The 'design return period' for a storm drain in urban areas is typically:
Choose the correct answer
Only 1 year (annual storm)
2–10 years for minor urban drains; 25–100 years for major systems — balances cost vs risk of flooding
1000 years for all drains
Same as drinking water design period (30 years)
Correct Answer
B. 2–10 years for minor urban drains; 25–100 years for major systems — balances cost vs risk of flooding
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Return period (T): probability of exceedance = 1/T per year. Urban stormwater drains: minor system = 2–10 years (nuisance flooding); major system (trunk drains, rivers in city) = 25–100 years (property damage). Bridge design on highways: 50 years (IRC:5); major river bridges: 100 years. The design storm has rainfall intensity corresponding to the selected return period from IDF curves.
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