In THE PERFECT STORM: The Science Behind Subrogating Catastrophic Flood Losses, hydrology expert witness Richard Van Bruggen writes:
The term “100-year flood” still seems to cause confusion among public lenders, professionals, and insurance companies. Many continue to believe it is a description of a flood that occurs only once every 100 years. In truth, the term “100-year flood” is an abbreviated way of describing the magnitude of a rainfall and subsequent flood event that has a one percent chance of occurring. It is important to note that the same statistical chances apply for any storm at any time and any given year. The “return period” (or recurrence interval) of an annual maximum flood event has a return period of X years if its magnitude is equaled or exceeded once, on the average, every X years. A reciprocal of X (1/X) is the exceedance probability of the event, meaning the probability that the event is equaled or exceeded in any one year. As an example, a 100-year return period (1/100) means that there is a 1% probability of an occurrence in any one year. A 10-year return period (1/10) means that there is a 10% probability of an occurrence in any one year. A 500-year return period (1/500) means that there is a 0.2% probability of such an occurrence in any one year. This is why many hydrologists have tried to change the terminology from “100-year flood” to a “1 percent flood”.