge, over which the liquid flows, is known as sill or crest of the notch and the sheet of liquid flowing over a notch (or a weir) are known as nappe or vein. A notch is, usually made of a metallic plate and is used to measure the discharge of liquids.

**Types of Notches**: There are many types of notches, depending upon their shapes. But the following are important from the subject point of view.

- Rectangular notch
- Triangular notch
- Trapezoidal notch
- Stepped notch

**Rectangular notch: **Consider a rectangular notch in one side of a tank over which water is flowing as shown in figure.

Let, H = Height of water above sill of notch

b = Width or length of the notch

C_{d} = Coefficient of discharge

**Triangular Notch:** A triangular notch is also called a V-notch. Consider a triangular notch, in one side of the tank, over which water is flowing as shown in figure.

Let, H = Height of the liquid above the apex of the notch

θ = Angle of the notch

C_{d} = Coefficient of discharge

**Trapezoidal Notch**: A trapezoidal notch is a combination of a rectangular notch and two triangular notches as shown in figure. It is, thus obvious that the discharge over such a notch will be the sum of the discharge over the rectangular and triangular notches.

Let,

= Height of the liquid above the sill of the notch

= Coefficient of discharge for the rectangular portion

= Coefficient of discharge for the triangular portion

= Breadth of the rectangular portion of the notch

= Angle, which the sides make with the vertical.

**Stepped Notch:** A stepped notch is a combination of rectangular notches as shown in figure. It is thus obvious that the discharge over such a notch will be the sum of the discharges over the different rectangular notches.

Let,

H_{1} = Height of the liquid above the sill of notch 1

b_{1} = Breadth of notch 1

H_{2}, b_{2} = Corresponding values for notch 2, and

C_{d }= Coefficient of discharge for both the notches