Flood Return Period

This formula estimates the average waiting time between floods at least as large as a chosen event, based on a ranked record of past floods. It comes from the Weibull plotting position, which treats the observed flood magnitudes as an ordered sample and assigns each rank an exceedance probability of about m/(n+1); the return period is the reciprocal of that probability. In practice, a larger return period means a rarer flood in the historical record.

Use this when you have a list of annual peak flood measurements and want a simple estimate of how rare a particular flood is. It is commonly used in hydrology, river engineering, and flood-risk studies to compare past events or size drainage and flood-control structures.

• Using the flood size in the formula instead of the rank m
• Forgetting that the largest event has rank 1, not rank n
• Interpreting return period as a fixed schedule rather than a long-term average