Abstract: The triangular weir or V-notch is preferable to the rectangular weir for the
measurement of wide range of flow. V-notch weir has the advantage that it can function for
very small flows and also measure reasonably larger flows as well. The flow regime in a weir
is said to be partially contracted when the contractions along the sides of the V-notch are
not fully developed due to proximity of the walls and bed of approach channel. Whereas a
weir which has an approach channel and whose bed and sides of the notch arc sufficiently
remote from the edges of the V-notch to allow for a sufficiently great approach velocity
component parallel to the weir face so that the contraction is fully developed is a fully
contracted weir. In case of a fully contracted weir Cd is fairly constant for a particular angle
of notch.
Theory:
Fig: Flow over a V-Notch
Consider the V-notch shown in the figure. Let H be the height of water surface and θ be the
angle of notch. The total theoretical discharge over the notch is given by
8
Qt = 15 √2𝑔 tan
𝜃
5
𝐻2
2
Let Qa be the actual discharge, Then the coefficient of discharge, Cd is given by
𝑄
Cd = 𝑄𝑎
𝑡
Apparatus: Steady water supply with a means of varying the flow rate, approach channel,
V-notch weir plate, flow rate measuring facility, point gauge for measuring H.
Procedure:
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•
•
•
•
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The weir plate was positioned at the end of approach channel, in a vertical plane, with
the sharp edge on the upstream side.
Water was admitted to channel until the water discharges over the weir plate.
The flow control valve was closed, and water was allowed to stop flowing over weir.
The point gauge was set to a datum reading.
The gauge was positioned about halfway between the notch plate and stilling baffle.
Water was admitted to the channel and flow control valve was allowed to obtain
heads, H, increasing in steps of 1 cm.
For each flow rate, conditions were stabilized, H was measured and recorded.
Readings of volume and time using the volumetric tank was taken to determine the
flow rate.
Data:
Angle of the notch, 𝜃 = 90°
K= 23.6
Cross-sectional area of the measuring tank= 1320 cm2
Initial point gauge reading, h2 = 9 cm
Final point gauge reading, h1= 6 cm
Difference in reading = 3 cm
Trail
1
Vol of
water, V
Collection
time, T
Effective
Head, H
sec
Actual
Discharge
Qa
cm3/sec
cm3
5280
Cd
cm
Theoretical
Discharge,
Qt
cm3/sec
19
278
3
367.89
0.76
No of
observation
1
2
3
4
Actual
discharge Qa
132
278
278
507.69
Effective head
H
2
4
3
2.3
Theoretical
discharge, Qt
133.4
755.2
367.89
Calculation:
8
K = 15 × √2 × 981 × tan
Qa =
5280
19
90°
2
= 23.6
= 278 cm3/s
5
Qt= 23.6× (3)2 = 367.89 cm3/s
Cd=
278
367.89
= .76
Discussion: The variation of the calculation was affected by personal and instrumental
errors.