Section 4: Soil and Water Conservation Engineering
Hydrology
Subtopic: Runoff estimation
Question Statement:
A watershed, with an area of 360 km², has a triangular shaped 4-h unit hydrograph with a base of 50 hours. The peak discharge of direct runoff hydrograph due to 3 cm of rainfall-excess in 4 hours from the watershed in m³ s⁻¹ is
Options:
(A) 13.33
(B) 40.00
(C) 120.00
(D) 160.00
Official Answer Key:
C
Step-by-Step Solution & Explanation
For a triangular UH, the area under the hydrograph equals the runoff volume for 1 cm of excess rainfall over the watershed: Volume(1cm) = Area×0.01m = (360×10⁶ m²)(0.01) = 3.6×10⁶ m³.
Triangle area = ½×base×peak ⇒ Peak(1cm) = 2×Volume/base = 2×3.6×10⁶/(50×3600s) = 40 m³/s.
Since UH ordinates scale linearly with excess rainfall depth: Peak(3cm) = 40×3 = 120.00 m³/s. Answer: (C).
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