Subtopic: Leaching requirement and salinity control
Question Statement:
Agricultural Drainage: Drainage coefficient; planning, design and layout of surface and sub- surface drainage systems; leaching requirement and salinity control; irrigation and drainage water quality and reuse; nonconventional drainage system.
In a drainage area of 15 ha, the slope and drainage coefficient are 0.4% and 11 mm/day. Manning's roughness coefficient is 0.016. The inside diameter (mm) of the corrugated plastic tubing used for drainage is
Options:
(A) 200.51
(B) 205.52
(C) 209.51
(D) 215.23
Official Answer Key:
C
Step-by-Step Solution & Explanation
Design discharge, Q = drainage coefficient × area = (0.011 m/day)/86400 s/day × (15×10,000 m²) = 1.2731×10⁻⁷ m/s × 150,000 m² = 0.01910 m³/s.
For a circular pipe flowing full, Manning's equation: Q = (1/n)·A·R^(2/3)·S^(1/2), with A = (π/4)D², R = D/4.
Solving iteratively for D with n = 0.016, S = 0.004:
D ≈ 0.2096 m = 209.6 mm ≈ 209.51 mm. Answer: (C) 209.51.
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