The peak of a 4 hour flood hydrograph is 240 m³/sec . If the rainfall excess is 80 mm and base flow which is constant is 40 m³/sec, then the peak of 4-hours unit hydrograph will be A)20 m³/sec B)25 m³/sec C)30 m³/sec D)35 m³/sec

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Quizplus · Accepted Answer

The peak of the 4-hour unit hydrograph is calculated by subtracting the base flow from the total flow and then dividing by the rainfall excess (in cm to match units with the flow rate which is in m^3/s). Given a total peak flow of 240 m^3/s, a base flow of 40 m^3/s, and a rainfall excess of 80 mm (or 8 cm), the calculation is as follows: (240 m^3/s - 40 m^3/s) / 8 cm = 200 m^3/s / 0.08 m = 2500 m^3/s/m. Since the unit hydrograph is defined per cm of rainfall, we divide by 100 to convert the depth from cm to m, resulting in a peak of 25 m^3/s/cm, which corresponds to option B.

The Peak of a 4 Hour Flood Hydrograph Is 240