Weather & Calamities

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Flood Encounter Probability

Chance of experiencing at least one flood of return period T over n years.

Formula

P = 1 - \left(1 - \frac{1}{T}\right)^{n}

Variables

PProbability of at least one event

TReturn period (years)

nDesign life (years)

Example

100-year flood over 30 years: P = 1−0.99³⁰ ≈ 26%

Did You Know?

Over a 30-year period there is about a 26% chance of the "100-year" flood occurring.

flood hydrology disaster risk

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