Subtopic: Planning, design and layout of surface and sub-surface drainage systems
Question Statement:
In a subsurface drainage network, 12 lateral drains each of 100 m long are laid at a spacing of 50 m. These lateral drains are connected to a collector drain. When the water table dropped 50 cm below the soil surface in 4 days, the average discharge at the outlet of the collector drain was found to be 12 L s⁻¹. The average drainable porosity of soil in per cent is ________. [round off to two decimal places]
Official Answer Key:
13.75 to 13.85
Step-by-Step Solution & Explanation
• Total drained area = 12 laterals × 100 m × 50 m spacing = 60,000 m²
• Total volume drained = Q×t = 12 L/s × (4×86400 s) = 4,147,200 L = 4147.2 m³
• Drainable porosity f = Volume/(Area × drop) = 4147.2/(60000×0.5) = 4147.2/30000 = 0.1382 = 13.82%
Computed value: 13.82% (within official range 13.75–13.85)
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