- 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.
- Classification of Notches and Weirs
The Notches are classified as:
- According to the shape of the opening:
(a) Rectangular Notch (b) Triangular Notch
(c) Trapezoidal Notch (d) Stepped Notch
- 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:
- According to the shape of the opening
(a) Rectangular Weir
(b) Triangular weir
(c) Trapezoidal weir (Cipolletti Weir)
- According to the shape of the crest:
(a) Sharp-crested weir
(b) Broad crested weir
(c) Narrow Crested Weir
(d) Ogee-shaped weir
- According to the effect of sides on the emerging nappe:
(a) Weir with end contraction
(b) Weir without End contraction
- Discharge over a notch or weir
(i) Rectangular notch/weir
Expression of discharge obtained for
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:
Broad crested Weir:
Ogee weir:
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