GATE 2017 CE (CE1) – Question 61

Hydrology · Streamflow measurements, unit hydrographs, hydrograph analysis · 2 marks · Numerical answer

A 2-hour storm produces a flood hydrograph peak of 200 m³/s with baseflow 20 m³/s. Spatially averaged rainfall is 2 cm over the storm and average loss rate is 0.4 cm/hour. Find the peak of the 2-hour unit hydrograph in m³/s per cm, to one decimal place.

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Correct answer: 149.0 to 151.0

Explanation

Effective rainfall depth is gross depth minus losses: $P_e=2-0.4(2)=1.2$ cm. Direct-runoff peak is total flood peak minus baseflow, $200-20=180$ m³/s. Dividing this direct-runoff hydrograph by 1.2 cm gives the 2-hour unit hydrograph, whose peak is $180/1.2=150.0$ m³/s per cm.

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