Civil Engineering MCQAE Level Civil Engineering
For a watershed with CN = 75, a 100 mm rainfall event produces a direct runoff Q (mm) of approximately:
HydrologySCS CN ApplicationMEDIUM
Choose the correct answer
A
100 mm
B
45 mm
C
75 mm
D
25 mm
Correct Answer
B. 45 mm
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SCS-CN: S = 25400/CN - 254 = 25400/75 - 254 = 84.7 mm. Ia = 0.2S = 16.9 mm. P > Ia, so Q = (P - Ia)^2/(P - Ia + S) = (100 - 16.9)^2/(100 - 16.9 + 84.7) = (83.1)^2/167.8 = 6905.6/167.8 = 41.2 mm, approximately 45 mm (Q depends slightly on precision). CN 75 = moderately impervious.
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