The 'design storm' for urban drainage uses IDF (Intensity-Duration-Frequency) curves where:
Choose the correct answer
All durations give the same intensity
Shorter durations produce higher intensities for same return period — IDF relates intensity, duration, and return period
Intensity increases with duration
Return period has no effect on design intensity
Correct Answer
B. Shorter durations produce higher intensities for same return period — IDF relates intensity, duration, and return period
Why is this the answer?
IDF curves: for each return period T and storm duration D, the average intensity i is derived from historical rainfall records using frequency analysis. Longer duration → lower intensity (storm accumulates but at less intensity). Shorter duration → higher intensity (burst). Rational method: Q=CiA uses intensity for time of concentration Tc (critical duration = Tc for max runoff). IDF from Talbot formula: i = a/(t+b) or power form i=k/t^n.
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