Section 4: Soil and Water Conservation Engineering
Soil and Water Erosion
Subtopic: Terraces and bunds
Question Statement:
In a semi-arid region with land slope of 3 %, contour bunding is planned to conserve and store the rain water. In 10 years return period, the daily effective rainfall is estimated as 20 cm maximum in the region. If the horizontal spacing between the bunds is 30 m, the designed height of the bund, in m, is __________. (Rounded off to 2 decimal places)
Official Answer Key:
0.58 to 0.62
Step-by-Step Solution & Explanation
The bund must impound the entire effective rainfall falling on the strip of land between two successive bunds.
Step 1 — water to be stored per metre length of bund:
V = effective rainfall depth × horizontal spacing
= 0.20 m × 30 m = 6 m³ per metre run
Step 2 — geometry of the ponded water. The water surface is horizontal while the ground rises at the land slope S = 3 % = 0.03. The ponded water therefore forms a triangular wedge of vertical depth h at the bund, extending upslope a horizontal distance:
x = h/S = h/0.03
Cross-sectional area of stored water:
A = ½ × base × height = ½ × (h/0.03) × h = h²/0.06
Step 3 — equate and solve:
h²/0.06 = 6
h² = 0.36
h = 0.60 m
(In practice a freeboard would be added to this hydraulic height.)
SECTION 5: IRRIGATION AND DRAINAGE ENGINEERING
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