Civil Engineering > GATE 2026 SET-1 > Hydrology
The intensity-duration relationship for a rainfall on a rectangular plot ABCD of area 7 ha (1 ha = 104 m2) can be modelled by the following equation:
I = 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 m3/h) of the storm-sewer at the outfall is _____ (in integer).
I = 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 m3/h) of the storm-sewer at the outfall is _____ (in integer).
Correct : 300 m³/h
This uses the Rational Method: Q = C × I × A, where the design intensity I is taken at the time of concentration tc — the longest travel time from any corner to the outfall, which is 25 minutes (corner D).
Design intensity: I = 25/(25+10) = 25/35 = 0.7143 cm/h = 0.007143 m/h
Area A = 7 ha = 70000 m²
Q = C × I × A = 0.60 × 0.007143 × 70000 = 300 m³/h
Using tc = 25 min gives the maximum flow from the entire contributing catchment. Using a shorter tc would give higher intensity but only a fraction of the area contributes — for a standard Rational Method application with uniform runoff coefficient over the whole area, tc = longest time = 25 min is used.
Correct answer: 300 m³/h
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