Weather & Calamities

Intermediate

Flood Return Period

The average interval between floods of a given size (Weibull plotting position).

Formula

T = \frac{n+1}{m}

Variables

TReturn period (years)

nNumber of years of record

mRank of the event (largest = 1)

Example

50 years of data, event ranked 1st: T = 51 years

Did You Know?

A "100-year flood" has a 1% chance each year — not one guaranteed per century.

flood hydrology disaster risk

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