GATE 2026 CE (CE1) – Question 64
The intensity-duration relationship for a rainfall on a rectangular plot ABCD of area 7 ha (1 ha = $10^4$ m$^2$) can be modelled by the following equation:
$$I = \frac{25}{(t + 10)}$$
where $I$ is rainfall intensity (in cm/h), and $t$ is the duration (in minutes) of rainfall. The average runoff coefficient over the area is 0.60. The time of entry (in minutes) to the outfall from the corners A, B, C, and D is 10, 20, 15 and 25, respectively.
The design flowrate (in m$^3$/h) of the storm-sewer at the outfall is _____ (*in integer*).
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Correct answer: 300
Explanation
The time of concentration is the longest time of entry, 25 minutes. The design intensity is $I = \frac{25}{25 + 10} = 0.714$ cm/h $= 0.00714$ m/h. By the rational method, $Q = CIA = 0.60 \times 0.00714 \times 70000 = 300$ m$^3$/h.