Bunds are to be constructed to conserve rainwater in a farm having 6% slope. If the horizontal interval between two bunds is 30 m and there is no loss of water, the required height of the bund to store rainwater from an 18 cm rainfall event with 10 years of return period in cm will be __________.
Official Answer Key:
80 to 81
Step-by-Step Solution & Explanation
• On a uniformly sloping field, water ponded behind a bund of height H forms a triangular wedge (in cross-section) bounded by the bund (vertical side H) and the sloping ground, extending upslope a horizontal distance H/s, where s = slope = 0.06.
• Cross-sectional storage area of this wedge = ½ × H × (H/s) = H²/(2s).
• With “no loss of water”, this storage must equal the entire rainfall volume falling on the 30 m interval (per unit width): Volume = rainfall depth × interval = 0.18 m × 30 m = 5.4 m² (per metre width).
• H²/(2×0.06) = 5.4 ⇒ H² = 5.4×0.12 = 0.648 ⇒ H = √0.648 ≈ 0.805 m = 80.5 cm, inside the accepted range 80–81.
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