GATE 2026 AR — Architecture – Question 45
A rectangular open concrete drain is 3 m wide with 1.5 m flow depth, bed slope 0.001 and Manning roughness 0.018. Find mean flow speed in m/s, to two decimals.
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Correct answer: 1.30 to 1.60
Explanation
Flow area A=3×1.5=4.5 m² and wetted perimeter P=3+2×1.5=6 m. Hydraulic radius is A/P=0.75 m. Manning’s equation gives v=(1/0.018)×0.75^(2/3)×√0.001=1.45022 m/s, or 1.45. The free top surface is not included in the wetted perimeter.