Work out flood return period instantly with clear inputs, formula shown and shareable results.
A 100 year flood has a 1% chance in any year, but over a 30 year mortgage the chance of at least one is 1 - 0.99^30 = 26%. The return period is an average recurrence interval, not a schedule, which is why two hundred-year floods can occur in consecutive years without contradiction.
Annual exceedance probability
p = 1 / return period
Risk over n years
risk = 1 - (1 - p)^n
Yes. Each year is independent, so consecutive occurrences are unlikely but entirely possible - the term describes long-run frequency.
Because the risk compounds across years. Over 70 years the chance of at least one 100 year event reaches 50%.