Section 4: Soil and Water Conservation Engineering
Hydrology
Subtopic: Hydrograph analysis, unit hydrograph theory and application
Question Statement:
An effective rainfall of 20 mm h⁻¹ occurs for 2 hours in a catchment. The time of concentration of the catchment is 1.5 hour. The peak of the resulting direct runoff hydrograph, in mm h⁻¹, is
Options:
(A) 10
(B) 20
(C) 30
(D) 40
Official Answer Key:
B
Step-by-Step Solution & Explanation
Peak (equilibrium) discharge is reached once the entire catchment is contributing simultaneously, i.e. once rainfall has lasted at least one time of concentration (tc = 1.5 h). Since the storm duration (2 h) exceeds tc, the catchment reaches full contribution and the runoff hydrograph rises to, and holds at, the rainfall excess rate itself, 20 mm h⁻¹, before receding once rainfall stops.
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