FLUID MECHANICS : Notches & weirs

By Shivendra Pratap|Updated : December 3rd, 2021
  1. NOTCHES AND WEIRS

1.1   Notch

  • A notch is a device used for measuring the rate of flow of a liquid through a small channel.
  • Notch can be defined as an opening in the side of a tank or a small channel in such a way that the liquid surface in the tank is below the top edge of the opening
  1. NOTCHES AND WEIRS

1.1   Notch

  • A notch is a device used for measuring the rate of flow of a liquid through a small channel.
  • Notch can be defined as an opening in the side of a tank or a small channel in such a way that the liquid surface in the tank is below the top edge of the opening.
  • Notch is generally made of metallic plate.

1.2     Weir

  • A weir is a concrete or masonry structure placed in the open channel over which the flow occurs.
  • It is generally in the form of a vertical wall, with a sharp edge at the top, running all the way across the channel.
  • Weir is of bigger size compared to Notch.
  • Weir is made of concrete or masonry structure.

Nappe or Vein The sheet of  water flowing through a notch or over a weir is called Nappe or Vein

Crest or Still The bottom edge of a notch or a top of a weir over which the water flows is known as the still or crest.

  1. Classification of Notches and Weirs

The Notches are classified as:

  1. According to the shape of the opening:

(a) Rectangular Notch                             (b) Triangular Notch

(c) Trapezoidal Notch                              (d) Stepped Notch                                                    

  1. According to the effect of the sides on the nappe:

      (a) Notch with end contraction.

      (b) Notch without end contraction or suppressed notch.

Weirs are classified as:

  1. According to the shape of the opening

      (a) Rectangular Weir

      (b) Triangular weir

      (c) Trapezoidal weir (Cipolletti Weir)

  1. According to the shape of the crest:

      (a) Sharp-crested weir

      (b) Broad crested weir

      (c) Narrow Crested Weir

      (d) Ogee-shaped weir

  1. According to the effect of sides on the emerging nappe:

      (a) Weir with end contraction

      (b) Weir without End contraction

  1. Discharge over a notch or weir

(i)  Rectangular notch/weir

Expression of discharge obtained for

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where, H = Height of water over the notch

            L = Length of the crest of the notch

         Cd1 = Coefficient of discharge for rectangular portion ABCD

         Cd2 = Coefficient of discharge for a triangular portion [FAD and BCE]

Cipoletti Weir or Notch:

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Broad crested Weir:

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Ogee weir:

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