After the extreme rainfall that accompanied Hurricane Harvey, many people were talking about 100-year or 500-year floods. What is meant by these terms? A 100-year event has a one percent chance of occurring in any given year. A 500-year event has a 0.2 percent chance of occurring in a year. Some assumptions are made in these calculations that may not always be true. First is that any extreme event in one year is independent of other years. Second is that the extreme events come from the similar climate events, such as summer thunderstorms or tropical storms. The third is that the event is based on the largest storm. The last is that the probability distribution is stationary. Only the third assumption is unlikely to create issues or uncertainty.
Mary Knapp, Weather Data Library