GATE 2025 CE (CE1) – Question 34
A water resources project with an expected life of 25 years has to be designed for an acceptable risk of 5% against a design flood. The return period for the design flood (in years) is _________________ (*rounded off to the nearest integer*).
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Correct answer: 488
Explanation
The risk is the chance of at least one exceedance in $n$ years, $R = 1 - \left(1 - \frac{1}{T}\right)^n$. So $0.05 = 1 - \left(1 - \frac{1}{T}\right)^{25}$, which gives $1 - \frac{1}{T} = 0.95^{1/25} = 0.997950$ and $T = 487.9$, about 488 years.