Section 4: Soil and Water Conservation Engineering
Soil and Water Erosion
Subtopic: Terraces and bunds
Question Statement:
Contour bunds are constructed on a land slope of 5% at a vertical interval of 1.35 m to store a 24 hour excess rainfall of 0.1 m. Minor effects due to side slopes of the bund are neglected in the calculation of storage volume of water behind the bund. The depth of impounding immediately behind the contour bund is
Options:
(A) 0.32 m
(B) 0.42 m
(C) 0.52 m
(D) 0.62 m
Official Answer Key:
C
Step-by-Step Solution & Explanation
Horizontal interval between bunds, HI = VI/slope = 1.35/0.05 = 27 m. Storage volume per unit contour length = HI × excess rainfall depth = 27×0.1 = 2.7 m² (cross-sectional area, per metre run). This water impounds as a wedge behind the bund with its free surface sloping back at the land slope S, so wedge cross-section area = d²/(2S), where d is the impounding depth at the bund. Setting d²/(2×0.05) = 2.7 → d² = 0.27 → d = 0.520 m.
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