GATE 2017 CE (CE1) – Question 34

Hydrology · Streamflow measurements, unit hydrographs, hydrograph analysis · 1 mark · Numerical answer

A 2-hour unit hydrograph has ordinates at times 0, 1, 2, 3, 4 hours of 0, 5, 12, 25, 41 m³/s respectively. Find the ordinate in m³/s of the corresponding 4-hour unit hydrograph at time 3 hours.

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Correct answer: 15 to 15

Explanation

A 4-hour unit rainfall excess can be formed by two consecutive 2-hour half-unit pulses. Therefore its hydrograph is half the sum of the 2-hour unit hydrograph and an identical copy delayed by 2 hours. At $t=3$ h, $U_4(3)=[U_2(3)+U_2(1)]/2=(25+5)/2=15$ m³/s.

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