Section 4: Soil and Water Conservation Engineering
Hydrology
Subtopic: Hydrograph analysis, unit hydrograph theory and application
Question Statement:
[Linked 54–55] The peak of a flood hydrograph due to a 1-h duration isolated storm in a catchment of area 13.5 km² is 135 m³ s⁻¹. Total depth of rainfall is 54 mm. Assume constant base flow 10 m³ s⁻¹ and phi-index 4 mm h⁻¹. The peak of the 1-h unit hydrograph for the catchment in m³ s⁻¹ is
Options:
(A) 15
(B) 20
(C) 25
(D) 30
Official Answer Key:
C
Step-by-Step Solution & Explanation
Effective rainfall (runoff-producing) = total rainfall − φ-index loss (over the 1-h storm duration) = 54 − 4×1 = 50 mm. Direct runoff peak = total hydrograph peak − base flow = 135 − 10 = 125 m³/s. The 1-h unit hydrograph (defined for 1 cm = 10 mm of rainfall excess) has peak = (direct runoff peak/effective rainfall in mm) × 10 mm = (125/50)×10 = 25 m³/s.
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