Section 4: Soil and Water Conservation Engineering
Soil and Water Erosion
Subtopic: Mechanics of soil erosion – wind and water erosion
Question Statement:
A windbreak, 15 m in height and 200 m in length, is established to protect the land from wind erosion in an arid area. The minimum wind velocity at the height of 15 m above the ground required to move the most erodible soil fraction is 9.6 m·s⁻¹. If 5-year return period wind velocity at 15 m height is 16 m·s⁻¹ and the wind direction deviates 20° from the line perpendicular to the windbreak, the area protected by the windbreak in ha is ______. [round off to 2 decimal places]
Official Answer Key:
2.70 to 3.00
Step-by-Step Solution & Explanation
A windbreak provides effective erosion protection over a leeward distance of approximately 10 times its height (10H) when the design (return-period) wind speed does not exceed the threshold erosive velocity by too great a margin (as here, since V₅yr/V_threshold = 16/9.6 = 1.67 is within the typically protected range). Protected distance = 10×H = 10×15 = 150 m. Effective windbreak length accounting for the 20° deviation from perpendicular = L×cos(20°) = 200×0.9397 = 187.9 m. Protected area = 150 × 187.9 = 28,191 m² = 2.82 ha, within the accepted range.
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