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Hydrology & Irrigation
Introduction and Basic Assumptions of Unit Hydrograph
The peak of a...
Question

The peak of a flood hydrograph due to a 6-h storm is 470 m3/s. The mean depth of rainfall is 8.0 cm. If average infiltration loss is 0.25 cm/h and a constant base flow of 15 m3/s, then the peak discharge of 6-h unit hydrograph for this catchment is

A
58.25 m3/s
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B
60.65 m3/s
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C
70 m3/s
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D
72.3 m3/s
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Solution

The correct option is C 70 m3/s
Duration of rainfall excess = 6 h

Mean depth of rainfall = 8 cm

Infiltration loss @ 0.25 cm/h for 6h = 0.25 x 6 = 1.5 cm

Rainfall excess = Mean depth of rainfall - Infiltration loss = 8 - 1.5 = 6.5 cm

Peak of flood hydrograph
=470 m3/s

Base flow
=15 m3/s

Peak flow DRH
=470−15=455 m3/s

Peak of 6 h unit hydrograph
=4556.5=70 m3/s

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