FLOW OVER NOTCHES  AND  WEIRS

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FLOW OVER NOTCHES  AND  WEIRS

 

 

Flow over notches and weirs There is no difference between a notch and a weir, except that the former is a small structure and has sharp edges.The sheet of water flowing through a notch or a weir is called as Nappe or Vein. The bottom of the notch or the top of weir over which the water flows, is known as the sill (or) crest and its height above the bottom of the tank or channel is known as the sill height or crest height.

NOTCH

A notch may be defined as an opening in the side of a tank or a channel in such a way that the liquid surface in the tank or channel is below the top edge of the opening. It is used to measured rate of flow of liquid through a channel or a tank. The notch is generally made with metallic plates.

CLASSIFICATION OF NOTCHES

The notches are classified, The shape of the opening is according to.

1. Rectangular notch

2. Triangular notch or V-notch

3. Trapezoidal notch, and

4. Stepped notch.
Notches may also be classified, according to the effect of the sides on the nappe, as
1. Notch with end contraction, and
2. End contraction or suppressed notch without being “notched”.

WEIR

A weir is a concrete or masonry structure placed across a river to raise the water level on the upstream side and to allow the excess water to flow over it to the downstream side, It is also used to measure rate of flow of water in rivers or streams.In a channel, if the crest length of the notch is less than the width of channel then it is known as notch with end contraction. But, if the crest length is equal to the width of channel then it is known as notch without end contraction.

CLASSIFICATION OF WEIRS

(a) The shape of the opening is being referred to.
1 Rectangular weir
2 Triangular weir, and
3. Trapezoidal weir.
4. Stepped weir

(b)The shape of the crest is according to.
1. Sharp-crested weir
2. Broad-crested weir
3. Narrow-crested weir
4. Ogee-shaped weir.

(c) The Sides Effect influences the issuance of Nappe.
1. Weir with end contraction and
2. Weir without end contraction.

d) The Discharge Conditions state that.
1.Freely discharging weir, (above crest)
2. Submerged or drowned weir( above crest)
If the liquid level on the downstream of the weir is lower than crest level, the weir is discharging free. When the liquid level on the downstream is higher than the crest level, the weit is submerged

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