Types of weir:

1. Rectangular

• Used to measure comparatively large discharges.
• May be either contracted or suppressed.
• Have sharp crest and are levelled on the downstream side only.

Some formulas for different rectangular weirs

1. For Suppressed rectangular weirs; Q = 0.0184 LH ^3/2

Where, Q= discharge in liter/sec.

             L = length of crest in cm

             H = Head over the weir crest in cm.

1. For two end contracted rectangular weirs; Q= 0.0184 ( L- 0.2 H ) H^3/2

• For one end contracted weir; Q= 0.0184 ( L- 0.1 H) H^3/2

1. Trapezoidal (Cipoletti weir)

• It is contracted trapezoidal weir in which each side of the notch has a slope of 1 horizontal to 4 vertical.
• Used to measure medium discharges.
• Discharge is measured by following formula

Q = 0.0186 LH ^3/2.

Where, Q= discharge in liter/sec.

             L = length of crest in cm

             H = Head over the weir crest in cm.

1. V-notched or triangular

• Used to measure small and medium sized streams .
• The advantage being its ability to measure small flows accurately.
• Has both sides sharp, bevelled from the down stream side only.

Q= 0.0138 H ^5/2 ltr/sec

Where, Q= discharge in liter/sec.

             H = Head over the weir crest in cm.

• Rectangular weir can be further divided into suppressed and in contracted weir.
• If instead the iron sheet, the weir is made up of bricks, it is called broad crested weir.
• A weir having thin edged crest such that the over flowing sheet of water has the minimum surface contact with the crest is called Sharp crested weir..

Some terminologies

a. End contraction : The horizontal distance from the end of the weir crest to the sides of the weir pond.

b. Bottom Contraction: The vertical distance from the weir crest to the bottom of the weir pond.

c. Weir Scale or gauge : The scale fastened on the side of weir on a stake in the weir crest to measure the head on the weir crest.

d. Nappe : The sheet of water which over flows a weir.

e. End contraction weir: When the crest length or width of weir notch is less than the channel width, it is called end contraction weir. (B > L).

f. Suppressed weir : When the crest extends across the full width of the channel and no contraction is produced, it is called as suppressed weir ( B=L).

g. Free flow: When the water surface down streams from the weir wall which is far enough below the crest so that air has access around the nappe, the flow is said to be free.

The basic formula to calculate the discharge through a weir is

Q = CLH^m

Where,

Q= discharge in liter/sec.

C= a coefficient and depend on the nature of the crest and approach conditions

L = length of the crest, cm

H = Head of the crest , cm

m = an exponent, depending upon the weir opening