Subtopic: Planning, design and layout of surface and sub-surface drainage systems
Question Statement:
In a sub-surface drainage system, tile drains are laid with a slope of 0.28% to carry a peak discharge of 3 litre s⁻¹ per drain. If the Manning's n is 0.011, the practical diameter of tile required is
Options:
(A) 50 mm
(B) 75 mm
(C) 100 mm
(D) 150 mm
Official Answer Key:
C
Step-by-Step Solution & Explanation
For a circular pipe flowing full, Manning's equation: Q = (1/n)A R^(2/3) S^(1/2), with A=πD²/4, R=D/4. Solving iteratively for Q=0.003 m³/s, n=0.011, S=0.0028 gives a theoretical diameter of ≈97.3 mm. Rounding up to the next commercially available (practical) tile size gives 100 mm.
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