Question Bank • QB_EXP_SWCE_MEA_005 NAT • 2 Marks

Section 4: Soil and Water Conservation Engineering

Flow Measurement: Weirs, Flumes & Orifices

Subtopic: 90 degree V-notch weir discharge

Question Statement:

A $90^\circ$ triangular V-notch weir is used to measure flow in an irrigation field channel. If the head over the crest is $H = 25\text{ cm}$ and coefficient of discharge $C_d = 0.60$, the discharge is ________ $\text{L/s}$ (round off to one decimal place).

Answer Key:

44.3

Step-by-Step Solution & Explanation

For a symmetrical triangular weir with vertex angle $\theta = 90^\circ$: $Q = \frac{8}{15} C_d \sqrt{2g} \tan\left(\frac{\theta}{2}\right) H^{5/2}$ Here $\theta/2 = 45^\circ$, $\tan(45^\circ) = 1$, $H = 0.25\text{ m}$: $Q = \frac{8}{15} \times 0.60 \times \sqrt{19.62} \times 1 \times (0.25)^{2.5} = 1.4174 \times (0.25)^{2.5}\text{ m}^3/\text{s}$ $Q = 0.0443\text{ m}^3/\text{s} = 44.3\text{ L/s}$
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