A 100-year flood can happen again next year
A return period is a way to express an estimated annual exceedance probability, not a timetable. This worked explanation uses a simplified model to show why several years of exposure matter. It does not estimate risk for a particular property.

1. Identify the event threshold and the source of its probability estimate. Flood depth, river flow and rainfall totals describe different thresholds; a label without one is incomplete.
2. For an annual exceedance probability p, the reciprocal return period is T = 1/p. Thus p = 0.01 corresponds to 100 years in this terminology, not a guaranteed interval.
3. Under constant probability and independent annual outcomes, the chance of at least one exceedance in n years is 1 − (1 − p)^n. State these assumptions beside the calculation.
4. Before a local decision, obtain current flood mapping and advice relevant to the site. Ask about uncertainty, changing conditions and the event definition rather than treating this example as a parcel assessment.

One year: p = 0.01: a 1% annual chance of exceeding the specified threshold.
Ten years — simplified model: 1 − 0.99^10 ≈ 9.56% for at least one exceedance.
Thirty years — simplified model: 1 − 0.99^30 ≈ 26.03% for at least one exceedance.
After an event: The model does not introduce a 99-year safe waiting period. Under its assumptions, next year still has a 1% chance.
Questions for a local assessment: Threshold __; source and date __; location __; model uncertainty __; changing exposure __; professional/local authority advice __.

One year: ____________________
Ten years — simplified model: ____________________
Thirty years — simplified model: ____________________
After an event: ____________________
Questions for a local assessment: ____________________
Annual independence and a constant p are modelling assumptions, not a claim that climate, catchments or flood estimates stay fixed. A probability is not a promise about the next event.

USGS — The 100-Year Flood: It’s All About Chance — https://www.usgs.gov/publications/100-year-flood-its-all-about-chance