3. Notch
A notch may be defined as an opening provided in one side of a tank
or a reservoir, with upstream liquid level below the top edge of the
opening.
A notch may have only the bottom edge and sides.
The bottom edge of north over which the liquid is flow is known as
sill or crest.
The sheet of liquid flowing over a notch is known as nappe or vein.
A notch is usually made of a metallic plate.
A notch is used to measure discharge of a small stream or canal.
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5. Discharge over a Rectangular notch
Consider a rectangular notch
in one side of a tank over
which is flowing.
It gives less accuracy.
The equation of Discharge
over a Rectangular notch
Q= .Cd. L.⅔ √(2g).H^(3/2)
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6. Discharge over a triangular Notch
A triangular notch is also called a
V-notch.
Consider a triangular notch, in
one side of a tank, over which
water is flowing.
It gives more accuracy when
discharge charge is small.
For 90° notch, the equation is
simple.
It requires only readings of H, to
find discharge.
The equation of Discharge over a
triangular notch
Q=8/15.Cd.√(2g).tan(Ѳ/2).H^(5/2)
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7. Discharge over a Trapezoidal notch
A trapezoidal notch is a combination
of a rectangular notch and two
triangular notches as shown in
figure.
Hence discharge over a trapezoidal
notch will be the sum of the
discharge of rectangular notch and
triangular notches.
Discharge over Trapezoidal notch=
discharge over rectangular notch+
discharge over triangular notch
Q= .Cd. L.⅔ √(2g).H^(3/2) + 8/15.Cd.
√(2g).tan( /2ᶿ ).H^(5/2)1 7
8. Weir
A weir may be defined as a structure constructed across a river or canal
to store water on the upstream side. Water flows over the crest of weir.
The top of a weir over which water flows is known as crest.
A Weir is made of cement concrete or masonary.
A weir is used to measure large discharge of rivers or big canals.
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9. Types of weirs
a) According to the shape
(a) Rectangular weir
(b) Cippoletti weir
b) According to the nature of discharge
(a) Ordinary weir
(b) submerged weir or drowned weir
c) According to the width of crest
(a) Narrow- crested weir
(b) Broad-crested weir
d) According to the nature of crest
(a) sharp crested weir
(b) ogee weir
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10. Cippoletti weir
The Cippoletti weir is a trapezoidal
weir, having side slopes 1 horizontal to
4 vertical.
The purpose of slope, on the sides, is
to obtain an increased discharge
through the triangular portions of the
weir, which otherwise would have
been decreased due to end
contractions in the case of rectangular
weirs.
Fransis formula for Cippoletti weir is,
Q=1.84 L . H^(3/2)
General equation of discharge for
Cippoletti weir,
Q= . Cd. L .⅔ √(2g). H^(3/2)
When velocity of approach is
considered,
Q= .Cd.L.⅔ √(2g).[H1^(3/2)-Ha^(3/2)1 10
11. Discharge over a Narrow-crested weir
A narrow crested weir is
hydraulically similar to an
ordinary weir or to a
rectangular weir.
The equation of discharge
over a narrow-crested weir
Q= .Cd. L .⅔ √(2g).
H^(3/2)
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12. Discharge over a Broad-crested weir
This is a weir having very broad
crest so that the flow of water
over crest may be compared to
the flow of water in a channel.
The equation of discharge over
a Broad crested weir,
Q=Cd. L. h. √(2g(H-h))
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13. Discharge over a sharp-crested weir
It is a special type of weir, having
a sharp crest. The water flowing
over the crest comes in contact
with the crest line and then
springs up from the crest and falls
as a trajectory as shown in figure.
The equation of Discharge over a
sharp-crested weir
Q= . Cd. L .⅔ √(2g). H^(3/2)
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14. Ogee weir
We know, in the case of a sharp
crested weir the nappe as it leaves
the crest springs on rises slightly at
the lower surface. In this way it
reaches a maximum rise of 0.115 H
above point ‘A’ and then falls. If the
space below the bottom surface of
the nappe be filled with masonary
or concrete, the consequent weir
formed is called ogee weir.
Is is generally used as a spillway of
a dam.
The equation of Ogee weir
Q= . Cd. L .⅔ √(2g). H^(3/2)1 14
15. Submerged weir or Drowned weir
A weir which the water level on the downstream
side of a weir is above the crest of the weir, is
called Submerged weir or Drowned weir.
The total discharge over the weir is obtained by
dividing the weir into two parts.
The portion of water between u/s and d/s water
surface may be treated as free weir and portion
between d/s water surface and crest of weir as a
drowned weir.
Q1= discharge through upper portion
Q= Cd. L.⅔ √(2g).(H-h)^(3/2)
Q2 = discharge through drowned portion
=Cd2* area of flow * velocity of flow
=Cd2* (L*h) * √(2g(H-h))
Total discharge
Q=Q1+Q2
= Cd. L.⅔ √(2g).(H-h)^(3/2)+ Cd2* (L*h) *
√(2g(H-h))
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16. Difference between Notch & Weir
NOTCH
A notch may be defined as an
opening provided in one side of a
tank or reservoir, with u/s liquid level
below the top edge of the opening.
The bottom edge of notch over
which water flows is known as sill or
crest.
A notch is usually made of a
metallic plate.
A notch is used to measure small
discharge of small stream or canal.
Notches are of small size.
WIER
A weir may be defined as a
structure constructed across a river
or canal to store water on the
upstream side.
The top of the weir over which
water flows is known as crest.
A weir is made of cement concrete
or masonary.
A weir is used to measure large
discharge of rivers and large canals.
Weirsd are of bigger size.
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