GATE 2020 CE (CE1) – Question 34
A 4 m wide rectangular channel carries 6 m³/s. Manning n = 0.02 and g = 9.81 m/s². Find critical velocity (m/s), to two decimals.
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Correct answer: 2.35 to 2.55
Explanation
Unit discharge is $q=6/4=1.5$ m²/s. Critical depth is $y_c=(q^2/g)^{1/3}$. Thus critical velocity is $q/y_c\approx\mathbf{2.45}$ m/s. Manning n is unnecessary for the critical condition.